自联接…用于现有列中值的新综合列

Question

我以前做过一次，但是不记得怎么做...我已经足够挣扎着，现在正在寻求帮助。

我有一个包含两列的表：Uuid和ProcessId ProcessId列当前具有两个值：ValueA和ValueB

我想做一个自我联接（完全外部联接？），以便与下表：

1234  ValueA
        1234  ValueB
        2345  ValueB
        3456  ValueA

我将得到如下结果：

Uuid    ValueA      ValueB
        1234    OK          OK
        2345    Missing     OK
        3456    OK          Missing

Answer 1

尝试这个：

SELECT Uuid, 
   CASE WHEN U.ValueA>0 THEN 'OK' ELSE 'Missing' END AS ValueA,
   CASE WHEN U.ValueB>0 THEN 'OK' ELSE 'Missing' END AS ValueB
FROM 
 (SELECT T.Uuid, 
     (SELECT COUNT(*) FROM MyTable AS M WHERE T.Uuid=M.UserID AND ValueField='ValueA') AS ValueA, 
     (SELECT COUNT(*) FROM MyTable AS M WHERE T.Uuid=M.UserID AND ValueField='ValueB') AS ValueB
  FROM (SELECT DISTINCT Uuid FROM MyTable) AS T
  GROUP BY Uuid) AS U
ORDER BY Uuid

Answer 2

根据索引，表大小和优化器的不同，使用一对自联接可能最快：

select TableU.Uuid, 
    if(count(TableA.ProcessId) > 0, 'OK', 'Missing') as ValueA, 
    if(count(TableB.ProcessId) > 0, 'OK', 'Missing') as ValueB 
from Table11301861 as TableU
left join Table11301861 as TableA on TableA.ProcessId = 'ValueA' and TableA.Uuid = TableU.Uuid
left join Table11301861 as TableB on TableB.ProcessId = 'ValueB' and TableB.Uuid = TableU.Uuid
group by TableU.Uuid