5.4TABLE

As a relational DBMS, Firebird stores data in tables. A table is a flat, two-dimensional structure containing any number of rows. Table rows are often called records.

All rows in a table have the same structure and consist of columns. Table columns are often called fields. A table must have at least one column. Each column contains a single type of SQL data.

This section describes how to create, alter and delete tables in a database.

5.4.1CREATE TABLE

Used forcreating a new table (relation)

Available inDSQL, ESQL

Syntax

   |CREATE [GLOBAL TEMPORARY] TABLE tablename
   |  [EXTERNAL [FILE] 'filespec']
   |  (<col_def> [, {<col_def> | <tconstraint>} ...])
   |  [ON COMMIT {DELETE | PRESERVE} ROWS]
   | 
   |<col_def> ::= <regular_col_def> | <computed_col_def>
   | 
   |<regular_col_def> ::=
   |  colname {<datatype> | domainname}
   |  [DEFAULT {<literal> | NULL | <context_var>}]
   |  [NOT NULL]
   |  [<col_constraint>]
   |  [COLLATE collation_name]
   | 
   |<computed_col_def> ::=
   |  colname [<datatype>]
   |  {COMPUTED [BY] | GENERATED ALWAYS AS} (<expression>)
   | 
   |<datatype> ::=
   |    {SMALLINT | INTEGER | BIGINT} [<array_dim>]
   |  | {FLOAT | DOUBLE PRECISION} [<array_dim>]
   |  | {DATE | TIME | TIMESTAMP} [<array_dim>]
   |  | {DECIMAL | NUMERIC} [(precision [, scale])] [<array_dim>]
   |  | {CHAR | CHARACTER} [VARYING] | VARCHAR} [(size)]
   |    [<array_dim>] [CHARACTER SET charset_name]
   |  | {NCHAR | NATIONAL {CHARACTER | CHAR}} [VARYING]
   |    [(size)] [<array_dim>]
   |  | BLOB [SUB_TYPE {subtype_num | subtype_name}]
   |    [SEGMENT SIZE seglen] [CHARACTER SET charset_name]
   |  | BLOB [(seglen [, subtype_num])]
   | 
   |<array_dim> ::= '[' [m:]n [, [m:]n ...] ']'
   | 
   |<col_constraint> ::=
   |  [CONSTRAINT constr_name]
   |    { PRIMARY KEY [<using_index>]
   |    | UNIQUE      [<using_index>]
   |    | REFERENCES other_table [(colname)] [<using_index>]
   |        [ON DELETE {NO ACTION | CASCADE | SET DEFAULT | SET NULL}]
   |        [ON UPDATE {NO ACTION | CASCADE | SET DEFAULT | SET NULL}]
   |    | CHECK (<check_condition>) }
   | 
   |<tconstraint> ::=
   |  [CONSTRAINT constr_name]
   |    { PRIMARY KEY (<col_list>) [<using_index>]
   |    | UNIQUE      (<col_list>) [<using_index>]
   |    | FOREIGN KEY (<col_list>)
   |        REFERENCES other_table [(<col_list>)] [<using_index>]
   |        [ON DELETE {NO ACTION | CASCADE | SET DEFAULT | SET NULL}]
   |        [ON UPDATE {NO ACTION | CASCADE | SET DEFAULT | SET NULL}]
   |    | CHECK (<check_condition>) }"
   | 
   |<col_list> ::= colname [, colname ...]
   | 
   |<using_index> ::= USING
   |  [ASC[ENDING] | DESC[ENDING]] INDEX indexname
   | 
   |<check_condition> ::=
   |    <val> <operator> <val>
   |  | <val> [NOT] BETWEEN <val> AND <val>
   |  | <val> [NOT] IN (<val> [, <val> ...] | <select_list>)
   |  | <val> IS [NOT] NULL
   |  | <val> IS [NOT] DISTINCT FROM <val>
   |  | <val> [NOT] CONTAINING <val>
   |  | <val> [NOT] STARTING [WITH] <val>
   |  | <val> [NOT] LIKE <val> [ESCAPE <val>]
   |  | <val> [NOT] SIMILAR TO <val> [ESCAPE <val>]
   |  | <val> <operator> {ALL | SOME | ANY} (<select_list>)
   |  | [NOT] EXISTS (<select_expr>)
   |  | [NOT] SINGULAR (<select_expr>)
   |  | (<check_condition>)
   |  | NOT <check_condition>
   |  | <check_condition> OR <check_condition>
   |  | <check_condition> AND <check_condition>
   | 
   |<operator> ::=
   |    <> | != | ^= | ~= | = | < | > | <= | >=
   |  | !< | ^< | ~< | !> | ^> | ~>
   | 
   |<val> ::=
   |    colname ['['array_idx [, array_idx ...]']']
   |  | <literal>
   |  | <context_var>
   |  | <expression>
   |  | NULL
   |  | NEXT VALUE FOR genname
   |  | GEN_ID(genname, <val>)
   |  | CAST(<val> AS <datatype>)
   |  | (<select_one>)
   |  | func([<val> [, <val> ...]])

Table 5.7CREATE TABLE Statement Parameters
ParameterDescription

tablename

Name (identifier) for the table. It may consist of up to 31 characters and must be unique in the database.

filespec

File specification (only for external tables). Full file name and path, enclosed in single quotes, correct for the local file system and located on a storage device that is physically connected to Firebird’s host computer.

colname

Name (identifier) for a column in the table. May consist of up to 31 characters and must be unique in the table.

datatype

SQL data type

col_constraint

Column constraint

tconstraint

Table constraint

constr_name

The name (identifier) of a constraint. May consist of up to 31 characters.

other_table

The name of the table referenced by the foreign key constraint

other_col

The name of the column in other_table that is referenced by the foreign key

literal

A literal value that is allowed in the given context

context_var

Any context variable whose data type is allowed in the given context

check_condition

The condition applied to a CHECK constraint, that will resolve as either true, false or NULL

collation

Collation

array_dim

Array dimensions

m, n

Integer numbers defining the index range of an array dimension

precision

The total number of significant digits that a value of the datatype can hold (1..18)

scale

The number of digits after the decimal point (0..precision)

size

The maximum size of a string in characters

charset_name

The name of a valid character set, if the character set of the column is to be different to the default character set of the database

subtype_num

BLOB subtype number

subtype_name

BLOB subtype mnemonic name

seglen

Segment size (max. 65535)

select_one

A scalar SELECT statement — selecting one column and returning only one row

select_list

A SELECT statement selecting one column and returning zero or more rows

select_expr

A SELECT statement selecting one or more columns and returning zero or more rows

expression

An expression resolving to a value that is is allowed in the given context

genname

Sequence (generator) name

func

Internal function or UDF

The CREATE TABLE statement creates a new table. Any user can create it and its name must be unique among the names of all tables, views and stored procedures in the database.

A table must contain at least one column that is not computed and the names of columns must be unique in the table.

A column must have either an explicit SQL data type, the name of a domain whose attributes will be copied for the column, or be defined as COMPUTED BY an expression (a calculated field).

A table may have any number of table constraints, including none.

5.4.1.1Making a Column Non-nullable

In Firebird, columns are nullable by default. The optional NOT NULL clause specifies that the column cannot take NULL in place of a value.

5.4.1.2Character Columns

You can use the CHARACTER SET clause to specify the character set for the CHAR, VARCHAR and BLOB (text subtype) types. If the character set is not specified, the character set specified during the creation of the database will be used by default. If no character set was specified during the creation of the database, the NONE character set is applied by default. In this case, data is stored and retrieved the way it was submitted. Data in any encoding can be added to such a column, but it is not possible to add this data to a column with a different encoding. No transliteration is performed between the source and destination encodings, which may result in errors.

The optional COLLATE clause allows you to specify the collation sequence for character data types, including BLOB SUB_TYPE TEXT. If no collation sequence is specified, the collation sequence that is default for the specified character set during the creation of the column is applied by default.

5.4.1.3Setting a DEFAULT Value

The optional DEFAULT clause allows you to specify the default value for the table column. This value will be added to the column when an INSERT statement is executed if no value was specified for it and that column was omitted from the INSERT command.

The default value can be a literal of a compatible type, a context variable that is type-compatible with the data type of the column, or NULL, if the column allows it. If no default value is explicitly specified, NULL is implied.

An expression cannot be used as a default value.

5.4.1.4Domain-based Columns

To define a column, you can use a previously defined domain. If the definition of a column is based on a domain, it may contain a new default value, additional CHECK constraints and a COLLATE clause that will override the values specified in the domain definition. The definition of such a column may contain additional column constraints (for instance, NOT NULL), if the domain does not have it.

Important

It is not possible to define a domain-based column that is nullable if the domain was defined with the NOT NULL attribute. If you want to have a domain that might be used for defining both nullable and non-nullable columns and variables, it is better practice to make the domain nullable and apply NOT NULL in the downstream column definitions and variable declarations.

5.4.1.5Calculated Fields

Calculated fields can be defined with the COMPUTED [BY] or GENERATED ALWAYS AS clause (according to the SQL:2003 standard). They mean the same. Describing the data type is not required (but possible) for calculated fields, as the DBMS calculates and stores the appropriate type as a result of the expression analysis. Appropriate operations for the data types included in an expression must be specified precisely.

If the data type is explicitly specified for a calculated field, the calculation result is converted to the specified type. This means, for instance, that the result of a numeric expression could be rendered as a string.

In a query that selects a COMPUTED BY column, the expression is evaluated for each row of the selected data.

Tip

Instead of a computed column, in some cases it makes sense to use a regular column whose value is evaluated in triggers for adding and updating data. It may reduce the performance of inserting/updating records, but it will increase the performance of data selection.

5.4.1.6Defining an ARRAY Column

  • If the column is to be an array, the base type can be any SQL data type except BLOB and ARRAY.

  • The dimensions of the array are specified between square brackets. (In the Syntax block these brackets appear in quotes to distinguish them from the square brackets that identify optional syntax elements.)

  • For each array dimension, one or two integer numbers define the lower and upper boundaries of its index range:

    • By default, arrays are 1-based. The lower boundary is implicit and only the upper boundary need be specified. A single number smaller than 1 defines the range num..1 and a number greater than 1 defines the range 1..num.

    • Two numbers separated by a colon (:) and optional whitespace, the second greater than the first, can be used to define the range explicitly. One or both boundaries can be less than zero, as long as the upper boundary is greater than the lower.

  • When the array has multiple dimensions, the range definitions for each dimension must be separated by commas and optional whitespace.

  • Subscripts are validated only if an array actually exists. It means that no error messages regarding invalid subscripts will be returned if selecting a specific element returns nothing or if an array field is NULL.

5.4.1.7Constraints

Four types of constraints can be specified. They are:

  • Primary key (PRIMARY KEY)

  • Unique key (UNIQUE)

  • Foreign key (REFERENCES)

  • CHECK constraint (CHECK)

Constraints can be specified at column level (column constraints) or at table level (table constraints). Table-level constraints are needed when keys (uniqueness constraint, Primary Key, Foreign Key) are to be formed across multiple columns and when a CHECK constraint involves other columns in the row besides the column being defined. Syntax for some types of constraint may differ slightly according to whether the constraint is being defined at column or table level.

  • A column-level constraint is specified during a column definition, after all column attributes except COLLATION are specified, and can involve only the column specified in that definition

  • Table-level constraints are specified after all of the column definitions. They are a more flexible way to set constraints, since they can cater for constraints involving multiple columns

  • You can mix column-level and table-level constraints in the same CREATE TABLE statement

The system automatically creates the corresponding index for a primary key (PRIMARY KEY), a unique key (UNIQUE) and a foreign key (REFERENCES for a column-level constraint, FOREIGN KEY REFERENCES for one at the table level).

5.4.1.7.1Names for Constraints and Their Indexes

Column-level constraints and their indexes are named automatically:

  • The constraint name has the form INTEG_n, where n represents one or more digits

  • The index name has the form RDB$PRIMARYn (for a primary key index), RDB$FOREIGNn (for a foreign key index) or RDB$n (for a unique key index). Again, n represents one or more digits.

Automatic naming of table-level constraints and their indexes follows the same pattern, unless the names are supplied explicitly.

5.4.1.7.1.1Named Constraints

A constraint can be named explicitly if the CONSTRAINT clause is used for its definition. While the CONSTRAINT clause is optional for defining column-level constraints, it is mandatory for table-level. By default, the constraint index will have the same name as the constraint. If a different name is wanted for the constraint index, a USING clause can be included.

5.4.1.7.1.2The USING Clause

The USING clause allows you to specify a user-defined name for the index that is created automatically and, optionally, to define the direction of the index — either ascending (the default) or descending.

5.4.1.7.2PRIMARY KEY

The PRIMARY KEY constraint is built on one or more key columns, each column having the NOT NULL constraint specified for it. The values across the key columns in any row must be unique. A table can have only one primary key.

  • A single-column Primary Key can be defined as a column level or a table-level constraint

  • A multi-column Primary Key must be specified as a table-level constraint

5.4.1.7.3The UNIQUE Constraint

The UNIQUE constraint defines the requirement of content uniqueness for the values in a key throughout the table. A table can contain any number of unique key constraints.

As with the Primary Key, the Unique constraint can be multi-column. If so, it must be specified as a table-level constraint.

5.4.1.7.3.1NULL in Unique Keys

Firebird’s SQL-99-compliant rules for UNIQUE constraints allow one or more NULLs in a column with a UNIQUE constraint. That makes it possible to define a UNIQUE constraint on a column that does not have the NOT NULL constraint.

For UNIQUE keys that span multiple columns, the logic is a little complicated:

  • Multiple rows having null in all the columns of the key are allowed

  • Multiple rows having keys with different combinations of nulls and non-null values are allowed

  • Multiple rows having the same key columns null and the rest filled with non-null values are allowed, provided the values differ in at least one column

  • Multiple rows having the same key columns null and the rest filled with non-null values that are the same in every column will violate the constraint

The rules for uniqueness can be summarised thus:

In principle, all nulls are considered distinct. However, if two rows have exactly the same key columns filled with non-null values, the NULL columns are ignored and the uniqueness is determined on the non-null columns as though they constituted the entire key.

Illustration

  |RECREATE TABLE t( x int, y int, z int, unique(x,y,z));
  |INSERT INTO t values( NULL, 1, 1 );
  |INSERT INTO t values( NULL, NULL, 1 );
  |INSERT INTO t values( NULL, NULL, NULL );
  |INSERT INTO t values( NULL, NULL, NULL ); -- Permitted
  |INSERT INTO t values( NULL, NULL, 1 );    -- Not permitted

5.4.1.7.4FOREIGN KEY

A Foreign Key ensures that the participating column(s) can contain only values that also exist in the referenced column(s) in the master table. These referenced columns are often called target columns. They must be the primary key or a unique key in the target table. They need not have a NOT NULL constraint defined on them although, if they are the primary key, they will, of course, have that constraint.

The foreign key columns in the referencing table itself do not require a NOT NULL constraint.

A single-column Foreign Key can be defined in the column declaration, using the keyword REFERENCES:

  |... ,
  |  ARTIFACT_ID INTEGER REFERENCES COLLECTION (ARTIFACT_ID),

The column ARTIFACT_ID in the example references a column of the same name in the table COLLECTIONS.

Both single-column and multi-column foreign keys can be defined at the table level. For a multi-column Foreign Key, the table-level declaration is the only option. This method also enables the provision of an optional name for the constraint:

  |...
  |  CONSTRAINT FK_ARTSOURCE FOREIGN KEY(DEALER_ID, COUNTRY)
  |    REFERENCES DEALER (DEALER_ID, COUNTRY),

Notice that the column names in the referenced (master) table may differ from those in the Foreign Key.

Note

If no target columns are specified, the Foreign Key automatically references the target table’s Primary Key.

5.4.1.7.4.1Foreign Key Actions

With the sub-clauses ON UPDATE and ON DELETE it is possible to specify an action to be taken on the affected foreign key column(s) when referenced values in the master table are changed:

NO ACTION

(the default) - Nothing is done

CASCADE

The change in the master table is propagated to the corresponding row(s) in the child table. If a key value changes, the corresponding key in the child records changes to the new value; if the master row is deleted, the child records are deleted.

SET DEFAULT

The Foreign Key columns in the affected rows will be set to their default values as they were when the foreign key constraint was defined.

SET NULL

The Foreign Key columns in the affected rows will be set to NULL.

The specified action, or the default NO ACTION, could cause a Foreign Key column to become invalid. For example, it could get a value that is not present in the master table, or it could become NULL while the column has a NOT NULL constraint. Such conditions will cause the operation on the master table to fail with an error message.

Example

  |...
  |  CONSTRAINT FK_ORDERS_CUST
  |    FOREIGN KEY (CUSTOMER) REFERENCES CUSTOMERS (ID)
  |      ON UPDATE CASCADE ON DELETE SET NULL

5.4.1.7.5CHECK Constraint

The CHECK constraint defines the condition the values inserted in this column must satisfy. A condition is a logical expression (also called a predicate) that can return the TRUE, FALSE and UNKNOWN values. A condition is considered satisfied if the predicate returns TRUE or value UNKNOWN (equivalent to NULL). If the predicate returns FALSE, the value will not be accepted. This condition is used for inserting a new row into the table (the INSERT statement) and for updating the existing value of the table column (the UPDATE statement) and also for statements where one of these actions may take place (UPDATE OR INSERT, MERGE).

Important

A CHECK constraint on a domain-based column does not replace an existing CHECK condition on the domain, but becomes an addition to it. The Firebird engine has no way, during definition, to verify that the extra CHECK does not conflict with the existing one.

CHECK conditions — whether defined at table level or column level — refer to table columns by their names. The use of the keyword VALUE as a placeholder, as in domain CHECK constraints, is not valid in the context of defining column constraints.

Examplewith two column-level constraints and one at table-level:

  |CREATE TABLE PLACES (
  |  ...
  |  LAT DECIMAL(9, 6) CHECK (ABS(LAT) <=  90),
  |  LON DECIMAL(9, 6) CHECK (ABS(LON) <= 180),
  |  ...
  |  CONSTRAINT CHK_POLES CHECK (ABS(LAT) < 90 OR LON = 0)
  |);

5.4.1.8Global Temporary Tables (GTT)

Global temporary tables have persistent metadata, but their contents are transaction-bound (the default) or connection-bound. Every transaction or connection has its own private instance of a GTT, isolated from all the others. Instances are only created if and when the GTT is referenced. They are destroyed when the transaction ends or on disconnection. The metadata of a GTT can be modified or removed using ALTER TABLE and DROP TABLE, respectively.

Syntax

  |CREATE GLOBAL TEMPORARY TABLE tablename
  |  (<column_def> [, {<column_def> | <table_constraint>} ...])
  |  [ON COMMIT {DELETE | PRESERVE} ROWS]

Syntax notes
  • ON COMMIT DELETE ROWS creates a transaction-level GTT (the default), ON COMMIT PRESERVE ROWS a connection-level GTT

  • An EXTERNAL [FILE] clause is not allowed in the definition of a global temporary table

5.4.1.8.1Restrictions on GTTs

GTTs can be dressed up with all the features and paraphernalia of ordinary tables (keys, references, indexes, triggers and so on) but there are a few restrictions:

  • GTTs and regular tables cannot reference one another

  • A connection-bound (PRESERVE ROWS) GTT cannot reference a transaction-bound (DELETE ROWS) GTT

  • Domain constraints cannot reference any GTT

  • The destruction of a GTT instance at the end of its life cycle does not cause any BEFORE/AFTER delete triggers to fire

Tip

In an existing database, it is not always easy to distinguish a regular table from a GTT, or a transaction-level GTT from a connection-level GTT. Use this query to find out what type of table you are looking at:

  |select t.rdb$type_name
  |from rdb$relations r
  |join rdb$types t on r.rdb$relation_type = t.rdb$type
  |where t.rdb$field_name = 'RDB$RELATION_TYPE'
  |and r.rdb$relation_name = 'TABLENAME'

For an overview of the types of all the relations in the database:

  |select r.rdb$relation_name, t.rdb$type_name
  |from rdb$relations r
  |join rdb$types t on r.rdb$relation_type = t.rdb$type
  |where t.rdb$field_name = 'RDB$RELATION_TYPE'
  |and coalesce (r.rdb$system_flag, 0) = 0

The RDB$TYPE_NAME field will show PERSISTENT for a regular table, VIEW for a view, GLOBAL_TEMPORARY_PRESERVE for a connection-bound GTT and GLOBAL_TEMPORARY_DELETE for a transaction_bound GTT.

5.4.1.9External Tables

The optional EXTERNAL [FILE] clause specifies that the table is stored outside the database in an external text file of fixed-length records. The columns of a table stored in an external file can be of any type except BLOB or ARRAY, although for most purposes, only columns of CHAR types would be useful.

All you can do with a table stored in an external file is insert new rows (INSERT) and query the data (SELECT). Updating existing data (UPDATE) and deleting rows (DELETE) are not possible.

A file that is defined as an external table must be located on a storage device that is physically present on the machine where the Firebird server runs and, if the parameter ExternalFileAccess in the firebird.conf configuration file is Restrict, it must be in one of the directories listed there as the argument for Restrict. If the file does not exist yet, Firebird will create it on first access.

Important

The ability to use external files for a table depends on the value set for the ExternalFileAccess parameter in firebird.conf:

  • If it is set to None (the default), any attempt to access an external file will be denied.

  • The Restrict setting is recommended, for restricting external file access to directories created explicitly for the purpose by the server administrator. For example:

    • ExternalFileAccess = Restrict externalfiles will restrict access to a directory named externalfiles directly beneath the Firebird root directory

    • ExternalFileAccess = d:\databases\outfiles; e:\infiles will restrict access to just those two directories on the Windows host server. Note that any path that is a network mapping will not work. Paths enclosed in single or double quotes will not work, either.

  • If this parameter is set to Full, external files may be accessed anywhere on the host file system. It creates a security vulnerability and is not recommended.

5.4.1.9.1External File Format

The row format of the external table is fixed length. There are no field delimiters: both field and row boundaries are determined by maximum sizes, in bytes, of the field definitions. It is important to keep this in mind, both when defining the structure of the external table and when designing an input file for an external table that is to import data from another application. The ubiquitous .csv format, for example, is of no use as an input file and cannot be generated directly into an external file.

The most useful data type for the columns of external tables is the fixed-length CHAR type, of suitable lengths for the data they are to carry. Date and number types are easily cast to and from strings whereas, unless the files are to be read by another Firebird database, the native data types will appear to external applications as unparseable alphabetti.

