Question

我有两张桌子：

 1) fk_tblUserEmployeeList
 2) fk_tblmanageremployeelist

在这两个表中，一列是常见的userid，我从UI获得userid。我想从第一张表中获得userid的记录。如果没有，请从第二个表中获取相同userid的记录..

我正在尝试使用CASE，但收到错误：

ERROR:  more than one row returned by a subquery used as an expression

以下是我的询问：

select case when ( select count(userid) 
                   from fk_tblUserEmployeeList 
                   where userid=1
                 ) > 1 
            then ( select employeeid 
                   from fk_tblUserEmployeeList 
                   where userid=1
                 )
            else ( select employeeid 
                   from fk_tblmanageremployeelist 
                   where userid=1
                 ) 
       end

Answer 1

使用您提供的代码，case的结果将包含多个row。由于先前select case，因此会出错。你不能select多个rows。

select

case when (select count(userid) from fk_tblUserEmployeeList where userid=1) > 1
    then (select employeeid from fk_tblUserEmployeeList where userid=1)
    else (select employeeid from fk_tblmanageremployeelist where userid=1)
end

因此，为避免这种情况，您可以使用IF。

它应该是这样的：

IF (select count(userid) from fk_tblUserEmployeeList where userid=1) > 1 THEN
    (select employeeid from fk_tblUserEmployeeList where userid=1)
ELSE
    (select employeeid from fk_tblmanageremployeelist where userid=1)
END IF;

Answer 2

尝试：

select employeeid 
from fk_tblUserEmployeeList 
where userid=1
union all
select employeeid 
from fk_tblmanageremployeelist
where userid=1 and
not exists (select null
            from fk_tblUserEmployeeList 
            where userid=1)

请注意，如果两个表中都没有用户，则不需要not exists子句。

Answer 3

    select employeeid 
    from fk_tblUserEmployeeList 
    where userid = 1
      and ( select count(*) 
            from fk_tblUserEmployeeList 
            where userid=1
          ) > 1
  union all
    select employeeid 
    from fk_tblmanageremployeelist 
    where userid = 1
      and ( select count(*) 
            from fk_tblUserEmployeeList 
            where userid = 1
          ) <= 1

Answer 4

我想这将是最简单和最快的：

SELECT COALESCE(
        (SELECT employeeid FROM fk_tblUserEmployeeList    WHERE userid = 1),
        (SELECT employeeid FROM fk_tblmanageremployeelist WHERE userid = 1))

如果userid不是 UNIQUE ，则必须添加一个方法来选择一行，例如：

SELECT COALESCE(
        (SELECT employeeid
         FROM   fk_tblUserEmployeeList
         WHERE  userid = 1
         ORDER  BY employeeid
         LIMIT  1),
        (SELECT employeeid
         FROM   fk_tblmanageremployeelist
         WHERE  userid = 1
         ORDER  BY employeeid
         LIMIT  1)
        )

如何在PostgreSQL中使用case获取多个记录？

4 个答案: