MySQL JOIN with LIMIT 1 on joined table

Question

I want to join two tables, but only get 1 record of table2 per record on table1

For example:

SELECT c.id, c.title, p.id AS product_id, p.title
FROM categories AS c
JOIN products AS p ON c.id = p.category_id

This would get me all records in products, which is not what I want. I want 1 [the first] product per category (I have a sort column in the products field).

How do I go about doing that?

Answer 1

The With clause would do the trick. Something like this:

WITH SELECTION AS (SELECT id FROM products LIMIT 1)
SELECT a.id, c.id, c.title FROM selection a JOIN categories c ON (c.id = a.id);

Answer 2

I would try something like this:

SELECT C.*,
      (SELECT P.id, P.title 
       FROM products as P
       WHERE P.category_id = C.id
       LIMIT 1)
FROM categories C

Answer 3

When using postgres you can use the DISTINCT ON syntex to limit the number of columns returned from either table.

Here is a sample of the code:

SELECT c.id, c.title, p.id AS product_id, p.title FROM categories AS c JOIN ( SELECT DISTINCT ON(p1.id) id, p1.title, p1.category_id FROM products p1 ) p ON (c.id = p.category_id)
The trick is not to join directly on the table with multiple occurrences of the id, rather, first create a table with only a single occurrence for each id

Answer 4

Accepted answer by @goggin13 looks wrong. Other solutions provided to-date will work, but suffer from the n+1 problem and as such, suffer a performance hit.

n+1 problem: If there are 100 categories, then we would have to do 1 select to get the categories, then for each of the 100 categories returned, we would need to do a select to get the products in that category. So 101 SELECT queries would be performed.

My alternative solution solves the n+1 problem and consequently should be significantly more performant as only 2 selects are being performed.

SELECT
  *
FROM
    (SELECT c.id, c.title, p.id AS product_id, p.title
    FROM categories AS c
    JOIN products AS p ON c.id = p.category_id
    ORDER BY c.id ASC) AS a 
GROUP BY id;

Answer 5

SELECT c.id, c.title, p.id AS product_id, p.title
FROM categories AS c
JOIN products AS p ON c.id = p.category_id
GROUP BY c.id

This will return the first data in products (equals limit 1)

Answer 6

Replace the tables with yours:

SELECT * FROM works w 
LEFT JOIN 
(SELECT photoPath, photoUrl, videoUrl FROM workmedias LIMIT 1) AS wm ON wm.idWork = w.idWork