MySQL 8.0

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CREATE TABLE wp_f7h0p6_users( id INT, user_login VARCHAR(100), user_email VARCHAR(100) )
INSERT INTO wp_f7h0p6_users VALUES
(1, 'johnd', 'johnd@example.com'),
(2, 'foob', 'foob@example.com')
CREATE TABLE wp_f7h0p6_usermeta( umeta_id INT,user_id INT, meta_key VARCHAR(100), meta_value VARCHAR(100))
INSERT INTO wp_f7h0p6_usermeta VALUES
(1,1,'first_name', 'John'),
(2,1,'last_name', 'Doe'),
(3,1,'billing_first_name','Defoe')
SELECT GROUP_CONCAT(
             CONCAT(
                    'MAX(CASE WHEN meta_key = "', meta_key,'" THEN meta_value END ) AS ',
                    meta_key
                    )
       )
  INTO @sql
  FROM ( SELECT DISTINCT meta_key FROM wp_f7h0p6_usermeta ) AS m;

SET @sql = CONCAT('SELECT user_login,',@sql,
                   ' FROM wp_f7h0p6_users U 
                     JOIN wp_f7h0p6_usermeta M 
                       ON U.ID = M.user_id
                    GROUP BY user_login'); 
                  
PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;  


user_login
first_name
last_name
billing_first_name


johnd
John
Doe
Defoe