MySQL 查询以根据自定义列名和值获取值
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【中文标题】MySQL 查询以根据自定义列名和值获取值【英文标题】：MySQL query to get values based on custom column name and values 【发布时间】：2021-07-24 06:59:11 【问题描述】：我有两张这样的桌子
wp_posts
ID post_date post_title post_type post_status
1 2020-12-10 ABC institute publish
2 2020-12-16 DEG institute publish
3 2020-12-11 XXY institute publish
4 2020-12-12 ABX institute publish
5 2020-12-24 ZYU institute publish
6 2020-12-28 ANM institute publish
7 2020-12-16 DDK institute publish
8 2020-12-30 LKI institute publish
9 2020-12-30 LKI institute publish
10 2020-12-31 LKI institute publish
11 2020-12-16 TGY institute publish
12 2020-12-16 MUN institute publish
wp_postmeta
meta_id post_id meta_key meta_value
1 1 country_id 10
2 1 registartion_by online
3 3 registartion_by offline
4 4 country_id 100
5 3 country_id 100
6 4 registartion_by online
7 5 registartion_by online
8 7 registartion_by offline
9 7 country_id 101
10 5 country_id 102
11 8 country_id 103
12 9 registartion_by online
13 8 registartion_by offline
14 9 country_id 10
15 10 country_id 104
16 10 registartion_by offline
17 11 country_id 101
18 11 registartion_by online
现在我想进行一些查询，以便最终输出显示为这样的内容
Date Country Offline_registration Online_registartion
2020-12-10 10 0 1
2020-12-11 100 1 0
2020-12-12 100 0 1
2020-12-16 101 1 1
2020-12-24 102 0 1
2020-12-30 103 0 1
2020-12-31 104 1 0
为了实现这一点，我做了一些这样的查询
SELECT date( posts.post_date ) as Date , postmeta.meta_value as Country
FROM wp_posts as posts INNER JOIN wp_postmeta as postmeta on posts.ID = postmeta.post_id WHERE posts.post_type = 'institute' AND posts.post_status = 'publish' AND postmeta.meta_key = 'country_id' GROUP BY postmeta.meta_value
但我无法获取注册类型计数的数据，即（在线/离线注册）。那么有人可以告诉我如何实现吗？
任何建议或意见都会非常可观。 谢谢。
【问题讨论】：
【参考方案1】：聚合两次
select p.post_date, m.country_id,
sum(case when registartion_by='offline' then 1 end) Offline_registartion,
sum(case when registartion_by='online' then 1 end) Online_registartion
from (
select post_id,
max(case when meta_key ='country_id' then meta_value end) country_id,
max(case when meta_key ='registartion_by' then meta_value end) registartion_by,
from wp_postmeta
group by post_id ) m
join wp_posts p on p.ID = m.post_id
group by post_date
在mysql中sum(case when registartion_by='offline' then 1 end)可以简写为sum(registartion_by='offline')
【讨论】：
【参考方案2】：我不知道你为什么从结果中排除了| 2020-12-30 | 10 | 0 | 1 |，所以我假设你正在寻找这样的东西：
DROP TABLE IF EXISTS wp_posts;
CREATE TABLE wp_posts
(id INT AUTO_INCREMENT PRIMARY KEY
,post_date DATE NOT NULL
,post_title CHAR(3) NOT NULL
);
INSERT INTO wp_posts VALUES
( 1,'2020-12-10','ABC'),
( 2,'2020-12-16','DEG'),
( 3,'2020-12-11','XXY'),
( 4,'2020-12-12','ABX'),
( 5,'2020-12-24','ZYU'),
( 6,'2020-12-28','ANM'),
( 7,'2020-12-16','DDK'),
( 8,'2020-12-30','LKI'),
( 9,'2020-12-30','LKI'),
(10,'2020-12-31','LKI'),
(11,'2020-12-16','TGY'),
(12,'2020-12-16','MUN');
DROP TABLE IF EXISTS wp_postmeta;
CREATE TABLE wp_postmeta
(meta_id INT AUTO_INCREMENT PRIMARY KEY
,post_id INT NOT NULL
,meta_key VARCHAR(20) NOT NULL
,meta_value VARCHAR(20) NOT NULL
,UNIQUE(post_id,meta_key)
);
INSERT INTO wp_postmeta VALUES
( 1, 1,'country_id',10),
( 2, 1,'registration_by','online'),
( 3, 3,'registration_by','offline'),
( 4, 4,'country_id','100'),
( 5, 3,'country_id','100'),
( 6, 4,'registration_by','online'),
( 7, 5,'registration_by','online'),
( 8, 7,'registration_by','offline'),
( 9, 7,'country_id','101'),
(10, 5,'country_id','102'),
(11, 8,'country_id','103'),
(12, 9,'registration_by','online'),
(13, 8,'registration_by','offline'),
(14, 9,'country_id','10'),
(15,10,'country_id','104'),
(16,10,'registration_by','offline'),
(17,11,'country_id','101'),
(18,11,'registration_by','online');
...
SELECT p.post_date
, MAX(CASE WHEN m.meta_key = 'country_id' THEN m.meta_value END) country
, MAX(CASE WHEN m.meta_key = 'registration_by' THEN CASE WHEN m.meta_value = 'offline' THEN 1 ELSE 0 END END) offline
, MAX(CASE WHEN m.meta_key = 'registration_by' THEN CASE WHEN m.meta_value = 'online' THEN 1 ELSE 0 END END) online
FROM wp_posts p
JOIN wp_postmeta m
ON m.post_id = p.id
GROUP
BY p.id
, p.post_date
ORDER
BY country
, post_date;
+------------+---------+---------+--------+
| post_date | country | offline | online |
+------------+---------+---------+--------+
| 2020-12-10 | 10 | 0 | 1 |
| 2020-12-30 | 10 | 0 | 1 |
| 2020-12-11 | 100 | 1 | 0 |
| 2020-12-12 | 100 | 0 | 1 |
| 2020-12-16 | 101 | 0 | 1 |
| 2020-12-16 | 101 | 1 | 0 |
| 2020-12-24 | 102 | 0 | 1 |
| 2020-12-30 | 103 | 1 | 0 |
| 2020-12-31 | 104 | 1 | 0 |
+------------+---------+---------+--------+
【讨论】：
【参考方案3】：您可以通过两次加入wp_postmeta 来做到这一点，一次用于国家/地区，一次用于注册：
SELECT date(p.post_date) as Date, pmc.meta_value as Country,
SUM( pmr.meta_value = 'online' ) as num_online,
SUM( pmr.meta_value = 'offline' ) as num_offline
FROM wp_posts p INNER JOIN
wp_postmeta pmc
ON pmc.post_id = p.id AND
pmc.meta_key = 'country_id' LEFT JOIN
wp_postmeta pmr
ON pmc.post_id = p.id AND
pmc.meta_key = 'registion_by'
WHERE p.post_type = 'institute' AND
p.post_status = 'publish'
GROUP BY date, country;
【讨论】：
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