Question

我有两张表有数百万条记录需要加入以显示不同语言的结果。我希望使用带有IF条件的查询创建一个视图，但我无法理解哪种方法是正确的。

我需要做的是：如果以下select对于单行具有NULL（换句话说，该行没有deutsch转换）...

SELECT one，two FROM tableA A LEFT JOIN tableB B ON（A.id = B.id）WHERE language ='de'order by xyz

...然后将英文版本放在同一行，所以选择：

SELECT one，two FROM tableA A LEFT JOIN tableB B ON（A.id = B.id）WHERE language ='en'order by xyz

谢谢！卢卡

Answer 1

您查询很难遵循，因为您没有别名。我假设它的结构如下：

select a.one. b.two
from tableA a left join
     tableb b
     on a.id = b.id
where b.language = 'de'
order by xyz

以下提供了适用于MySQL的逻辑：

SELECT a.one, 
       (case when (select COUNT(*) from b where b.id = a.id and b.language = 'de' and b.two is null) = 0
             then bde.two
             else bend.two
        end) as two
FROM tableA A LEFT JOIN
     tableB Bde
     ON (A.id=B.id) and
        bde.language='de' left join
     tableB ben
     on ben.language = 'en'
order by xyz

这假设NULL值存储在B表中。如果它通过连接丢失了，那就更难了，比如：

SELECT a.one, 
       (case when (select COUNT(*) from a a2 join b b2 on a2.id = b2.id and b2.language = 'de' and b2.two is null) = 0
             then bde.two
             else bend.two
        end) as two
FROM tableA A LEFT JOIN
     tableB Bde
     ON (A.id=B.id) and
        bde.language='de' left join
     tableB ben
     on ben.language = 'en'
order by xyz

在Mysql查询中使用IF

1 个答案: