Each database has a counter that is incremented for each insert or update operation that is performed on a table that contains a timestamp column within the database. This counter is the database timestamp. This tracks a relative time within a database, not an actual time that can be associated with a clock. A table can have only one timestamp column. Every time that a row with a timestamp column is modified or inserted, the incremented database timestamp value is inserted in the timestamp column. This property makes a timestamp column a poor candidate for keys, especially primary keys. Any update made to the row changes the timestamp value and, therefore, changes the key value. If the column is in a primary key, the old key value is no longer valid, and foreign keys referencing the old value are no longer valid. If the table is referenced in a dynamic cursor, all updates change the position of the rows in the cursor. If the column is in an index key, all updates to the data row also generate updates of the index.

You can use the timestamp column of a row to easily determine whether any value in the row has changed since the last time it was read. If any change is made to the row, the timestamp value is updated. If no change is made to the row, the timestamp value is the same as when it was previously read. To return the current timestamp value for a database, use @@DBTS.

The Transact-SQL timestamp data type is different from the timestamp data type defined in the SQL-2003 standard. The SQL-2003 timestamp data type is equivalent to the Transact-SQL datetime data type.

rowversion is the synonym for the timestamp data type and is subject to the behavior of data type synonyms. In DDL statements, use rowversion instead of timestamp wherever possible. For more information, see Data Type Synonyms (Transact-SQL).

In a CREATE TABLE or ALTER TABLE statement, you do not have to specify a column name for the timestamp data type, for example:

CREATE TABLE ExampleTable (PriKey int PRIMARY KEY, timestamp);

If you do not specify a column name, the Microsoft SQL Server 2005 Database Engine generates the timestamp column name; however, the rowversion synonym does not follow this behavior. When you use rowversion, you must specify a column name.

Note:
Duplicate timestamp values can be generated by using the SELECT INTO statement in which a timestamp column is in the SELECT list. We do not recommend using timestamp in this manner.

A nonnullable timestamp column is semantically equivalent to a binary(8) column. A nullable timestamp column is semantically equivalent to a varbinary(8) column.

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