[SQL]删除约束

来源：http://www.archonsystems.com/devblog/2012/05/25/how-to-drop-a-column-with-a-default-value-constraint-in-sql-server/

DECLARE @ConstraintName nvarchar(200);

SELECT @ConstraintName = Name
FROM SYS.DEFAULT_CONSTRAINTS
WHERE PARENT_OBJECT_ID = OBJECT_ID('__TableName__')
  AND PARENT_COLUMN_ID = (
    SELECT column_id
    FROM SYS.COLUMNS
    WHERE NAME = N'__ColumnName__'
      AND object_id = OBJECT_ID(N'__TableName__')
  );
IF @ConstraintName IS NOT NULL
BEGIN
  EXEC('ALTER TABLE __TableName__ DROP CONSTRAINT ' + @ConstraintName);
END

另一方法（修改字段类型）

IF EXISTS(SELECT name FROM SYSOBJECTS WHERE name = '__TableName__' AND xtype='U')
BEGIN
  DECLARE @name VARCHAR(100) = (
    SELECT TOP 1 name
    FROM SYSOBJECTS
    WHERE name LIKE 'DF\_\_%' ESCAPE '\' AND xtype='D' AND id=(SELECT TOP 1 cdefault FROM SYSCOLUMNS WHERE id=OBJECT_ID('__TableName__') AND name='__ColumnName__')
  );

  IF(@name IS NOT NULL)
  BEGIN
    EXEC('ALTER TABLE [DBO].[__TableName__] DROP CONSTRAINT ' + @name);
    EXEC('ALTER TABLE [DBO].[__TableName__] ALTER COLUMN [__ColumnName__] TEXT NULL');
  END;
  ELSE
  BEGIN
    ALTER TABLE [DBO].[__TableName__] ALTER COLUMN [__ColumnName__] TEXT NULL;
  END;
END;

PS：将__TableName__和__ColumnName__替换你的表名和字段名