合并两个表（按日期排序）

Question

I have two tables posts and replies . What I'm currently doing is only displaying the 25 most recent posts on the main page of my website with the following query:

SELECT p.post_id
     , p.user_id
     , p.message
     , p.datetime
     , u.username 
  FROM posts p 
  LEFT 
  JOIN users u 
    ON p.user_id = u.user_id 
 ORDER 
    BY p.datetime DESC
 LIMIT 25

What I want to do is get the 25 most recent posts and replies (combined). For example, I want to output this on the main page:

User A posted "blah blah blah" on Feb. 3, 2014.
User B posted "yeah yeah yeah" on Feb. 2, 2014.
User A replied "this is a reply" on Feb. 2, 2014.
User C posted "some post" on Feb. 1, 2014.

How would I do this?

Answer 1

You can use a union query , then limit the results of that. I'm assuming your replies table has the same structure:

SELECT results.*, u.username FROM (
    SELECT p.post_id, p.user_id, p.message, p.datetime FROM posts p
    UNION    
    SELECT r.post_id, r.user_id, r.message, r.datetime FROM replies r 
) as results  
LEFT JOIN users u ON results.user_id = u.user_id
ORDER BY results.datetime DESC LIMIT 25

Answer 2

If you have same structure for replies table, you can use union like below

SELECT p.post_id, p.user_id, p.message, p.datetime AS postTime, u.username FROM posts p LEFT JOIN users u ON p.user_id = u.user_id 
UNION
SELECT r.reply_id, r.user_id, r.message, r.datetime AS postTime, u.username FROM replies r LEFT JOIN users u ON r.user_id = u.user_id
ORDER BY postTime DESC LIMIT 25