将多行中多个MySql表中的数据合并到SELECT语句的一行中

Question

Note: This is not a duplicate of: mySQL - Create a New Table Using Data and Columns from Three Tables . That question is about taking data from multiple tables and actually creating a new table with that data. I'm just looking to select already existing data. The answer on that question is very specific to the question that was asked, and doesn't really help me since the code in the answer is not explained. So no, this is not "the exact same question" at all.

I have a question about joining two MySQL tables that I cannot seem to find an answer to. The usual types of joins do not seem to be what I'm after. perhaps this is not possible, I'm not sure.

So I have my first table like this (tables simplified for example purposes):

ID    Name    Email               Phone
1     John    john@example.com    000-123-4567
2     Jane    jane@example.com    000-890-1234

My second table is arranged this way:

ID   UserID    meta_key            meta_value
1    1         location_review     4
2    1         condition_review    1
3    1         price_review        5
4    2         location_review     4
5    2         condition_review    2
6    2         price_review        4

I am working with some existing software, just making some modifications, so ideally I can do what I need without needing to change how the software is written too much.

I am trying to be able to join these tables so they look like this:

    ID    Name    Email               Phone          location_review    condition_review    price_review
    1     John    john@example.com    000-123-4567   4                  1                   5
    2     Jane    jane@example.com    000-890-1234   4                  2                   4

So I need to add the rows from Table2 that have the UserID to the data already in Table1.

Since this is existing software there is already some code in place to join these two tables, it is here:

SELECT * 
FROM wpnf_comments as comment 
    INNER JOIN wpnf_commentmeta AS meta 
WHERE comment.comment_post_ID = $prop_ID 
AND meta.meta_key = 'rating' 
AND meta.comment_id = comment.comment_ID 
AND ( comment.comment_approved = 1 
    OR comment.user_id = $userID 
    )

What I am doing is: There used to be only a single "rating" value, stored in the meta table. You can see it in the above Join where it says:

AND meta.meta_key = 'rating'

However, I am trying to replace that with 3 separate ratings. So the join above, that used to work when there was only a single rating, doesn't really work when I have multiple ratings.

I would very much appreciate any insight into what I need to do to fix this. I am also using PHP for this, if that matters.

EDIT

This is the code I have so far. it's showing all of the correct column names I need now, but the actual values/data is not showing up. It's returning 0 rows.

SELECT 
  * 
FROM 
  wpnf_comments t1 
  INNER JOIN 
  (SELECT 
      comment_id as comment_id2, 
      MAX(IF(meta_key = 'location-rating', meta_value, NULL)) AS location_rating, 
      MAX(IF(meta_key = 'condition-rating', meta_value, NULL)) AS condition_rating, 
      MAX(IF(meta_key = 'maintenance-rating', meta_value, NULL)) AS maintenance_rating, 
      MAX(IF(meta_key = 'professionalism-rating', meta_value, NULL)) AS professionalism_rating, 
      MAX(IF(meta_key = 'contact-rating', meta_value, NULL)) AS contact_rating, 
      MAX(IF(meta_key = 'availability-rating', meta_value, NULL)) AS availability_rating, 
      MAX(IF(meta_key = 'responsiveness-rating', meta_value, NULL)) AS responsiveness_rating, 
      MAX(IF(meta_key = 'price-rating', meta_value, NULL)) AS price_rating 
   FROM 
      wpnf_commentmeta 
   GROUP BY comment_id) temp_t 
      ON t1.comment_post_ID = temp_t.comment_id2

Answer 1

Using MySQL pivot tables

SELECT 
      temp_t.user_id, t1.Name, t1.Email, t1.Phone, temp_t.location_review, temp_t.condition_review, temp_t.price_review 
    FROM 
      table_1st t1 
      INNER JOIN 
      (SELECT 
          UserID as user_id, 
          MAX(IF(meta_key = 'location_review', meta_value, NULL)) AS location_review, 
          MAX(IF(meta_key = 'condition_review', meta_value, NULL)) AS condition_review, 
          MAX(IF(meta_key = 'price_review', meta_value, NULL)) AS price_review 
       FROM 
          table_2nd 
       GROUP BY UserID) temp_t 
          ON t1.ID = temp_t.user_id