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')

第二条select查询出来的结果

2.合并查询结果

1. 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') 

2. 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

3. 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

4. 运行结果

5. 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