Question

I am new to sql and I really tried to solve this problem on my own, but no success…
Hope someone can help.

I have 3 tables:

Auth_user

+--------------+--------------+------+-----+---------+----------------+
| Field        | Type         | Null | Key | Default | Extra          |
+--------------+--------------+------+-----+---------+----------------+
| id           | int(11)      | NO   | PRI | NULL    | auto_increment |
| username     | varchar(30)  | NO   | UNI | NULL    |                |
| first_name   | varchar(30)  | NO   |     | NULL    |                |
| last_name    | varchar(30)  | NO   |     | NULL    |                |
| email        | varchar(75)  | NO   |     | NULL    |                |
| password     | varchar(128) | NO   |     | NULL    |                |
| is_staff     | tinyint(1)   | NO   |     | NULL    |                |
| is_active    | tinyint(1)   | NO   |     | NULL    |                |
| is_superuser | tinyint(1)   | NO   |     | NULL    |                |
| last_login   | datetime     | NO   |     | NULL    |                |
| date_joined  | datetime     | NO   |     | NULL    |                |
+--------------+--------------+------+-----+---------+----------------+

catalog_presents

+-------------+----------------------+------+-----+---------+----------------+
| Field       | Type                 | Null | Key | Default | Extra          |
+-------------+----------------------+------+-----+---------+----------------+
| id          | int(11)              | NO   | PRI | NULL    | auto_increment |
| name        | varchar(48)          | NO   |     | NULL    |                |
| slug        | varchar(50)          | NO   | UNI | NULL    |                |
| amount      | smallint(5) unsigned | NO   |     | 0       |                |
| points      | smallint(5) unsigned | NO   |     | 500     |                |
| created_at  | datetime             | NO   |     | NULL    |                |
| updated_at  | datetime             | NO   |     | NULL    |                |
| active      | tinyint(1)           | NO   |     | NULL    |                |
| image       | varchar(100)         | YES  |     | NULL    |                |
| description | longtext             | YES  |     | NULL    |                |
+-------------+----------------------+------+-----+---------+----------------+

catalog_orders

+------------+----------------------+------+-----+---------+----------------+
| Field      | Type                 | Null | Key | Default | Extra          |
+------------+----------------------+------+-----+---------+----------------+
| id         | int(11)              | NO   | PRI | NULL    | auto_increment |
| user_id    | int(11)              | NO   | MUL | NULL    |                |
| status     | smallint(5) unsigned | NO   |     | 0       |                |
| present_id | int(11)              | NO   | MUL | NULL    |                |
| address_id | int(11)              | NO   | MUL | NULL    |                |
| created_at | datetime             | NO   |     | NULL    |                |
| updated_at | datetime             | NO   |     | NULL    |                |
+------------+----------------------+------+-----+---------+----------------+

I am trying to select a user and check whether he ordered a particular present and write to the row accordingly, then check another one, and another one and so on.

Basically I would like the results to look like this:

id present1 present2 present3 present4
1 1 0 0
2 1 1 1
3 0 0 0
4 1 0 1

where 1 is user placed an order on that type of present and 0 is he didn’t.

My query looks like this

select auth_user.id, case when present_id = 1 from auth_user 
left join catalog_orders on catalog_orders.user_id = auth_user.id 
left join catalog_presents on catalog_presents.id = catalog_orders.present_id

The problem is that all different orders by users are sorted in multiple rows like this:

id present1 present2 present3 present4
1 1 0 0
2 1 0 0
2 0 1 0
2 0 0 1
3 0 0 0
4 1 0 0
4 0 0 1

Can somebody please help me solve this? Thanks in advance!

Answer 1

This is a kind of pivot table.

You are on the right path, but you need to combine your results down to a single row per user.

Use an aggregate MAX() per auth_user.id to collapse them into a single row. The CASE supplies a zero or one for each present, and the MAX() then selects the greatest value of all rows for the auth_user.id, which will be 1 if the present was bought and 0 otherwise.

SELECT
  auth_user.id,
  MAX(CASE WHEN present_id = 1 THEN 1 ELSE 0 END) AS present1,
  MAX(CASE WHEN present_id = 2 THEN 1 ELSE 0 END) AS present2,
  MAX(CASE WHEN present_id = 3 THEN 1 ELSE 0 END) AS present3,
  MAX(CASE WHEN present_id = 4 THEN 1 ELSE 0 END) AS present4
FROM
   auth_user
   LEFT JOIN catalog_orders on catalog_orders.user_id = auth_user.id 
   LEFT JOIN catalog_presents on catalog_presents.id = catalog_orders.present_id
GROUP BY auth_user.id
ORDER BY auth_user.id

Specifically for MySQL, you don’t need the CASE since the boolean comparison present_id = 1 will return a 1 or 0 on its own. This is not portable to all other RDBMS though. The method above is preferred.

SELECT
  auth_user.id,
  /* MySQL ok, not all other RDBMS will do this - boolean comparison returns 1 or 0 */
  MAX(present_id = 1) AS present1,
  MAX(present_id = 2) AS present2,
  MAX(present_id = 3) AS present3,
  MAX(present_id = 4) AS present4
FROM
   auth_user
   LEFT JOIN catalog_orders on catalog_orders.user_id = auth_user.id 
   LEFT JOIN catalog_presents on catalog_presents.id = catalog_orders.present_id
GROUP BY auth_user.id
ORDER BY auth_user.id