Db2 Identity Column

Summary: in this tutorial, you will learn how to use DB2 identity column to define an identity column for a table.

Introduction to Db2 identity column

When you create a new table and use the GENERATED AS IDENTITY option for a column, this column will become an identity column.

An identity column contains a unique integer for each row in the table. When you insert a new row into the table, Db2 automatically generates a sequential integer for the identity column. Thus, identity columns are ideal for the primary key columns such as book id (book_id) or publisher id (publisher_id).

The following shows the syntax of declaring an identity column:

column_name data_type 
	GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY 
	[( identity_option) ]

In this syntax:

First, specify the data type for the identity column. The data type can be SMALLINT, INT, and BIGINT.

Second, use either GENERATED ALWAYS or GENERATED BY DEFAULT option.

• For the GENERATED ALWAYS option, Db2 will always generate a sequential integer for the identity column. Any attempt to insert a value into the identity column with GENERATED ALWAYS option will result in an error.
• On the other hand, for the GENERATED BY DEFAULT option, DB2 will only generate the sequential integer when you don’t provide the value for the identity column. If you insert a value into the identity column with the GENERATED BY DEFAULT option, Db2 will use your value instead of using the system generated one.

Third, specify the identity column’s options:

(
	START WITH starting_value
	INCREMENT BY increment_value
	[MINVALUE min_value]
	[MAXVALUE max_value]
	[CYCLE | NO CYCLE]
)

The identity option allows you to specify the starting value in START WITH clause and increment value in the INCREMENT BY.

If the increment value is positive, you will have an ascending sequence like 1, 2, 3, … In case it is negative, then you will have a descending sequence e.g., -1, -2, -3, …

The MINVALUE and MAXVALUE options allow you to specify the minimum and maximum values that Db2 will generate.

The CYCLE or NOCYCLE option determines whether Db2 should restart the values when it has generated all the possible values.

For example, if you use CYCLE option and the sequence is 1, 2, 3, then Db2 will return 1 if it has generated 3. However, if you use the NO CYCLE option, Db2 will raise an error instead.

Notice that a table can have one and only one identity column in a table in Db2.

Db2 identity column examples

Let’s take some examples of using identity columns to get a better understanding.

1) Db2 identity column example

First, create a new table named t1 with the id column as an identity column.

CREATE TABLE t1(
	id INT GENERATED BY DEFAULT AS IDENTITY 
		(START WITH 10 INCREMENT BY 10),
	c1 VARCHAR(10),
	PRIMARY KEY(id)
);

The value of the id column will start with 10 and increment by 10.

Second, use the following INSERT statement to insert three rows into the t1 table:

INSERT INTO 
	t1(c1) 
VALUES
	('A'),('B'),('C');

Third, view data from the t1 table using the following SELECT statement:

SELECT * FROM t1;

Here is the output:

ID          C1         
----------- ---------- 
10          A          
20          B          
30          C  

2) Db2 identity column with CYCLE example

First, create a new table named t2 whose id column is an identity column.

CREATE TABLE t2(
	id INT GENERATED ALWAYS AS IDENTITY
		(START WITH -1,
		INCREMENT BY 1,
		CYCLE,
		MINVALUE -1,
		MAXVALUE 2),
	c2 VARCHAR(10)
);

Second, insert seven rows into the t2 table:

INSERT INTO 
	t2(c2)
VALUES('A'),('B'),('C'),('D'),('E'),('F');

Third, query data from the t2 table:

SELECT * FROM t2;

Here is the output:

ID          C2         
----------- ----
-1          A          
0           B          
1           C          
2           D          
-1          E          
0           F          
1           G          

In this example, the id column’s value starts with -1 and has an increment of one.

Because the MAXVALUE is 2 and CYCLE option is specified, the sequence is -1, 0, 1 ,2, -1, 0, 1 …

In this tutorial, you have learned how to use the Db2 identity column to define an identity column for a table.