Multiple similar SELECTs in one statement - mysql

Question

I have these SELECT statements in SQL:
this:
SELECT
*
FROM
products
WHERE
product_category = '12'
LIMIT
3
and this:
SELECT
*
FROM
products
WHERE
product_category = '36'
LIMIT
3
and this:
SELECT
*
FROM
products
WHERE
product_id IN ('3178','3181','7403')
LIMIT
3
As you can see they are very similar, what I want is to run these 3 statement effectively, point of that is that whole result should be 9 rows long (because 3x3), and firstly should be displayed three products from category 12, then second three products should be displayed from category 36 and the last three products should be products with IDs 3178,3181,7403.
I know that I can use UNION like this:
(SELECT
*
FROM
products
WHERE
product_category = '12'
LIMIT
3)
UNION
(SELECT
*
FROM
products
WHERE
product_category = '36'
LIMIT
3)
UNION
(SELECT
*
FROM
products
WHERE
product_id IN ('3178','3181','7403')
LIMIT
3)
LIMIT 9
But I wonder, if there is more effective way, because these statements are mostly copies.

Answer 1

for mysql version 8.0 or greater and you can reduce one subquery
select col1,col2,col3 from (SELECT *, row_number() over(partition by product_category order by product_category) rn
FROM
products
WHERE
product_category in( 12,36)
) a where a.rn<=3
union
SELECT
col1,col2,col3
FROM
products
WHERE
product_id IN ('3178','3181','7403') order by product_id
LIMIT
3

Answer 2

In MySQL 8+, you can use window functions:
SELECT p.*
FROM (SELECT p.*,
ROW_NUMBER() OVER (PARTITION BY product_category,
product_id IN (3178, 3181, 7403)
ORDER BY product_category
) as seqnum
FROM products p
WHERE product_category IN (12, 36) OR
product_id IN (3178, 3181, 7403)
) p
WHERE seqnum <= 3
ORDER BY (product_category = 12) DESC,
(product_category = 36) DESC;

Answer 3

UNION ALL
UNION alone will try to merge similar records as one. With UNION ALL, will show everything.

Answer 4

Perhaps if you really want to make this more efficient you have to look at it a slightly different way and not limit yourself to the query, but think about updating your database structure.
For example, you could add a new column in products table to handle the group order.
| product_id | product_category | product_group |
|------------|------------------|---------------|
| 3178 | | c |
| | 12 | a |
| | 36 | b |
| 3181 | | c |
| 7403 | | c |
And get the result with a simple query :
SELECT * from products
WHERE product_group in (a, b, c)
ORDER BY product_group
Depending on why you're limiting to just 3 results per group, it probably makes more sense anyway to use code to manage which of the entries you want to return (so having some code to set the product_group field), not simply the 3 first entries from your table, in no specific order.