Keywords: Identity column attributes, generated-column-spec

# generated-column-spec

    [ GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY
    [ ( START WITH IntegerConstant
    [ ,INCREMENT BY IntegerConstant] ) ]  ]  ]

#### Identity column attributes

For SMALLINT, INT, and BIGINT columns with identity attributes, Derby automatically assigns increasing integer values to the column. Identity column attributes behave like other defaults in that when an insert statement does not specify a value for the column, Derby automatically provides the value. However, the value is not a constant; Derby automatically increments the default value at insertion time.

The IDENTITY keyword can only be specified if the data type associated with the column is one of the following exact integer types.

- SMALLINT
- INT
- BIGINT

There are two kinds of identity columns in Derby: those which are GENERATED ALWAYS and those which are GENERATED BY DEFAULT.

GENERATED ALWAYS
  An identity column that is GENERATED ALWAYS will increment the default value on every insertion and will store the incremented value into the column. Unlike other defaults, you cannot insert a value directly into or update an identity column that is GENERATED ALWAYS. Instead, either specify the DEFAULT keyword when inserting into the identity column, or leave the identity column out of the insertion column list altogether. For example:

      create table greetings
      	(i int generated always as identity, ch char(50));
      insert into greetings values (DEFAULT, 'hello');
      insert into greetings(ch) values ('bonjour');

  Automatically generated values in a GENERATED ALWAYS identity column are unique. Creating an identity column does not create an index on the column.
GENERATED BY DEFAULT

  An identity column that is GENERATED BY DEFAULT will only increment and use the default value on insertions when no explicit value is given. Unlike GENERATED ALWAYS columns, you can specify a particular value in an insertion statement to be used instead of the generated default value.

  To use the generated default, either specify the DEFAULT keyword when inserting into the identity column, or just leave the identity column out of the insertion column list. To specify a value, included it in the insertion statement. For example:

      create table greetings
      	(i int generated by default as identity, ch char(50));
      -- specify value "1":
      insert into greetings values (1, 'hi');
      -- use generated default
      insert into greetings values (DEFAULT, 'salut');
      -- use generated default
      insert into greetings(ch) values ('bonjour');

  Note that unlike a GENERATED ALWAYS column, a GENERATED BY DEFAULT column does not guarantee uniqueness. Thus, in the above example, the `hi` and `salut` rows will both have an identity value of "1", because the generated column starts at "1" and the user-specified value was also "1". To prevent duplication, especially when loading or importing data, create the table using the START WITH value which corresponds to the first identity value that the system should assign. To check for this condition and disallow it, you can use a primary key or unique constraint on the GENERATED BY DEFAULT identity column.

By default, the initial value of an identity column is 1, and the amount of the increment is 1. You can specify non-default values for both the initial value and the interval amount when you define the column with the key words STARTS WITH and INCREMENT BY. And if you specify a negative number for the increment value, Derby decrements the value with each insert. If this value is 0, or positive, Derby increments the value with each insert.

The maximum and minimum values allowed in identity columns are determined by the data type of the column. Attempting to insert a value outside the range of values supported by the data type raises an exception.

Table 1. Maximum and Minimum Values for Columns with Generated Column Specs
Data type | Maximum Value | Minimum Value
--- | --- | ---
SMALLINT | 32767 (java.lang.Short.MAX_VALUE) | -32768 (java.lang.Short.MIN_VALUE)
INT | 2147483647 (java.lang.Integer.MAX_VALUE) | -2147483648 (java.lang.Integer.MIN_VALUE)
BIGINT | 9223372036854775807 (java.lang.Long.MAX_VALUE) | -9223372036854775808 (java.lang.Long.MIN_VALUE)

Automatically generated values in an identity column are unique. Use a primary key or unique constraint on a column to guarantee uniqueness. Creating an identity column does not create an index on the column.

The `IDENTITY_VAL_LOCAL` function is a non-deterministic function that returns the most recently assigned value for an identity column. See IDENTITY_VAL_LOCAL function for more information.

Note: Specify the schema, table, and column name using the same case as those names are stored in the system tables--that is, all upper case unless you used delimited identifiers when creating those database objects.

Derby keeps track of the last increment value for a column in a cache. It also stores the value of what the next increment value will be for the column on disk in the AUTOINCREMENTVALUE column of the SYS.SYSCOLUMNS system table. Rolling back a transaction does not undo this value, and thus rolled-back transactions can leave "gaps" in the values automatically inserted into an identity column. Derby behaves this way to avoid locking a row in SYS.SYSCOLUMNS for the duration of a transaction and keeping concurrency high.

When an insert happens within a triggered-SQL-statement, the value inserted by the triggered-SQL-statement into the identity column is available from ConnectionInfo only within the trigger code. The trigger code is also able to see the value inserted by the statement that caused the trigger to fire. However, the statement that caused the trigger to fire is not able to see the value inserted by the triggered-SQL-statement into the identity column. Likewise, triggers can be nested (or recursive). An SQL statement can cause trigger T1 to fire. T1 in turn executes an SQL statement that causes trigger T2 to fire. If both T1 and T2 insert rows into a table that cause Derby to insert into an identity column, trigger T1 cannot see the value caused by T2's insert, but T2 can see the value caused by T1's insert. Each nesting level can see increment values generated by itself and previous nesting levels, all the way to the top-level SQL statement that initiated the recursive triggers. You can only have 16 levels of trigger recursion.

#### Example

    create table greetings
      (i int generated by default as identity (START WITH 2, INCREMENT BY 1),
      ch char(50));
    -- specify value "1":
    insert into greetings values (1, 'hi');
    -- use generated default
    insert into greetings values (DEFAULT, 'salut');
    -- use generated default
    insert into greetings(ch) values ('bonjour');

Parent topic: CREATE TABLE statement
Related reference
column-definition