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

通过该查询，我可以在 test_name 列上计算两个表之间的不匹配。但是我想在motorcontroltests表的此查询中添加 WHERE 子句。而且我不知道该条款放在何处。我尝试过，但总是会出错。

多谢。

Answer 1

我不知道如何处理where子句，但可以避免使用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'
                 );

您可以直接通过JOIN来表达它：

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 2

在连接之后使用where子句，以后再使用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

我认为您可以通过使用join来实现

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 3

尝试将b.version和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

尽管您可以将查询编写为：

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;

但是，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;

也许更清楚地是：

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，表别名是表名称的缩写，而不是任意字母。这样可以更轻松地理解和修改查询。

在查询中添加WHERE子句

4 个答案: