利用带关联子查询Update语句更新数据的方法
Update是T-sql中再简单不过的语句了,update table set column=expression [where condition],我们都会用到。但update的用法不仅于此,真正在开发的时候,灵活恰当地使用update可以达到事半功倍的效果。
假定有表Table1(a,b,c)和Table2(a,c),现在Table1中有些记录字段c为null,要根据字段a在Table2中查找,取出字段a相等的字段c的值来更新Table1。一种常规的思路,通过游标遍历Table1中字段c为null的所有记录,在循环体内查找Table2并进行更新,即用游标Cursor的形式。测试sql语句如下:
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--1.创建测试表
create TABLE Table1
(
a varchar (10),
b varchar (10),
c varchar (10),
CONSTRAINT [PK_Table1] PRIMARY KEY CLUSTERED
(
a ASC
)
) ON [ PRIMARY ]
create TABLE Table2
(
a varchar (10),
c varchar (10),
CONSTRAINT [PK_Table2] PRIMARY KEY CLUSTERED
(
a ASC
)
) ON [ PRIMARY ]
GO
--2.创建测试数据
Insert into Table1 values ( '赵' , 'asds' , null )
Insert into Table1 values ( '钱' , 'asds' , '100' )
Insert into Table1 values ( '孙' , 'asds' , '80' )
Insert into Table1 values ( '李' , 'asds' , null )
Insert into Table2 values ( '赵' , '90' )
Insert into Table2 values ( '钱' , '100' )
Insert into Table2 values ( '孙' , '80' )
Insert into Table2 values ( '李' , '95' )
GO
select * from Table1
--3.通过游标方式更新
declare @ name varchar (10)
declare @score varchar (10)
declare mycursor cursor for select a from Table1 where c is null
open mycursor
fetch next from mycursor into @ name
while(@@fetch_status = 0)
BEGIN
select @score=c from Table2 where a=@ name
update Table1 set c = @score where a = @ name
fetch next from mycursor into @ name
END
close mycursor
deallocate mycursor
GO
--4.显示更新后的结果
select * from Table1
GO
--5.删除测试表
drop TABLE Table1
drop TABLE Table2
|
虽然用游标可以实现,但代码看起来很复杂,其实用Update根据子关联来更新只要一条语句就可以搞定了,测试代码如下:
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--1.创建测试表
create TABLE Table1
(
a varchar (10),
b varchar (10),
c varchar (10),
CONSTRAINT [PK_Table1] PRIMARY KEY CLUSTERED
(
a ASC
)
) ON [ PRIMARY ]
create TABLE Table2
(
a varchar (10),
c varchar (10),
CONSTRAINT [PK_Table2] PRIMARY KEY CLUSTERED
(
a ASC
)
) ON [ PRIMARY ]
GO
--2.创建测试数据
Insert into Table1 values ( '赵' , 'asds' , null )
Insert into Table1 values ( '钱' , 'asds' , '100' )
Insert into Table1 values ( '孙' , 'asds' , '80' )
Insert into Table1 values ( '李' , 'asds' , null )
Insert into Table2 values ( '赵' , '90' )
Insert into Table2 values ( '钱' , '100' )
Insert into Table2 values ( '孙' , '80' )
Insert into Table2 values ( '李' , '95' )
GO
select * from Table1
--3.通过Update方式更新
Update Table1 set c = ( select c from Table2 where a = Table1.a) where c is null
GO
--4.显示更新后的结果
select * from Table1
GO
--5.删除测试表
drop TABLE Table1
drop TABLE Table2
|
参考资料:也许是被忽略的update语句,update 子查询
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