What is the role of the identity column in sql

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The function of the identification column in sql is to enable the computer to automatically generate the value of the identity field for the records in the table as required.

Definition and characteristics of identity column

The identity column in SQL Server is also called identifier column, which is traditionally called self-incrementing column.

This kind of column has the following three characteristics:

1. The data type of the column is a numeric type without decimal.

2. During the Insert operation, the value of the column is generated by the system according to a certain rule, and null values are not allowed.

3. The column values are not duplicated and have the function of identifying each row in the table. Each table can only have one identification column.

Because of the above characteristics, the identity column is widely used in the design of database.

The composition and function of identification column

To create an identity column, you typically specify three things:

1. Type (type)

In SQL Server 2000, the identity column type must be numeric, as follows:

Decimal 、 int 、 numeric 、 smallint 、 bigint 、 tinyint

It should be noted that when selecting decimal and numeric, the number of decimal places must be zero

Also note the numerical range of all representations for each data type

2. Seed (seed)

Is the value assigned to the first row of the table, default to 1

3. Incremental increment (increment)

The increment between two adjacent identity values, defaults to 1.

The identity column allows the computer to automatically generate the value of the identity field for the records in the table as required.

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