SQL Server: drop table primary key, without knowing its name

You’ll have to use dynamic SQL for that, since ALTER TABLE does not accept variables or subqueries.

CREATE TABLE PKTest ( ID INT PRIMARY KEY ) ;

DECLARE @SQL VARCHAR(4000)
SET @SQL = 'ALTER TABLE PKTEST DROP CONSTRAINT |ConstraintName| '

SET @SQL = REPLACE(@SQL, '|ConstraintName|', 
                         ( SELECT   name
                             FROM     sysobjects
                            WHERE    xtype="PK"
                              AND parent_obj = OBJECT_ID('PKTest')
                          ))

EXEC (@SQL)

DROP TABLE PKTest