Question

在下面的查询中，我想在CASE的WHEN中添加一个AND条件，然后才可能吗？

例如WHEN'r'和table1.name =“jones”那么'非常高'

SELECT table1.id, table1.name,
   CASE table1.event
     WHEN 'r' THEN 'very high'
     WHEN 't' THEN 'very low'
     ELSE (SELECT table2.risk FROM table2 WHERE table2.value <= table1.value
           ORDER BY table2.value DESC LIMIT 1)
   END AS risk
FROM table1
ORDER BY FIELD( table1.event, 'r', 'f', 't' ), table1.value DESC

Answer 1

您可以像这样重写您的语句以实现您想要的目标

SELECT table1.id, table1.name,
   CASE 
     WHEN table1.event = 'r' AND table1.name = 'jones' THEN 'very high'
     WHEN table1.event = 't' AND table1.name = 'smith' THEN 'very low'
     ELSE (SELECT table2.risk FROM table2 WHERE table2.value <= table1.value
           ORDER BY table2.value DESC LIMIT 1)
   END AS risk
FROM table1
ORDER BY FIELD( table1.event, 'r', 'f', 't' ), table1.value DESC

请注意，您需要在table1.event语句后删除CASE。 documentation here

Answer 2

任何评估为布尔值（true或false）的内容都可以在WHEN语句的CASE条件下。因此，您可以将'r'替换为：

('r' AND table1.name='jones')

考虑到这一点，您可能不得不在table1.event

之后丢失CASE

SELECT table1.id, table1.name,
    CASE 
        WHEN (table1.event = 'r' AND table1.name='Jones') THEN 'very high'
        WHEN table1.event = 't' THEN 'very low'
        ELSE (SELECT table2.risk 
              FROM table2 
              WHERE table2.value <= table1.value 
              ORDER BY table2.value DESC LIMIT 1)
    END AS risk
FROM table1
ORDER BY FIELD( table1.event, 'r', 'f', 't' ), table1.value DESC

Answer 3

从case <column> when <value> then ...切换到case when <condition> then ...：

CASE 
WHEN table1.event = 'r' AND table1.active = 1 THEN 'very high'
...

那时的情况，但是当时和之前有AND条件

3 个答案: