将一列中的多行选择为不同的列（SQL）

Question

我在MySQL中有一个名为wp_usermeta的表：（这是一个wordpress应用程序，但我不想仅通过MySQL与之通过wordpress与数据库进行交互）

umeta_id    user_id     meta_key      meta_value
-------------------------------------------------------
1           1           nickname      Rymdblomma
2           1           firstname     Nile
3           1           lastname      White
4           1           description   Coolest person

依此类推。 我要选择所有这4个，但改为4列。 同时，我正在从其他表中选择数据，并创建一个具有所有属性的临时表。

SELECT  wp_users.ID AS wp_id,
        wp_users.user_email AS email,
        wp_users.user_login AS username,
        wp_users.user_registered AS regDate,
        wp_blogs.path AS subname,
        wp_usermeta.meta_value AS firstname, 
        wp_usermeta.meta_value AS lastname
FROM    wp_users
INNER JOIN wp_blogs ON wp_users.ID = wp_blogs.blog_id
LEFT JOIN wp_usermeta ON wp_users.ID = wp_usermeta.user_id AND 
(wp_usermeta.umeta_id = 2 OR wp_usermeta.umeta_id = 3);

（我想包括所有meta_values，而不仅仅是名字和姓氏，但出于测试目的，我将其限制为2）

这将返回如下内容：

wp_id    email    username    regDate    subname    firstname    lastname
-------------------------------------------------------------------------
1        blabla   blabla      blabla     blabla     Nile         Nile
1        blabla   blabla      blabla     blabla     White        White

依此类推。 但是我不想重复行，而是想要这样的东西：

wp_id    email    username    regDate    subname    firstname    lastname
-------------------------------------------------------------------------
1        blabla   blabla      blabla     blabla     Nile         White

我试图在任何地方搜索任何帮助，但还没有找到任何东西。 任何帮助，将不胜感激。

Answer 1

一种方法使用条件聚合：

SELECT u.ID AS wp_id, u.user_email AS email, u.user_login AS username,
       u.user_registered AS regDate,
       b.path AS subname,
       MAX(CASE WHEN um.umeta_id = 2 THEN um.meta_value END) as firstname,
       MAX(CASE WHEN um.umeta_id = 3 THEN um.meta_value END) as lastname
FROM wp_users u INNER JOIN
     wp_blogs b
     ON u.ID = b.blog_id LEFT JOIN
     wp_usermeta umf
     ON u.ID = um.user_id AND 
        um.umeta_id IN (2, 3)
GROUP BY u.ID, u.user_email, u.user_login,
         u.user_registered, b.path;

Answer 2

要将wp_usermeta中的名字和姓氏放入一行，只需使用不同的umeta_id =连接条件将表连接两次

SELECT  wp_users.ID AS wp_id,
        wp_users.user_email AS email,
        wp_users.user_login AS username,
        wp_users.user_registered AS regDate,
        wp_blogs.path AS subname,
        metafirstname.meta_value AS firstname, 
        metalastname.meta_value AS lastname
FROM    wp_users
INNER JOIN wp_blogs ON wp_users.ID = wp_blogs.blog_id
LEFT JOIN wp_usermeta metafirstname ON wp_users.ID = metafirstname.user_id AND metafirstname.umeta_id = 2
LEFT JOIN wp_usermeta metalastname ON wp_users.ID = metalastname.user_id AND metalastname.umeta_id = 3;