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DECLARE @name VARCHAR(MAX)
SELECT @name = CONSTRAINT_NAME
FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE
WHERE TABLE_NAME = '<TblName>'
AND COLUMN_NAME = '<ColumnName>'
AND TABLE_SCHEMA = '<Schema>'
declare @sql varchar(MAX) = 'ALTER TABLE <TblName> DROP CONSTRAINT [' + @name + ']'
select @sql
EXEC (@sql)