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'

返回的是

total
4
0

这是正确的数据，我期待的是这个

total     converts
  4           0

Answer 1

您不需要UNION个查询来执行此操作。 SELECT A UNION SELECT B返回A行，后跟B行（重复数据删除;如果您想要来自两个数据集的所有行），请使用UNION ALL）

你想要的是这样的：

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

执行此操作的其他方法是使用case ... end表达式，如果1返回status='6'：

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

Answer 2

不需要UNION，一次性完成。

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作为条件聚合：

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

MySQL将布尔值视为整数，1为真，0为假。您可以只计算值的总和以获得计数。