两个表 t_val, t_val2，以id为键, 将 t_val2中的val与 t_val中的val比较，如果相等result 为true, 不相等为false
表： t_val
id val
----------- --------------------------------------------------
1 A
2 B
3 AC
4 C
5 AD
(5 row(s) affected)
表： t_val2
id val result
----------- ------------------------------- --------------------------------------------------
1 A NULL
2 A NULL
3 AB NULL
4 D NULL
5 AD NULL
CREATE TABLE [dbo].[t_val](
[id] [int] NOT NULL,
[val] [nvarchar](50) NOT NULL,
CONSTRAINT [PK_t_val] PRIMARY KEY CLUSTERED
(
[id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
CREATE TABLE [dbo].[t_val2](
[id] [int] NOT NULL,
[val] [nvarchar](50) NOT NULL,
[result] [nvarchar](50) NULL,
CONSTRAINT [PK_t_val2] PRIMARY KEY CLUSTERED
(
[id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
insert into t_val(id, val) values(1, 'A')
insert into t_val(id, val) values(2, 'B')
insert into t_val(id, val) values(3, 'AC')
insert into t_val(id, val) values(4, 'C')
insert into t_val(id, val) values(5, 'AD')
insert into t_val2(id, val) values(1, 'A')
insert into t_val2(id, val) values(2, 'A')
insert into t_val2(id, val) values(3, 'AB')
insert into t_val2(id, val) values(4, 'D')
insert into t_val2(id, val) values(5, 'AD')
-- 用它我可以计算出来，但我不知道怎么去更新result。
select a.id, result=(case b.val when a.val then 'true' else 'false' end from t_val a left join t_val2 b on a.id=b.id
update t_val2 set result=t.result
from (select a.id, result=(case b.val when a.val then 'true' else 'false' end) from t_val a left join t_val2 b on a.id=b.id) t
where id=t.id
------解决思路----------------------
UPDATE
t_val2
SET
result = CASE WHEN t_val2.val = t_val.val THEN 'true' ELSE 'false' END
FROM