sql server 判断及增加列的默认值约束

IF NOT EXISTS ( SELECT name FROM sysobjects WHERE id = ( SELECT syscolumns.cdefault FROM sysobjects     INNER JOIN syscolumns ON sysobjects.Id=syscolumns.Id     WHERE sysobjects.name=N'tableName' AND syscolumns.name=N'columnName' ) )BEGIN        ALTER TABLE tableName add constraint DF_columnName DEFAULT N'N' for columnName;      PRINT N'New add DF_columnName';ENDELSEBEGIN    PRINT N'DEFAULT FOR columnName HAD EXISTS';END