MySQL JOIN with LIMIT 1 on joined table

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 tha开发者_JAVA百科t?

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 (this query also will work in PostgresQL and it is pretty fast, see my update below):

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
 )

PS. I did the performance test of the query vs other proposed here, and this query is the best option yet!

UPDATE (2022-07-20, PostgresSQL)

I'm not working with mySQL for a while already, so, I decided to test the performance of my solution (which actually works perfect in both MySQL and PostgresQL) with solution provided by @Gravy in PostgresQL v.12.9.

For that I decided to create a dummy tables and data with 100 categories and 100000 products. You can check the code on this gist

I run my query above and it took only 13ms to run.

After I slightly modified (for postgres) the query from @Gravy:

SELECT
  id,
  category_title,
  (array_agg(product_title))[1]  
FROM
    (SELECT c.id, c.title AS category_title, p.id AS product_id, p.title AS product_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, category_title;

and run it too. It took more than 150ms in my machine. Which is >10x times slower.

In defense of @gravy's solution, I agree with n+1 problem. But, in this particular case, usually the number of products is way larger than categories. So, running through each category is way less expensive than running through each product as in @Gravy's query.

By the way, if your table has 1mln products with 100 categories, the speed of my query is still the same (between 9-17ms), but the query from [@Gravy] takes more than 2 seconds to run

In resume, at this moment, my query is the most performant and optimal solution for the current task.

Feel free to comment.

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;

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)

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;

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);

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 = (                               
  SELECT MIN(p1.id) FROM products AS p1
  WHERE c.id=p1.category_id
 )

Using MIN or MAX in a subquery will make your query run much faster.

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

Another example with 3 nested tables: 1/ User 2/ UserRoleCompanie 3/ Companie

• 1 user has many UserRoleCompanie.
• 1 UserRoleCompanie has 1 user and 1 Company
• 1 Companie has many UserRoleCompanie

SELECT 
u.id as userId, 
u.firstName,
u.lastName,
u.email,
urc.id ,
urc.companieRole,
c.id as companieId,
c.name as companieName
FROM User as u 
JOIN UserRoleCompanie as urc ON u.id = urc.userId
    AND urc.id = (
        SELECT urc2.id
        FROM UserRoleCompanie urc2 
        JOIN Companie ON urc2.companieId = Companie.id
        AND urc2.userId = u.id 
        AND Companie.isPersonal = false
        order by Companie.createdAt DESC
        
        limit 1
    )
    
LEFT JOIN Companie as c ON urc.companieId = c.id

+---------------------------+-----------+--------------------+---------------------------+---------------------------+--------------+---------------------------+-------------------+
| userId                    | firstName | lastName           | email                     | id                        | companieRole | companieId                | companieName      |
+---------------------------+-----------+--------------------+---------------------------+---------------------------+--------------+---------------------------+-------------------+
| cjjt9s9iw037f0748raxmnnde | henry     | pierrot            | henry@gmail.com           | cjtuflye81dwt0748e4hnkiv0 | OWNER        | cjtuflye71dws0748r7vtuqmg | leclerc           |

In my opinion, this is the best answer (making it general):

SELECT  
TB1.Id  
FROM Table1 AS TB1  
INNER JOIN Table2 AS TB2 ON (TB1.Id = TB2.Id_TB1)  
    AND TB2.Id = (  
        SELECT Id  
        FROM Table2  
        WHERE TB1.Id = Id_TB1  
        ORDER BY Table2.Id DESC  
        LIMIT 1  
    )  

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

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

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