如何取所有 COUNT 的总和？答案

【问题标题】：How to take SUM of all COUNTs?如何取所有 COUNT 的总和？
【发布时间】：2018-06-07 02:38:57
【问题描述】：

我是mysql 的新手。

我有一个查询要从不同的表中获取计数，它工作正常。但是，现在我想将所有 3 个查询 COUNT 合并为 1，例如 SUM (COUNT1, COUNT2, COUNT3)

select COUNT(*) as count1, 0 as count2, 0 as count3 from table1 
UNION ALL 
select 0 as count1, COUNT(*) as count2, 0 as count3 from table2 
UNION ALL 
select 0 as count1, 0 as count2, COUNT(*) as count3 from table3

例如count1 = 10，count2 = 20 和 count3 = 15

然后，我想得到 sum = 45

【问题讨论】：

标签： mysql

【解决方案1】：

您可以将UNION 放在子查询中，然后将SUM() 放在计数中。但是没有必要在每个联合查询中将它们放在单独的列中。

SELECT SUM(count) AS total
FROM (
    SELECT COUNT(*) AS count FROM table1
    UNION ALL
    SELECT COUNT(*) AS count FROM table2
    UNION ALL
    SELECT COUNT(*) AS count FROM table3
) AS subquery

【讨论】：

【解决方案2】：

要从您的表中获取所有计数的总和，您可以将查询编写为

select 
(select COUNT(*) from table1) +
(select COUNT(*) from table2) +
(select COUNT(*) from table3) as total

按照您的方法，您可以将联合查询包装为子选择，然后对结果应用总和

SELECT SUM(count1) + SUM(count2) + SUM(count3) total
FROM (
    SELECT COUNT(*) AS count1, 0 AS count2, 0 AS count3 FROM table1 
    UNION ALL 
    SELECT 0 AS count1, COUNT(*) AS count2, 0 AS count3 FROM table2
    UNION ALL 
    SELECT 0 AS count1, 0 AS count2, COUNT(*) AS count3 FROM table3
) t

【讨论】：