The TSEQUAL function can only be used in a WHERE clause and is most commonly used as part of an UPDATE statement.
Although the TSEQUAL function can be used to compare two ordinary TIMESTAMP values, the purpose of TSEQUAL is to determine
whether or not a row has been modified by another connection by comparing two special Transact-SQL TIMESTAMP values.
In a single-row UPDATE statement using TSEQUAL, if timestamp-expression-1 is equal to timestamp-expression-2 and one of these values refers to a column declared with DEFAULT TIMESTAMP and the other refers to the value of the column
when the row was last fetched, then the row has not changed since it was fetched and TSEQUAL returns TRUE. If the row was
changed by another user, its timestamp has been modified and the TSEQUAL function returns FALSE. If the TSEQUAL function returns
FALSE in this situation, the UPDATE is not performed. The application can determine that no rows were updated by examining
the number of rows affected, for example by using @@rowcount. If no rows were affected, the application can assume that the
row was modified by another user and that it needs to be re-fetched.
Suppose you create a TIMESTAMP column Products.LastUpdated to store the timestamp for the last time the row was updated. The
following example uses the TSEQUAL function to change a row value. An update is applied to the row only when the row has not
been changed since it was last fetched.
SELECT LastUpdated into old_ts_value
WHERE ID = '300';
SET Color = 'Yellow'
WHERE ID = '300'
AND TSEQUAL( LastUpdated, old_ts_value );