SQL SERVER 修改帶有默認值約束的列類型

SQL SERVER 修改帶有默認值約束的列類型時，由於默認值約束導致執行sql失敗，需要先刪除默認值約束，修改類型，在加上默認值約束。

在建表時加默認值創建的默認值約束由於名稱不知道沒法直接drop通過以下語句直接完成修改類型操作

DECLARE @dfname NVARCHAR(50),
        @dropsql NVARCHAR(100)
SELECT @dfname=NAME FROM sysobjects WHERE id = (SELECT syscolumns.cdefault FROM sysobjects INNER JOIN syscolumns ON sysobjects.Id=syscolumns.Id 
    WHERE upper(SYSCOLUMNS.NAME) = '@tablecolumn' AND upper(SYSOBJECTS.NAME) = '@table')
    IF LEN(@dfname)>0
    SET @dropsql='ALTER TABLE @table DROP CONSTRAINT '+ CAST(@dfname AS NVARCHAR(50))
    IF LEN(@dfname)>0
    EXEC sp_executesql @dropsql
GO
IF  EXISTS (SELECT 1  FROM SYSCOLUMNS INNER JOIN SYSOBJECTS ON SYSOBJECTS.ID = SYSCOLUMNS.ID 
    WHERE upper(SYSCOLUMNS.NAME) = '@tablecolumn' AND upper(SYSOBJECTS.NAME) = '@table')
ALTER TABLE @table ALTER COLUMN @tablecolumn float 
GO
IF  EXISTS (SELECT 1  FROM SYSCOLUMNS INNER JOIN SYSOBJECTS ON SYSOBJECTS.ID = SYSCOLUMNS.ID 
    WHERE upper(SYSCOLUMNS.NAME) = '@tablecolumn' AND upper(SYSOBJECTS.NAME) = '@table')
ALTER TABLE @table add default 0  for @tablecolumn
GO

 如果默認值約束是制定名稱的就比較簡單；先根據名稱直接drop就可以

IF  EXISTS (SELECT 1  FROM sysobjects WHERE upper(NAME) = '@dfname' AND id = (SELECT syscolumns.cdefault FROM sysobjects INNER JOIN syscolumns ON sysobjects.Id=syscolumns.Id 
    WHERE upper(SYSCOLUMNS.NAME) = '@tablecolumn' AND upper(SYSOBJECTS.NAME) = '@table')
ALTER TABLE @table DROP CONSTRAINT @dfname
GO
IF  EXISTS (SELECT 1  FROM SYSCOLUMNS INNER JOIN SYSOBJECTS ON SYSOBJECTS.ID = SYSCOLUMNS.ID 
    WHERE upper(SYSCOLUMNS.NAME) = '@tablecolumn' AND upper(SYSOBJECTS.NAME) = '@table')
ALTER TABLE @table ALTER COLUMN @tablecolumn float 
GO
IF  EXISTS (SELECT 1  FROM SYSCOLUMNS INNER JOIN SYSOBJECTS ON SYSOBJECTS.ID = SYSCOLUMNS.ID 
    WHERE upper(SYSCOLUMNS.NAME) = '@tablecolumn' AND upper(SYSOBJECTS.NAME) = '@table')
ALTER TABLE @table add @dfname default 0  for @tablecolumn
GO