Of course, there are ways to manipulate typed data so as to generate output files from Firebird that can be read directly as input files to other applications, using stored procedures, with or without employing external tables. Such techniques are beyond the scope of a language reference. Here, we provide some guidelines and tips for producing and working with simple text files, since the external table feature is often used as an easy way to produce or read transaction-independent logs that can be studied off-line in a text editor or auditing application.

5.4.1.9.1.1Row Delimiters

Generally, external files are more useful if rows are separated by a delimiter, in the form of a newline sequence that is recognised by reader applications on the intended platform. For most contexts on Windows, it is the two-byte 'CRLF' sequence, carriage return (ASCII code decimal 13) and line feed (ASCII code decimal 10). On POSIX, LF on its own is usual; for some MacOSX applications, it may be LFCR. There are various ways to populate this delimiter column. In our example below, it is done by using a BEFORE INSERT trigger and the internal function ASCII_CHAR.

5.4.1.9.1.2External Table Example

For our example, we will define an external log table that might be used by an exception handler in a stored procedure or trigger. The external table is chosen because the messages from any handled exceptions will be retained in the log, even if the transaction that launched the process is eventually rolled back because of another, unhandled exception. For demonstration purposes, it has just two data columns, a time stamp and a message. The third column stores the row delimiter:

  |CREATE TABLE ext_log
  |  EXTERNAL FILE 'd:\externals\log_me.txt' (
  |  stamp CHAR (24),
  |  message CHAR(100),
  |  crlf CHAR(2) -- for a Windows context
  |);
  |COMMIT;

Now, a trigger, to write the timestamp and the row delimiter each time a message is written to the file:

   |SET TERM ^;
   |CREATE TRIGGER bi_ext_log FOR ext_log
   |ACTIVE BEFORE INSERT
   |AS
   |BEGIN
   |  IF (new.stamp is NULL) then
   |    new.stamp = CAST (CURRENT_TIMESTAMP as CHAR(24));
   |  new.crlf = ASCII_CHAR(13) || ASCII_CHAR(10);
   |END ^
   |COMMIT ^
   |SET TERM ;^

Inserting some records (which could have been done by an exception handler or a fan of Shakespeare):

  |insert into ext_log (message)
  |values('Shall I compare thee to a summer''s day?');
  |insert into ext_log (message)
  |values('Thou art more lovely and more temperate');

The output:

  |2015-10-07 15:19:03.4110Shall I compare thee to a summer's day?
  |2015-10-07 15:19:58.7600Thou art more lovely and more temperate

5.4.1.10CREATE TABLE Examples

  1. Creating the COUNTRY table with the primary key specified as a column constraint.

      |CREATE TABLE COUNTRY (
      |  COUNTRY COUNTRYNAME NOT NULL PRIMARY KEY,
      |  CURRENCY VARCHAR(10) NOT NULL
      |);
    
  2. Creating the STOCK table with the named primary key specified at the column level and the named unique key specified at the table level.

      |CREATE TABLE STOCK (
      |  MODEL     SMALLINT NOT NULL CONSTRAINT PK_STOCK PRIMARY KEY,
      |  MODELNAME CHAR(10) NOT NULL,
      |  ITEMID    INTEGER NOT NULL,
      |  CONSTRAINT MOD_UNIQUE UNIQUE (MODELNAME, ITEMID)
      |);
    
  3. Creating the JOB table with a primary key constraint spanning two columns, a foreign key constraint for the COUNTRY table and a table-level CHECK constraint. The table also contains an array of 5 elements.

       |CREATE TABLE JOB (
       |  JOB_CODE        JOBCODE NOT NULL,
       |  JOB_GRADE       JOBGRADE NOT NULL,
       |  JOB_COUNTRY     COUNTRYNAME,
       |  JOB_TITLE       VARCHAR(25) NOT NULL,
       |  MIN_SALARY      NUMERIC(18, 2) DEFAULT 0 NOT NULL,
       |  MAX_SALARY      NUMERIC(18, 2) NOT NULL,
       |  JOB_REQUIREMENT BLOB SUB_TYPE 1,
       |  LANGUAGE_REQ    VARCHAR(15) [1:5],
       |  PRIMARY KEY (JOB_CODE, JOB_GRADE),
       |  FOREIGN KEY (JOB_COUNTRY) REFERENCES COUNTRY (COUNTRY)
       |  ON UPDATE CASCADE
       |  ON DELETE SET NULL,
       |  CONSTRAINT CHK_SALARY CHECK (MIN_SALARY < MAX_SALARY)
       |);
    
  4. Creating the PROJECT table with primary, foreign and unique key constraints with custom index names specified with the USING clause.

       |CREATE TABLE PROJECT (
       |  PROJ_ID     PROJNO NOT NULL,
       |  PROJ_NAME   VARCHAR(20) NOT NULL UNIQUE USING DESC INDEX IDX_PROJNAME,
       |  PROJ_DESC   BLOB SUB_TYPE 1,
       |  TEAM_LEADER EMPNO,
       |  PRODUCT     PRODTYPE,
       |  CONSTRAINT PK_PROJECT PRIMARY KEY (PROJ_ID) USING INDEX IDX_PROJ_ID,
       |  FOREIGN KEY (TEAM_LEADER) REFERENCES EMPLOYEE (EMP_NO)
       |    USING INDEX IDX_LEADER
       |);
    
  5. Creating the SALARY_HISTORY table with two computed fields. The first one is declared according to the SQL:2003 standard, while the second one is declared according to the traditional declaration of computed fields in Firebird.

