将 WHERE 子句添加到查询

Question

我有以下查询：

select count(case when test_nameb is null then 1 end) 
from ( select a.test_name as test_namea, b.test_name as test_nameb 
       from list_52 a 
       left join motorcontroltests b 
       on a.test_name=b.test_name) x

通过这个查询，我可以计算 test_name 列上两个 table 之间的不匹配。但我想在 table motorcontroltests 的查询中添加一个 WHERE 子句。而且我不知道把这个条款放在哪里。我试过了，但总是出错。

提前致谢。

Answer 1

只需在加入后使用 where 子句，然后 on

select count(case when test_nameb is null then 1 end) 
from ( select a.test_name as test_namea, b.test_name as test_nameb 
       from list_52 a 
       left join motorcontroltests b 
       on a.test_name=b.test_name    
      where b.test_name is not null --assuming you want to null check---put your condition
     ) x

但我认为你可以使用 join

select sum(case when b.test_name is null then 1 else 0 end) 
from list_52 a left join 
     motorcontroltests b 
     on a.test_name=b.test_name

Answer 2

我不知道如何处理 where 子句，但你可以避免 subquery :

select count(*)
from list_52 a
where not exists (select 1 
                  from motorcontroltests b 
                  where a.test_name = b.test_name and b.version = '2.0'
                 );

可以直接表达为 via JOIN :

select sum(case when b.test_name is null then 1 else 0 end) 
from list_52 a left join 
     motorcontroltests b 
     on a.test_name = b.test_name and  b.version = '2.0';

Answer 3

尝试将 b.version 与 JOIN 放在一起：

SELECT SUM(CASE WHEN b.test_name IS NULL THEN 1 ELSE 0 END) Count
FROM list_52 a left join motorcontroltests b on a.test_name=b.test_name 
and b.version='2.0'

Answer 4

尽管您可以将查询写成：

select count(case when test_nameb is null then 1 end)
from (select l.test_name as test_namea, mct.test_name as test_nameb
      from list_52 l left join
           motorcontroltests mct
           on l.test_name = mct.test_name and mct.version = '2.0'
     ) x;

然而，这将更简单地写为：

select count(*)
from list_52 l left join
     motorcontroltests mct
     on l.test_name = mct.test_name and mct.version = '2.0'
where mct.test_name is null;

也许更清楚：

select count(*)
from list_52 l
where not exists (select 1
                  from motorcontroltests mct
                  where l.test_name = mct.test_name and mct.version = '2.0'
                 );

请注意 table 别名是 table 名称的缩写，而不是任意字母。这使得理解和修改查询变得更加容易。

将 WHERE 子句添加到查询

Add WHERE clause to a query

sql

where