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

What about this?

SELECT c.id, c.title, (SELECT id from products AS p 
                            WHERE c.id = p.category_id 
                            ORDER BY ... 
                            LIMIT 1)
   FROM categories AS c;

Answer 2

Assuming you want product with MIN()imial value in sort column, it would look something like this.

SELECT 
  c.id, c.title, p.id AS product_id, p.title
FROM 
  categories AS c
INNER JOIN (
  SELECT
    p.id, p.category_id, p.title
  FROM
    products AS p
  CROSS JOIN (
    SELECT p.category_id, MIN(sort) AS sort
    FROM products
    GROUP BY category_id
  ) AS sq USING (category_id)
) AS p ON c.id = p.category_id

Answer 3

I like more another approach described in a similar question: https://stackoverflow.com/a/11885521/2215679

This approach is better especially in case if you need to show more than one field in SELECT. To avoid Error Code: 1241. Operand should contain 1 column(s) or double sub-select for each column.

For your situation the Query should looks like:

SELECT
 c.id,
 c.title,
 p.id AS product_id,
 p.title AS product_title
FROM categories AS c
JOIN products AS p ON
 p.id = (                                 --- the PRIMARY KEY
  SELECT p1.id FROM products AS p1
  WHERE c.id=p1.category_id
  ORDER BY p1.id LIMIT 1
 )