如何在 db2 中使用 join 进行更新

Posted 2023-03-23

【中文标题】如何在 db2 中使用 join 进行更新

【英文标题】：How to update in db2 with join

【发布时间】：2017-01-19 16:01:54

【问题描述】：

我想做一个更新，只更新受影响的值，但我的更新查询有一个连接，所以where exists 不起作用。

update semester_workload tA
set tA.account = (SELECT DISTINCT (t2.WORKING - t2.WORKLOAD) FROM SEMESTER_WORKLOAD t1
        join SEMESTER_WORKLOAD t2 on t1.ID_LECTURER = t2.ID_LECTURER
        WHERE t2.ACADEMIC_SEMESTER = 'WS10' AND tA.ID_SW = t2.ID_SW);
where exists (SELECT 1 FROM SEMESTER_WORKLOAD t1        
        join SEMESTER_WORKLOAD t2 on t1.ID_LECTURER = t2.ID_LECTURER
        WHERE t2.ACADEMIC_SEMESTER = 'WS10' AND tA.ID_SW = t2.ID_SW);

此代码不起作用，因为它显然无法识别 join 子句后的 where exists。

【问题讨论】：

不相关，但是：distinct 不是一个函数。括号没用

【参考方案1】：

只是一个简单的语法错误。 where exists 子句之前有一个多余的分号：

update semester_workload tA
set tA.account = (SELECT DISTINCT (t2.WORKING - t2.WORKLOAD) FROM SEMESTER_WORKLOAD t1
        join SEMESTER_WORKLOAD t2 on t1.ID_LECTURER = t2.ID_LECTURER
        WHERE t2.ACADEMIC_SEMESTER = 'WS10' AND tA.ID_SW = t2.ID_SW) -- ; remove this
where exists (SELECT 1 FROM SEMESTER_WORKLOAD t1        
        join SEMESTER_WORKLOAD t2 on t1.ID_LECTURER = t2.ID_LECTURER
        WHERE t2.ACADEMIC_SEMESTER = 'WS10' AND tA.ID_SW = t2.ID_SW);

【参考方案2】：

你有一个“;”过量。请注意，您不需要加入同一个表三次，您可以这样做：

UPDATE SEMESTER_WORKLOAD t1
set t1.account = 
(
   SELECT t2.WORKING - t2.WORKLOAD FROM SEMESTER_WORKLOAD t2
   WHERE 
   (t1.ID_LECTURER, t1.ID_SW, 'WS10')=(t2.ID_LECTURER, t2.ID_SW, t2.ACADEMIC_SEMESTER) 
   FETCH FIRST ROWS ONLY
)
where exists 
(
   SELECT * FROM SEMESTER_WORKLOAD t2
   WHERE 
   (t1.ID_LECTURER, t1.ID_SW, 'WS10')=(t2.ID_LECTURER, t2.ID_SW, t2.ACADEMIC_SEMESTER) 
)