Question

我必须在此步骤中创建一个查询：

update t1
set c3='000'
from t_loop t1
join t_loop t2
  on t1.c2=t2.c1
where t1.c3 is null
  and t2.c3 is not null

while (@@rowcount>0)
    update t1
    set c3='000'
    from t_loop t1
    join t_loop t2
      on t1.c2=t2.c1
    where t1.c3 is null
      and t2.c3 is not null

此代码检查列c1具有c3值的位置，然后对每个匹配的c2进行循环，估计其c3。

例如：

select *
into [t_loop]
from
(
    select 'v1'c1,'v2'c2,NULL c3 union all
    select 'v10','v9',NULL union all
    select 'v2','v3',NULL union all
    select 'v3','v2','000'union all
    select 'v4','v2',NULL union all
    select 'v5',NULL ,NULL union all
    select 'v6',NULL ,NULL union all
    select 'v7','v1',NULL union all
    select 'v8','v7',NULL union all
    select 'v9',NULL ,NULL union all
    select 'va','vb',NULL union all
    select 'vb','vc',NULL union all
    select 'vc','vb',NULL union all
    select 'vd',NULL ,NULL union all
    select 've',NULL ,NULL union all
    select 'vf','vb','000'
)t

然后结果是：

c1  c2  c3
v1  v2  000
v10 v9  NULL
v2  v3  000
v3  v2  000
v4  v2  000
v5  NULL    NULL
v6  NULL    NULL
v7  v1  000
v8  v7  000
v9  NULL    NULL
va  vb  NULL
vb  vc  NULL
vc  vb  NULL
vd  NULL    NULL
ve  NULL    NULL
vf  vb  000

我试过平板电脑但是我不能这样做......有人可以帮助我吗？

!!解决了！！

使用 gofr1

的帖子这里是CTE解释

--CTE explosion

--query 1
    SELECT  t1.c1,
            t1.c2
    from t_loop t1
    join t_loop t2
      on t1.c2=t2.c1
    where t1.c3 is null
      and t2.c3 is not null
    union all
--query 2
    SELECT  t1.c1,
            t1.c2
    from t_loop t1
    join --cte -> replace cte with the first query (query 1)
    (
        SELECT  t1.c1,
                t1.c2
        from t_loop t1
        join t_loop t2
          on t1.c2=t2.c1
        where t1.c3 is null
          and t2.c3 is not null
    )
     t2
      on t1.c2=t2.c1
    where t1.c3 is null
    union all
--query 3
    SELECT  t1.c1,
            t1.c2
    from t_loop t1
    join --cte ->  replace cte with the second query (query 2)
    (
        SELECT  t1.c1,
                t1.c2
        from t_loop t1
        join 
        (
            SELECT  t1.c1,
                    t1.c2
            from t_loop t1
            join t_loop t2
              on t1.c2=t2.c1
            where t1.c3 is null
              and t2.c3 is not null
    )
     t2
      on t1.c2=t2.c1
    where t1.c3 is null
    ) t2
      on t1.c2=t2.c1
    where t1.c3 is null

Answer 1

使用CTE：

;WITH cte AS (
    SELECT  t1.c1,
            t1.c2,
            '000' as c3
    from t_loop t1
    join t_loop t2
      on t1.c2=t2.c1
    where t1.c3 is null
      and t2.c3 is not null
    UNION ALL
    SELECT  t1.c1,
            t1.c2,
            '000' as c3
    from t_loop t1
    join cte t2
      on t1.c2=t2.c1
    where t1.c3 is null
      and t2.c3 is not null
)


UPDATE t
SET c3 = c.c3
FROM [dbo].[t_loop] t
INNER JOIN cte c
    ON t.c1 = c.c1
        and t.c2 = c.c2

SELECT *
FROM  [dbo].[t_loop]

输出与您提供的相同。

Answer 2

GO 16将更新16次。

UPDATE [t_loop]
SET c3 = '000'
WHERE
[t_loop].c3 IS NULL AND
EXISTS
(
    SELECT 1 FROM dbo.[t_loop] t
    WHERE 
        t.c1 = [t_loop].c2 AND
        t.c3 IS NOT NULL 
)
GO 16 -- SELECT COUNT(*) FROM [t_loop]

结果：

c1   c2   c3
---- ---- ----
v1   v2   000
v10  v9   NULL
v2   v3   000
v3   v2   000
v4   v2   000
v5   NULL NULL
v6   NULL NULL
v7   v1   000
v8   v7   000
v9   NULL NULL
va   vb   NULL
vb   vc   NULL
vc   vb   NULL
vd   NULL NULL
ve   NULL NULL
vf   vb   000

其他选择：

DECLARE @Counter INT = 1
WHILE @Counter < (SELECT COUNT(*) FROM [t_loop])
BEGIN
    UPDATE [t_loop]
    SET c3 = '000'
    WHERE
    [t_loop].c3 IS NULL AND
    EXISTS
    (
        SELECT 1 FROM dbo.[t_loop] t
        WHERE 
            t.c1 = [t_loop].c2 AND
            t.c3 IS NOT NULL 
    )

    SET @Counter += 1
END

使用自联接编写更新查询

2 个答案: