MySQL 查询合并
如果我们需要将两个select语句的结果作为一个整体显示出来，我们就需要用到union或者union all关键字。union(或称为联合)的作用是将多个结果合并在一起显示出来。

注意：两个列表中的字段要一样才可以合并(顺序也要一样)

满足条件：

1、两个select查询的列的数量必须相同；

2、每个列的数据类型需要相似；

1.先写两条select
第一条select：

SELECT
    DATE_FORMAT(add_time, '%Y-%m-%d') as 'add_time',
    COUNT(add_time) as 'reach_intention'
FROM
    table1
where
## where 条件可以根据自己实际情况来定
    DATE_FORMAT(add_time, '%Y-%m-%d') > DATE_FORMAT(date_sub(now(), interval 1 month), '%Y-%m-%d')
group by
    DATE_FORMAT(add_time, '%Y-%m-%d')

这是查询出来的结果

第二条select：

select
    DATE_FORMAT(add_date, '%Y-%m-%d') as 'add_time' ,
    COUNT(add_date)  as 'post_release'
from
    table2
where
## where 条件可以根据自己实际情况来定
    DATE_FORMAT(add_date, '%Y-%m-%d') > DATE_FORMAT(date_sub(now(), interval 1 month), '%Y-%m-%d')
group by
    DATE_FORMAT(add_date, '%Y-%m-%d')

2.合并查询结果
先把两条select用union all连接起来

SELECT
        DATE_FORMAT(add_time, '%Y-%m-%d') as 'add_time', COUNT(add_time) as 'reach_intention' ,  '' as 'post_release'
    FROM
        table1
    where
        DATE_FORMAT(add_time, '%Y-%m-%d') > DATE_FORMAT(date_sub(now(), interval 30 day), '%Y-%m-%d')
    group by
        DATE_FORMAT(add_time, '%Y-%m-%d')
UNION ALL
    select
        DATE_FORMAT(add_date, '%Y-%m-%d') as 'add_time' , '' as 'reach_intention' ,  COUNT(add_date) as 'post_release'
    from
        table2
    where
        DATE_FORMAT(add_date, '%Y-%m-%d') > DATE_FORMAT(date_sub(now(), interval 30 day), '%Y-%m-%d')
    group by
        DATE_FORMAT(add_date, '%Y-%m-%d')

在外层嵌套一个select

## 括号里面放 用 union all 连接的select
select * from () test
## 这里要给表起个别名 不然会报 Every derived table must have its own alias 每个派生表都必须有自己的别名

把union all 连接的select 放到括号里面去然后运行

select
    *
from
(
    SELECT
        DATE_FORMAT(add_time, '%Y-%m-%d') as 'add_time', COUNT(add_time) as 'reach_intention' ,  '' as 'post_release'
    FROM
        table1
    where
        DATE_FORMAT(add_time, '%Y-%m-%d') > DATE_FORMAT(date_sub(now(), interval 30 day), '%Y-%m-%d')
    group by
        DATE_FORMAT(add_time, '%Y-%m-%d')
UNION ALL
    select
        DATE_FORMAT(add_date, '%Y-%m-%d') as 'add_time' , '' as 'reach_intention' ,  COUNT(add_date) as 'post_release'
    from
        table2
    where
        DATE_FORMAT(add_date, '%Y-%m-%d') > DATE_FORMAT(date_sub(now(), interval 30 day), '%Y-%m-%d')
    group by
        DATE_FORMAT(add_date, '%Y-%m-%d') 
) test

把结果按日期分组排序。

使用 group by 和 order by 关键字

select
    add_time,
    sum(reach_intention) as 'reach_intention' ,
    sum(post_release) as 'post_release'
from
    (
    SELECT
        DATE_FORMAT(add_time, '%Y-%m-%d') as 'add_time', COUNT(add_time) as 'reach_intention' , '' as 'post_release'
    FROM
        table1
    where
        DATE_FORMAT(add_time, '%Y-%m-%d') > DATE_FORMAT(date_sub(now(), interval 30 day), '%Y-%m-%d')
    group by
        DATE_FORMAT(add_time, '%Y-%m-%d')
UNION ALL
    select
        DATE_FORMAT(add_date, '%Y-%m-%d') as 'add_time' , '' as 'reach_intention' , COUNT(add_date) as 'post_release'
    from
        table2
    where
        DATE_FORMAT(add_date, '%Y-%m-%d') > DATE_FORMAT(date_sub(now(), interval 30 day), '%Y-%m-%d')
    group by
        DATE_FORMAT(add_date, '%Y-%m-%d') ) test
group by
    test.add_time
order by
    test.add_time desc

结果如下

总结
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