generated always

create table test
(
    id int generated always as identity(start with 1, increment by 1, maxvalue 100),         
    name varchar(50)
)

DB2字段类型研究 --- INTEGER或INT(大整型)

说明：大整型是四个字节的整数，精度为 10 位。大整型的范围从 -2,147,483,648 到 2,147,483,647。

增加字段：ALTER TABLE <tableName> ADD COLUMN <columnName> integer

修改字段类型：ALTER TABLE <tableName> ALTER <columnName> SET DATA TYPE integer

删除字段：ALTER TABLE <tableName> DROP <columnName>

验证字段范围：

insert into aaa(col_interger) values(-2147483648) --成功
insert into aaa(col_interger) values(-2147483649) --失败
insert into aaa(col_interger) values(2147483647)  --成功
insert into aaa(col_interger) values(2147483648)  --失败

Column                         Type      Type
name                           schema    name               Length   Scale Nulls
------------------------------ --------- ------------------ -------- ----- ------
<columnName>                   SYSIBM    INTEGER                   4     0 Yes

generated-column-definition Specifies a generated value for the column.

default-clause

Specifies a default value for the column.

WITH

An optional keyword.

DEFAULT

Provides a default value in the event a value is not supplied on INSERT or is specified as DEFAULT on INSERT or UPDATE. If a default value is not specified following the DEFAULT keyword, the default value depends on the data type of the column as shown in "ALTER TABLE".

If a column is defined as XML, a default value cannot be specified (SQLSTATE 42613). The only possible default is NULL.

If the column is based on a column of a typed table, a specific default value must be specified when defining a default. A default value cannot be specified for the object identifier column of a typed table (SQLSTATE 42997).

If a column is defined using a distinct type, then the default value of the column is the default value of the source data type cast to the distinct type.

If a column is defined using a structured type, the default-clausecannot be specified (SQLSTATE 42842).

Omission of DEFAULT from a column-definition results in the use of the null value as the default for the column. If such a column is defined NOT NULL, then the column does not have a valid default.

default-values

Specific types of default values that can be specified are as follows.

constant

Specifies the constant as the default value for the column. The specified constant must:

• represent a value that could be assigned to the column in accordance with the rules of assignment
• not be a floating-point constant unless the column is defined with a floating-point data type
• be a numeric constant or a decimal floating-point special value if the data type of the column is a decimal floating-point. Floating-point constants are first interpreted as DOUBLE and then converted to decimal floating-point if the target column is DECFLOAT. For DECFLOAT(16) columns, decimal constants having precision greater than 16 digits will be rounded using the rounding modes specified by the CURRENT DECFLOAT ROUNDING MODE special register.
• not have nonzero digits beyond the scale of the column data type if the constant is a decimal constant (for example, 1.234 cannot be the default for a DECIMAL(5,2) column)
• be expressed with no more than 254 bytes including the quote characters, any introducer character such as the X for a hexadecimal constant, and characters from the fully qualified function name and parentheses when the constant is the argument of acast-function

datetime-special-register

Specifies the value of the datetime special register (CURRENT DATE, CURRENT TIME, or CURRENT TIMESTAMP) at the time of INSERT, UPDATE, or LOAD as the default for the column. The data type of the column must be the data type that corresponds to the special register specified (for example, data type must be DATE when CURRENT DATE is specified).

user-special-register

Specifies the value of the user special register (CURRENT USER, SESSION_USER, SYSTEM_USER) at the time of INSERT, UPDATE, or LOAD as the default for the column. The data type of the column must be a character string with a length not less than the length attribute of a user special register. Note that USER can be specified in place of SESSION_USER and CURRENT_USER can be specified in place of CURRENT USER.

CURRENT SCHEMA

Specifies the value of the CURRENT SCHEMA special register at the time of INSERT, UPDATE, or LOAD as the default for the column. If CURRENT SCHEMA is specified, the data type of the column must be a character string with a length greater than or equal to the length attribute of the CURRENT SCHEMA special register.

NULL

Specifies NULL as the default for the column. If NOT NULL was specified, DEFAULT NULL may be specified within the same column definition but will result in an error on any attempt to set the column to the default value.

cast-function

This form of a default value can only be used with columns defined as a distinct type, BLOB or datetime (DATE, TIME or TIMESTAMP) data type. For distinct type, with the exception of distinct types based on BLOB or datetime types, the name of the function must match the name of the distinct type for the column. If qualified with a schema name, it must be the same as the schema name for the distinct type. If not qualified, the schema name from function resolution must be the same as the schema name for the distinct type. For a distinct type based on a datetime type, where the default value is a constant, a function must be used and the name of the function must match the name of the source type of the distinct type with an implicit or explicit schema name of SYSIBM. For other datetime columns, the corresponding datetime function may also be used. For a BLOB or a distinct type based on BLOB, a function must be used and the name of the function must be BLOB with an implicit or explicit schema name of SYSIBM.

constant

Specifies a constant as the argument. The constant must conform to the rules of a constant for the source type of the distinct type or for the data type if not a distinct type. If the cast-function is BLOB, the constant must be a string constant.

datetime-special-register

Specifies CURRENT DATE, CURRENT TIME, or CURRENT TIMESTAMP. The source type of the distinct type of the column must be the data type that corresponds to the specified special register.

user-special-register

Specifies CURRENT USER, SESSION_USER, or SYSTEM_USER. The data type of the source type of the distinct type of the column must be a string data type with a length of at least 8 bytes. If the  cast-function is BLOB, the length attribute must be at least 8 bytes.

