一点sql语法

--删除有默认值约束的字段
IF COL_LENGTH('tableName', 'columnName') IS NOT NULL
BEGIN
    declare @tableName varchar(100)='tableName'  
    declare @columnName varchar(100)='columnName'            
    declare @constraintName varchar(200)  
  
    select @constraintName=b.name from syscolumns a,sysobjects b where a.id=object_id(@tableName)   
    and b.id=a.cdefault and a.name=@columnName and b.name like 'DF%'  
      
    exec('alter table '+@tableName+' drop constraint '+@constraintName)  
    exec ('alter table ' + @tableName + ' drop column ' + @columnName )  
END
GO