       |CREATE TABLE SALARY_HISTORY (
       |  EMP_NO         EMPNO NOT NULL,
       |  CHANGE_DATE    TIMESTAMP DEFAULT 'NOW' NOT NULL,
       |  UPDATER_ID     VARCHAR(20) NOT NULL,
       |  OLD_SALARY     SALARY NOT NULL,
       |  PERCENT_CHANGE DOUBLE PRECISION DEFAULT 0 NOT NULL,
       |  SALARY_CHANGE  GENERATED ALWAYS AS
       |    (OLD_SALARY * PERCENT_CHANGE / 100),
       |  NEW_SALARY     COMPUTED BY
       |    (OLD_SALARY + OLD_SALARY * PERCENT_CHANGE / 100)
       |);
    
  6. Creating a connection-scoped global temporary table.

      |CREATE GLOBAL TEMPORARY TABLE MYCONNGTT (
      |  ID  INTEGER NOT NULL PRIMARY KEY,
      |  TXT VARCHAR(32),
      |  TS  TIMESTAMP DEFAULT CURRENT_TIMESTAMP)
      |ON COMMIT PRESERVE ROWS;
    
  7. Creating a transaction-scoped global temporary table that uses a foreign key to reference a connection-scoped global temporary table. The ON COMMIT sub-clause is optional because DELETE ROWS is the default.

      |CREATE GLOBAL TEMPORARY TABLE MYTXGTT (
      |  ID        INTEGER NOT NULL PRIMARY KEY,
      |  PARENT_ID INTEGER NOT NULL REFERENCES MYCONNGTT(ID),
      |  TXT       VARCHAR(32),
      |  TS        TIMESTAMP DEFAULT CURRENT_TIMESTAMP
      |) ON COMMIT DELETE ROWS;
    

5.4.2ALTER TABLE

Used foraltering the structure of a table.

Available inDSQL, ESQL

Syntax

    |ALTER TABLE tablename
    |  <operation> [, <operation> ...]
    | 
    |<operation> ::=
    |    ADD <col_def>
    |  | ADD <tconstraint>
    |  | DROP colname
    |  | DROP CONSTRAINT constr_name
    |  | ALTER [COLUMN] colname <col_mod>
    | 
    |<col_def> ::= <regular_col_def> | <computed_col_def>
    | 
    |<regular_col_def> ::=
    |  colname {<datatype> | domainname}
    |  [DEFAULT {<literal> | NULL | <context_var>}]
    |  [NOT NULL]
    |  [<col_constraint>]
    |  [COLLATE collation_name]
    | 
    |<computed_col_def> ::=
    |  colname [<datatype>]
    |  {COMPUTED [BY] | GENERATED ALWAYS AS} (<expression>)
    | 
    |<col_mod> ::= <regular_col_mod> | <computed_col_mod>
    | 
    |<regular_col_mod> ::=
    |    TO newname
    |  | POSITION newpos
    |  | TYPE {<datatype> | domainname}
    |  | SET DEFAULT {<literal> | NULL | <context_var>}
    |  | DROP DEFAULT
    | 
    |<computed_col_mod> ::=
    |    TO newname
    |  | POSITION newpos
    |  | [TYPE <datatype>] {COMPUTED [BY] | GENERATED ALWAYS AS} (<expression>)
    | 
    |<datatype> ::=
    |    {SMALLINT | INTEGER | BIGINT} [<array_dim>]
    |  | {FLOAT | DOUBLE PRECISION} [<array_dim>]
    |  | {DATE | TIME | TIMESTAMP} [<array_dim>]
    |  | {DECIMAL | NUMERIC} [(precision [, scale])] [<array_dim>]
    |  | {CHAR | CHARACTER} [VARYING] | VARCHAR} [(size)]
    |    [<array_dim>] [CHARACTER SET charset_name]
    |  | {NCHAR | NATIONAL {CHARACTER | CHAR}} [VARYING]
    |    [(size)] [<array_dim>]
    |  | BLOB [SUB_TYPE {subtype_num | subtype_name}]
    |    [SEGMENT SIZE seglen] [CHARACTER SET charset_name]
    |  | BLOB [(seglen [, subtype_num])]
    | 
    |<array_dim> ::= '[' [m:]n [,[m:]n ...] ']'
    | 
    |<col_constraint> ::=
    |  [CONSTRAINT constr_name]
    |    { PRIMARY KEY [<using_index>]
    |    | UNIQUE      [<using_index>]
    |    | REFERENCES other_table [(colname)] [<using_index>]
    |        [ON DELETE {NO ACTION | CASCADE | SET DEFAULT | SET NULL}]
    |        [ON UPDATE {NO ACTION | CASCADE | SET DEFAULT | SET NULL}]
    |    | CHECK (<check_condition>) }
    | 
    |<tconstraint> ::=
    |  [CONSTRAINT constr_name]
    |    { PRIMARY KEY (<col_list>) [<using_index>]
    |    | UNIQUE      (<col_list>) [<using_index>]
    |    | FOREIGN KEY (<col_list>)
    |        REFERENCES other_table [(<col_list>)] [<using_index>]
    |        [ON DELETE {NO ACTION | CASCADE | SET DEFAULT | SET NULL}]
    |        [ON UPDATE {NO ACTION | CASCADE | SET DEFAULT | SET NULL}]
    |    | CHECK (<check_condition>) }
    | 
    |<col_list> ::= colname [, colname ...]
    | 
    |<using_index> ::= USING
    |  [ASC[ENDING] | DESC[ENDING]] INDEX indexname
    | 
    |<check_condition> ::=
    |    <val> <operator> <val>
    |  | <val> [NOT] BETWEEN <val> AND <val>
    |  | <val> [NOT] IN (<val> [, <val> ...] | <select_list>)
    |  | <val> IS [NOT] NULL
    |  | <val> IS [NOT] DISTINCT FROM <val>
    |  | <val> [NOT] CONTAINING <val>
    |  | <val> [NOT] STARTING [WITH] <val>
    |  | <val> [NOT] LIKE <val> [ESCAPE <val>]
    |  | <val> [NOT] SIMILAR TO <val> [ESCAPE <val>]
    |  | <val> <operator> {ALL | SOME | ANY} (<select_list>)
    |  | [NOT] EXISTS (<select_expr>)
    |  | [NOT] SINGULAR (<select_expr>)
    |  | (<search_condition>)
    |  | NOT <search_condition>
    |  | <search_condition> OR <search_condition>
    |  | <search_condition> AND <search_condition>
    | 
    |<operator> ::=
    |    <> | != | ^= | ~= | = | < | > | <= | >=
    |  | !< | ^< | ~< | !> | ^> | ~>
    | 
    |<val> ::=
    |    colname ['['array_idx [, array_idx ...]']']
    |  | <literal>
    |  | <context_var>
    |  | <expression>
    |  | NULL
    |  | NEXT VALUE FOR genname
    |  | GEN_ID(genname, <val>)
    |  | CAST(<val> AS <datatype>)
    |  | (<select_one>)
    |  | func([<val> [, <val> ...]])

