我应该在这个MYSQL查询中使用JOIN或UNION吗？

Question

I am trying to display data from two different tables ordered by the time the data was created.

Here are the 2 tables I am pulling data from:

active_projects

+------------+------------+---------+-------+-------------+
| project_id | ts_created | user_id | title | description |
+------------+------------+---------+-------+-------------+

likes

+---------+------------+------------+
| user_id | project_id | ts_created |
+---------+------------+------------+

The desired result is all of the data retrieved from these columns displayed properly, ordered by this column:

+------------+
| ts_created |
+------------+

This is the query that I am currently using:

$sql = "SELECT *
        FROM (SELECT project_id AS ppid, 
                     ts_created AS pts,
                     user_id AS puid,
                     title AS ptitle,
                     description AS pdesc
              FROM active_projects 
              WHERE user_id = 12)
        UNION
             (SELECT id AS lid, 
                     user_id AS luid,
                     project_id AS lpid,
                     ts_created AS lts
              FROM likes 
              WHERE user_id = 12)";

But, it's not working the way I want to. I've been trying to debug what's going on but right now it's just saying that it's not actually retrieving any of the data.

I have been researching JOIN and UNION but am not sure which one I should be using for this query. Any assistance is greatly appreciated.

EDIT: The query is now displaying properly using @Samuel's answer. But, now the results are being displayed 3 times per row. I am doing this at the end of the query:

$act = mysql_query($sql);

Then, when I echo out the results I am using a while loop likes this:

while ($t = mysql_fetch_row($act)) {
    echo $t[1];
}

Any ideas as to why each row is being displayed 3 times?

Answer 1

Your query is quite awful in terms of runtime performance and it does not produce the result you wish. Probably more useful:

$sql = "
    SELECT * FROM active_projects
    LEFT JOIN likes ON (active_projects.user_id = likes.user_id)
    WHERE user_id = 12";

Untested, but something like that is what you need. This should create a result of all entries from active_project for the user_id 12 left joined with the table likes.

Answer 2

First your query has a problem. You can't do select * from (select ...) . there should be 'where' clause before calling another sub query.

You should use join instead of union as union is used to show results for tables with the same column names/types (aliases) while join uses a foreign key from the other table to show a 'joined' output.

Do the following:

SELECT A.project_id AS ppid, 
       A.ts_created AS pts,
       A.user_id AS puid,
       A.title AS ptitle,
       A.description AS pdesc,
       B.id AS lid, 
       B.user_id AS luid,
       B.project_id AS lpid,
       B.ts_created AS lts
FROM active_projects A
LEFT JOIN likes B ON
       A.user_id = B.user_id
WHERE A.user_id = 12 ORDER BY B.ts_created;

Actually, you can choose between, RIGHT JOIN and LEFT JOIN.