SQL Union查询错误

Question

我正在尝试加入两个计数查询

SELECT COUNT(*) AS total FROM clients WHERE addedby = 1
UNION
SELECT COUNT(*) AS converts FROM clients WHERE addedby = 1 AND status = '6'

这返回的是

total
4
0

这是正确的数据，我期望的是

total     converts
  4           0

Answer 1

您不需要UNION查询即可执行此操作。 SELECT A UNION SELECT B返回A的行，然后是B的行（去重复）；如果要从两个数据集中获取所有行，请使用UNION ALL 。

您想要的是这样的：

select 
    (select count(*) from clients where addedby=1) as total,
    (select count(*) from clients where addedby=1 and status='6') as converts

另一种方法是使用case ... end表达式，如果status='6'则返回1 ：

select 
    count(*) from clients, 
    sum(case when status='6' then 1 else 0 end) as converts
from clients

Answer 2

无需UNION ，只需一次即可完成。

SELECT COUNT(*) as total,
       SUM(CASE status WHEN '6' THEN 1 ELSE 0 END) as converts
FROM clients;

Answer 3

编写此查询的最简单方法是条件聚合：

select count(*) as total, sum(status = '6') as converts
from cleints
where addedby = 1;

MySQL将布尔值视为整数，其中1为true， 0为false。 您可以对这些值进行求和以获得计数。