CURRENT SCHEMA

Specifies the value of the CURRENT SCHEMA special register. The data type of the source type of the distinct type of the column must be a character string with a length greater than or equal to the length attribute of the CURRENT SCHEMA special register. If the cast-function is BLOB, the length attribute must be at least 8 bytes.

EMPTY_CLOB(), EMPTY_DBCLOB(), or EMPTY_BLOB()

Specifies a zero-length string as the default for the column. The column must have the data type that corresponds to the result data type of the function.

If the value specified is not valid, an error is returned (SQLSTATE 42894).

GENERATED

Indicates that DB2® generates values for the column. GENERATED must be specified if the column is to be considered an IDENTITY column  or a ROW CHANGE TIMESTAMP column.

ALWAYS

Specifies that DB2 will always generate a value for the column when a row is inserted into the table, or whenever the result value of the  generation-expression changes. The result of the expression is stored in the table. GENERATED ALWAYS is the recommended value unless data propagation or unload and reload operations are being done. GENERATED ALWAYS is the required value for generated columns.

BY DEFAULT

Specifies that DB2 will generate a value for the column when a row is inserted, or updated specifying the DEFAULT clause, unless an explicit value is specified. BY DEFAULT is the recommended value when using data propagation or performing an unload and reload operation.

Although not explicitly required, to ensure uniqueness of the values, define a unique single-column index on generated IDENTITY columns.

AS IDENTITY

Specifies that the column is to be the identity column for this table. A table can only have a single IDENTITY column (SQLSTATE 428C1). The IDENTITY keyword can only be specified if the data type associated with the column is an exact numeric type with a scale of zero, or a user-defined distinct type for which the source type is an exact numeric type with a scale of zero (SQLSTATE 42815). SMALLINT, INTEGER, BIGINT, or DECIMAL with a scale of zero, or a distinct type based on one of these types, are considered exact numeric types. By contrast, single- and double-precision floating points are considered approximate numeric data types. Reference types, even if represented by an exact numeric type, cannot be defined as identity columns.

An identity column is implicitly NOT NULL. An identity column cannot have a DEFAULT clause (SQLSTATE 42623).

START WITH  numeric-constant

Specifies the first value for the identity column. This value can be any positive or negative value that could be assigned to this column (SQLSTATE 42815), without nonzero digits existing to the right of the decimal point (SQLSTATE 428FA). The default is MINVALUE for ascending sequences, and MAXVALUE for descending sequences. This value is not necessarily the value that would be cycled to after reaching the maximum or minimum value for the identity column. The START WITH clause can be used to start the generation of values outside the range that is used for cycles. The range used for cycles is defined by MINVALUE and MAXVALUE.

INCREMENT BY  numeric-constant

Specifies the interval between consecutive values of the identity column. This value can be any positive or negative value that could be assigned to this column (SQLSTATE 42815), and does not exceed the value of a large integer constant (SQLSTATE 42820), without nonzero digits existing to the right of the decimal point (SQLSTATE 428FA).

If this value is negative, this is a descending sequence. If this value is 0, or positive, this is an ascending sequence. The default is 1.

NO MINVALUE  or MINVALUE

Specifies the minimum value at which a descending identity column either cycles or stops generating values, or an ascending identity column cycles to after reaching the maximum value.

NO MINVALUE

For an ascending sequence, the value is the START WITH value, or 1 if START WITH was not specified. For a descending sequence, the value is the minimum value of the data type of the column. This is the default.

MINVALUE  numeric-constant

Specifies the numeric constant that is the minimum value. This value can be any positive or negative value that could be assigned to this column (SQLSTATE 42815), without nonzero digits existing to the right of the decimal point (SQLSTATE 428FA), but the value must be less than or equal to the maximum value (SQLSTATE 42815).

NO MAXVALUE  or MAXVALUE

Specifies the maximum value at which an ascending identity column either cycles or stops generating values, or a descending identity column cycles to after reaching the minimum value.

NO MAXVALUE

For an ascending sequence, the value is the maximum value of the data type of the column. For a descending sequence, the value is the START WITH value, or -1 if START WITH was not specified. This is the default.

MAXVALUE  numeric-constant

Specifies the numeric constant that is the maximum value. This value can be any positive or negative value that could be assigned to this column (SQLSTATE 42815), without nonzero digits existing to the right of the decimal point (SQLSTATE 428FA), but the value must be greater than or equal to the minimum value (SQLSTATE 42815).

NO CYCLE  or CYCLE

Specifies whether this identity column should continue to generate values after generating either its maximum or minimum value.

NO CYCLE

Specifies that values will not be generated for the identity column once the maximum or minimum value has been reached. This is the default.

CYCLE

Specifies that values continue to be generated for this column after the maximum or minimum value has been reached. If this option is used, after an ascending identity column reaches the maximum value, it generates its minimum value; or after a descending sequence reaches the minimum value, it generates its maximum value. The maximum and minimum values for the identity column determine the range that is used for cycling.

When CYCLE is in effect, DB2 may generate duplicate values for an identity column. Although not explicitly required, a unique, single-column index should be defined on the generated column to ensure uniqueness of the values, if unique values are desired. If a unique index exists on such an identity column and a non-unique value is generated, an error occurs (SQLSTATE 23505).

NO CACHE  or CACHE

Specifies whether to keep some pre-allocated values in memory for faster access. If a new value is needed for the identity column, and there are none available in the cache, then the end of the new cache block must be logged. However, when a new value is needed for the identity column, and there is an unused value in the cache, then the allocation of that identity value is faster, because no logging is necessary. This is a performance and tuning option.

NO CACHE

Specifies that values for the identity column are not to be pre-allocated.

When this option is specified, the values of the identity column are not stored in the cache. In this case, every request for a new identity value results in synchronous I/O to the log.

CACHE  integer-constant

Specifies how many values of the identity sequence are to be pre-allocated and kept in memory. When values are generated for the identity column, pre-allocating and storing values in the cache reduces synchronous I/O to the log.

If a new value is needed for the identity column and there are no unused values available in the cache, the allocation of the value involves waiting for I/O to the log. However, when a new value is needed for the identity column and there is an unused value in the cache, the allocation of that identity value can happen more quickly by avoiding the I/O to the log.

In the event of a database deactivation, either normally or due to a system failure, all cached sequence values that have not been used in committed statements arelost; that is, they will never be used. The value specified for the CACHE option is the maximum number of values for the identity column that could be lost in case of database deactivation. (If a database is not explicitly activated, using the ACTIVATE command or API, when the last application is disconnected from the database, an implicit deactivation occurs.)

The minimum value is 2 (SQLSTATE 42815). The default value is CACHE 20.

NO ORDER  or ORDER

Specifies whether the identity values must be generated in order of request.

NO ORDER

Specifies that the values do not need to be generated in order of request. This is the default.

ORDER

Specifies that the values must be generated in order of request.

FOR EACH ROW ON UPDATE AS ROW CHANGE TIMESTAMP

Specifies that the column is a timestamp column for the table. A value is generated for the column in each row that is inserted, and for any row in which any column is updated. The value that is generated for a ROW CHANGE TIMESTAMP column is a timestamp that corresponds to the insert or update time for that row. If multiple rows are inserted or updated with a single statement, the value of the ROW CHANGE TIMESTAMP column might be different for each row.

A table can only have one ROW CHANGE TIMESTAMP column (SQLSTATE 428C1). If data-type is specified, it must be TIMESTAMP or TIMESTAMP(6) (SQLSTATE 42842). A ROW CHANGE TIMESTAMP column cannot have a DEFAULT clause (SQLSTATE 42623). NOT NULL must be specified for a ROW CHANGE TIMESTAMP column (SQLSTATE 42831).

GENERATED ALWAYS AS ( generation-expression)

Specifies that the definition of the column is based on an expression. (If the expression for a GENERATED ALWAYS column includes a user-defined external function, changing the executable for the function (such that the results change for given arguments) can result in inconsistent data. This can be avoided by using the SET INTEGRITY statement to force the generation of new values.) The  generation-expression cannot contain any of the following (SQLSTATE 42621):

• Subqueries
• XMLQUERY or XMLEXISTS expressions
• Column functions
• Dereference operations or DEREF functions
• User-defined or built-in functions that are non-deterministic
• User-defined functions using the EXTERNAL ACTION option
• User-defined functions that are not defined with NO SQL
• Host variables or parameter markers
• Special registers and built-in functions that depend on the value of a special register
• Global variables
• References to columns defined later in the column list
• References to other generated columns
• References to columns of type XML

The data type for the column is based on the result data type of thegeneration-expression. A CAST specification can be used to force a particular data type and to provide a scope (for a reference type only). If data-type is specified, values are assigned to the column according to the appropriate assignment rules. A generated column is implicitly considered nullable, unless the NOT NULL column option is used. The data type of a generated column and the result data type of the generation-expression must have equality defined (see "Assignments and comparisons"). This excludes columns and generation expressions of type LOB data types, XML, structured types, and distinct types based on any of these types (SQLSTATE 42962).