Table 5.8ALTER TABLE Statement Parameters
ParameterDescription

tablename

Name (identifier) of the table

operation

One of the available operations altering the structure of the table

colname

Name (identifier) for a column in the table, max. 31 characters. Must be unique in the table.

newname

New name (identifier) for the column, max. 31 characters. Must be unique in the table.

newpos

The new column position (an integer between 1 and the number of columns in the table)

col_constraint

Column constraint

tconstraint

Table constraint

constr_name

The name (identifier) of a constraint. May consist of up to 31 characters.

other_table

The name of the table referenced by the foreign key constraint

literal

A literal value that is allowed in the given context

context_var

A context variable whose type is allowed in the given context

check_condition

The condition of a CHECK constraint that will be satisfied if it evaluates to TRUE or UNKNOWN/NULL

collation

Name of a collation sequence that is valid for charset_name, if it is supplied with datatype or, otherwise, is valid for the default character set of the database

array_dim

Array dimensions

m, n

Integer numbers defining the index range of an array dimension

precision

The total number of significant digits that a value of the datatype can hold (1..18)

scale

The number of digits after the decimal point (0..precision)

size

The maximum size of a string in characters

charset_name

The name of a valid character set, if the character set of the column is to be different to the default character set of the database

subtype_num

BLOB subtype number

subtype_name

BLOB subtype mnemonic name

seglen

Segment size (max. 65535)

select_one

A scalar SELECT statement — selecting one column and returning only one row

select_list

A SELECT statement selecting one column and returning zero or more rows

select_expr

A SELECT statement selecting one or more columns and returning zero or more rows

expression

An expression resolving to a value that is is allowed in the given context

genname

Sequence (generator) name

func

Internal function or UDF

The ALTER TABLE statement changes the structure of an existing table. With one ALTER TABLE statement it is possible to perform multiple operations, adding/dropping columns and constraints and also altering column specifications.

Multiple operations in an ALTER TABLE statement are separated with commas.

5.4.2.1Version Count Increments

Some changes in the structure of a table increment the metadata change counter (version count) assigned to every table. The number of metadata changes is limited to 255 for each table. Once the counter reaches the 255 limit, you will not be able to make any further changes to the structure of the table without resetting the counter.

5.4.2.2The ADD Clause

With the ADD clause you can add a new column or a new table constraint. The syntax for defining the column and the syntax of defining the table constraint correspond with those described for CREATE TABLE statement.

Effect on Version Count
  • Each time a new column is added, the metadata change counter is increased by one

  • Adding a new table constraint does not increase the metadata change counter

Points to Be Aware of
  1. Be careful about adding a new column with the NOT NULL constraint set. It may lead to breaking the logical integrity of data, since you will have existing records containing NULL in a non-nullable column. When adding a non-nullable column, it is recommended either to set a default value for it or to update the column in existing rows with a non-null value.

  2. When a new CHECK constraint is added, existing data is not tested for compliance. Prior testing of existing data against the new CHECK expression is recommended.

5.4.2.3The DROP Clause

The DROP <column name> clause deletes the specified column from the table. An attempt to drop a column will fail if anything references it. Consider the following items as sources of potential dependencies:

  • column or table constraints

  • indexes

  • stored procedures and triggers

  • views

Effect on Version Count
  • Each time a column is dropped, the table’s metadata change counter is increased by one.

5.4.2.4The DROP CONSTRAINT Clause

The DROP CONSTRAINT clause deletes the specified column-level or table-level constraint.

A PRIMARY KEY or UNIQUE key constraint cannot be deleted if it is referenced by a FOREIGN KEY constraint in another table. It will be necessary to drop that FOREIGN KEY constraint before attempting to drop the PRIMARY KEY or UNIQUE key constraint it references.

Effect on Version Count
  • Deleting a column constraint or a table constraint does not increase the metadata change counter.

5.4.2.5The ALTER [COLUMN] Clause

With the ALTER [COLUMN] clause, attributes of existing columns can be modified without the need to drop and re-add the column. Permitted modifications are:

  • change the name (does not affect the metadata change counter)

  • change the data type (increases the metadata change counter by one)

  • change the column position in the column list of the table (does not affect the metadata change counter)

  • delete the default column value (does not affect the metadata change counter)

  • set a default column value or change the existing default (does not affect the metadata change counter)

  • change the type and expression for a computed column (does not affect the metadata change counter)

5.4.2.6Renaming a Column: the TO Keyword

The TO keyword with a new identifier renames an existing column. The table must not have an existing column that has the same identifier.

