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The DROP CONSTRAINT Clause

The DROP CONSTRAINT clause deletes the specified column-level or table-level constraint.

A PRIMARY KEY or UNIQUE key constraint cannot be deleted if it is referenced by a FOREIGN KEY constraint in another table.It will be necessary to drop that FOREIGN KEY constraint before attempting to drop the PRIMARY KEY or UNIQUE key constraint it references.

Effect on Version Count

  • Deleting a column constraint or a table constraint does not increase the metadata change counter.

The ALTER [COLUMN] Clause

With the ALTER [COLUMN] clause, attributes of existing columns can be modified without the need to drop and re-add the column.Permitted modifications are:

  • change the name (does not affect the metadata change counter)

  • change the data type (increases the metadata change counter by one)

  • change the column position in the column list of the table (does not affect the metadata change counter)

  • delete the default column value (does not affect the metadata change counter)

  • set a default column value or change the existing default (does not affect the metadata change counter)

  • change the type and expression for a computed column (does not affect the metadata change counter)

  • set the NOT NULL constraint (does not affect the metadata change counter)

  • drop the NOT NULL constraint (does not affect the metadata change counter)

  • change the type of an identity column, or change an identity column to a regular column

  • restart an identity column

  • change the increment of an identity column