同一张桌子上有多个选择

Question

我试图弄清楚它是在同一查询中进行选择的一种方法。 我正在处理wordpress数据库，并且有这些SQL查询可以自己运行：

SELECT `meta_value` AS 'Name' FROM `wp_postmeta` WHERE `meta_key` =     "myfield1" ORDER BY `post_id` ASC
SELECT `meta_value` AS 'Department' FROM `wp_postmeta` WHERE `meta_key` = "myfield2" ORDER BY `post_id` ASC
SELECT `meta_value` AS 'Location' FROM `wp_postmeta` WHERE `meta_key` = "myfield3" ORDER BY `post_id` ASC
SELECT `meta_value` AS 'Job Title' FROM `wp_postmeta` WHERE `meta_key` = "myfield4" ORDER BY `post_id` ASC
SELECT `meta_value` AS 'Social Number' FROM `wp_postmeta` WHERE `meta_key` = "myfield5" ORDER BY `post_id` ASC

但我想将它们全部放在同一个查询中，以便稍后可以在报表上导出数据。 这是我尝试过的：

SELECT  
    (SELECT `meta_value` FROM `wp_postmeta` WHERE `meta_key` = "myfield1" ORDER BY `post_id` ASC) As name,
    (SELECT `meta_value` FROM `wp_postmeta` WHERE `meta_key` = "myfield2" ORDER BY `post_id` ASC) AS department,
    (SELECT `meta_value` FROM `wp_postmeta` WHERE `meta_key` = "myfield3" ORDER BY `post_id` ASC) AS location,
    (SELECT `meta_value` FROM `wp_postmeta` WHERE `meta_key` = "myfield4" ORDER BY `post_id` ASC) AS jobtitle,
    (SELECT `meta_value` FROM `wp_postmeta` WHERE `meta_key` = "myfield5" ORDER BY `post_id` ASC) AS socnumber

但这会使命令不同步。 有人知道如何解决这个问题吗？

#2014 - Commands out of sync; you can't run this command now 

编辑：我正在查询wp_postmeta表，它具有4列meta_id，post_id，meta_key和meta_value列。 我从添加的自定义字段（称为myfield1，保存在meta_key列中）中选择meta_value。 我按post_id排序，post_id是wordpress中帖子的ID，因此我按时间顺序排序。

但是我有5个自定义字段，从myfield1到myfield5。 我想对所有这些查询一次。

Answer 1

一种方法是使用条件聚合：

SELECT MAX(CASE WHEN `meta_key` = 'myfield1' THEN meta_value END) as Name,
       MAX(CASE WHEN `meta_key` = 'myfield2' THEN meta_value END) as Department,
       MAX(CASE WHEN `meta_key` = 'myfield3' THEN meta_value END) as Location,
       MAX(CASE WHEN `meta_key` = 'myfield4' THEN meta_value END) as JobTitle,
       MAX(CASE WHEN `meta_key` = 'myfield5' THEN meta_value END) as SocialNumber
FROM `wp_postmeta`
WHERE `meta_key` = "myfield5" 
GROPU BY post_id
ORDER B post_id ASC;

Answer 2

您不能选择别名不同的同一列（“ meta_value” AS“名称” ...等）并将结果作为一组来获取。 该列只能有一个名称！

您可以做的最好的事情是这样的：

SELECT `meta_value`, `meta_key`
FROM `wp_postmeta`
WHERE `meta_key` IN ("myfield1", "myfield2", "myfield3", "myfield4", "myfield5")
ORDER BY `post_id` ASC

哪一个都将在meta_key列中为您提供所有值在一个结果集中，因此您可以分辨出哪个是哪个。

Answer 3

要获得具有五列的结果集（每个自定义字段一个），您必须将表自身连接五次...

像这样：

SELECT
    pm1.meta_value AS 'Name' 
    pm2.meta_value AS 'Department'
    pm3.meta_value AS 'Location'
    pm4.meta_value AS 'Job Title'
    pm5.meta_value AS 'Social Number'

 FROM `wp_postmeta` pm1
    JOIN `wp_postmeta` pm2 ON pm1.post_id = pm2.post_id
    JOIN `wp_postmeta` pm3 ON pm1.post_id = pm3.post_id
    JOIN `wp_postmeta` pm4 ON pm1.post_id = pm4.post_id
    JOIN `wp_postmeta` pm5 ON pm1.post_id = pm5.post_id

WHERE
    pm1.meta_key = "myfield1"
    AND pm2.meta_key = "myfield2"
    AND pm3.meta_key = "myfield3"
    AND pm4.meta_key = "myfield4"
    AND pm5.meta_key = "myfield5"

 ORDER BY `post_id` ASC

我尚未检查SQL语法，因此，如果出现语法错误，请原谅！

假设这是mySQL？