It will not be possible to change the name of a column that is included in any constraint: PRIMARY KEY, UNIQUE key, FOREIGN KEY, column constraint or the CHECK constraint of the table.

Renaming a column will also be disallowed if the column is used in any trigger, stored procedure or view.

5.4.2.7Changing the Data Type of a Column: the TYPE Keyword

The keyword TYPE changes the data type of an existing column to another, allowable type. A type change that might result in data loss will be disallowed. As an example, the number of characters in the new type for a CHAR or VARCHAR column cannot be smaller than the existing specification for it.

If the column was declared as an array, no change to its type or its number of dimensions is permitted.

The data type of a column that is involved in a foreign key, primary key or unique constraint cannot be changed at all.

5.4.2.8Changing the Position of a Column: the POSITION Keyword

The POSITION keyword changes the position of an existing column in the notional left-to-right layout of the record.

Numbering of column positions starts at 1.

  • If a position less than 1 is specified, an error message will be returned

  • If a position number is greater than the number of columns in the table, its new position will be adjusted silently to match the number of columns.

5.4.2.9The DROP DEFAULT and SET DEFAULT Clauses

The optional DROP DEFAULT clause deletes the default value for the column if it was put there previously by a CREATE TABLE or ALTER TABLE statement.

  • If the column is based on a domain with a default value, the default value will revert to the domain default

  • An execution error will be raised if an attempt is made to delete the default value of a column which has no default value or whose default value is domain-based

The optional SET DEFAULT clause sets a default value for the column. If the column already has a default value, it will be replaced with the new one. The default value applied to a column always overrides one inherited from a domain.

5.4.2.10The COMPUTED [BY] or GENERATED ALWAYS AS Clauses

The data type and expression underlying a computed column can be modified using a COMPUTED [BY] or GENERATED ALWAYS AS clause in the ALTER TABLE ALTER [COLUMN] statement. Converting a regular column to a computed one and vice versa are not permitted.

5.4.2.11Attributes that Cannot Be Altered

The following alterations are not supported:

  • Enabling or disabling the NOT NULL constraint on a column

  • Changing the default collation for a character type column

Only the table owner and administrators have the authority to use ALTER TABLE.

5.4.2.12Examples Using ALTER TABLE

  1. Adding the CAPITAL column to the COUNTRY table.

      |ALTER TABLE COUNTRY
      |  ADD CAPITAL VARCHAR(25);
    
  2. Adding the CAPITAL column with the UNIQUE constraint and deleting the CURRENCY column.

      |ALTER TABLE COUNTRY
      |  ADD CAPITAL VARCHAR(25) NOT NULL UNIQUE,
      |  DROP CURRENCY;
    
  3. Adding the CHK_SALARY check constraint and a foreign key to the JOB table.

      |ALTER TABLE JOB
      |  ADD CONSTRAINT CHK_SALARY CHECK (MIN_SALARY < MAX_SALARY),
      |  ADD FOREIGN KEY (JOB_COUNTRY) REFERENCES COUNTRY (COUNTRY);
    
  4. Setting default value for the MODEL field, changing the type of the ITEMID column and renaming the MODELNAME column.

      |ALTER TABLE STOCK
      |  ALTER COLUMN MODEL SET DEFAULT 1,
      |  ALTER COLUMN ITEMID TYPE BIGINT,
      |  ALTER COLUMN MODELNAME TO NAME;
    
  5. Changing the computed columns NEW_SALARY and SALARY_CHANGE.

      |ALTER TABLE SALARY_HISTORY
      |  ALTER NEW_SALARY GENERATED ALWAYS AS
      |    (OLD_SALARY + OLD_SALARY * PERCENT_CHANGE / 100),
      |  ALTER SALARY_CHANGE COMPUTED BY
      |    (OLD_SALARY * PERCENT_CHANGE / 100);
    

See alsoSection 5.4.1, “CREATE TABLE, Section 5.4.3, “DROP TABLE, Section 5.3.1, “CREATE DOMAIN

5.4.3DROP TABLE

Used fordeleting a table

Available inDSQL, ESQL

Syntax

  |DROP TABLE tablename

Table 5.9DROP TABLE Statement Parameter
ParameterDescription

tablename

Name (identifier) of the table

The DROP TABLE statement deletes an existing table. If the table has dependencies, the DROP TABLE statement will fail with an execution error.

When a table is dropped, all triggers for its events and indexes built for its fields will be deleted as well.

Only the table owner and administrators have the authority to use DROP TABLE.

ExampleDeleting the COUNTRY table.

  |DROP TABLE COUNTRY;

See alsoSection 5.4.1, “CREATE TABLE, Section 5.4.2, “ALTER TABLE, Section 5.4.4, “RECREATE TABLE

5.4.4RECREATE TABLE

Used forcreating a new table (relation) or recreating an existing one

Available inDSQL

Syntax

  |RECREATE [GLOBAL TEMPORARY] TABLE tablename
  |  [EXTERNAL [FILE] 'filespec']
  |  (<col_def> [, {<col_def> | <tconstraint>} ...])
  |  [ON COMMIT {DELETE | PRESERVE} ROWS]

See the CREATE TABLE section for the full syntax of CREATE TABLE and descriptions of defining tables, columns and constraints.

RECREATE TABLE creates or recreates a table. If a table with this name already exists, the RECREATE TABLE statement will try to drop it and create a new one. Existing dependencies will prevent the statement from executing.

ExampleCreating or recreating the COUNTRY table.

  |RECREATE TABLE COUNTRY (
  |  COUNTRY COUNTRYNAME NOT NULL PRIMARY KEY,
  |  CURRENCY VARCHAR(10) NOT NULL
  |);

See alsoSection 5.4.1, “CREATE TABLE, Section 5.4.3, “DROP TABLE