SELECT query where LIMIT is a distinct count of repeating key - mysql

Question

I have a problem with selecting specific amount of data. The problem is that one of the keys have the same repeated value.
--------------------
| id | name | key |
--------------------
| 1 | alfa | a |
| 2 | alfa | b |
| 3 | alfa | c |
| 4 | beal | a |
| 5 | beal | b |
| 6 | gala | c |
| 7 | gala | d |
| 8 | delt | a |
| 9 | ceta | a |
--------------------
In this situation I want to select three individual names. For example I want to limit distinct name to 3 positions to get this result:
SAMPLE DUMP CODE:
SELECT * in Table
WHERE `name` LIKE '%al%'
LIMIT BY DISTINCT
`name`, 3
------ RESULT ------
| 1 | alfa | a |
| 2 | alfa | b |
| 3 | alfa | c |
| 4 | beal | a |
| 5 | beal | b |
| 6 | gala | c |
| 7 | gala | d |
--------------------
I will be glad for help.

Answer 1

Without window functions:
select *
from (
select distinct name
from mytable
where `name` like '%al%'
order by name
limit 3
) n
natural join mytable
db-fiddle
If you don't like NATURAL JOINs you can also use
select t.*
from (
select distinct name
from mytable
where `name` like '%al%'
order by name
limit 3
) n
join mytable t on t.name = n.name
If window functions are supported, you can use DENSE_RANK():
with cte as (
select *,
dense_rank() over (order by name) as dr
from mytable
where `name` like '%al%'
)
select id, name, `key`
from cte
where dr <= 3
db-fiddle
I prefer the LIMIT 3 subquery, since it can stop the index scan (depending on optimizer) after three distinct names are found.

Answer 2

MySQL 8.0 solution utilizing Window functions is as follows:
SELECT
dt.id, dt.name, dt.`key`
FROM
(
SELECT
ROW_NUMBER() OVER (PARTITION BY name ORDER BY id) AS rn,
id,
name,
`key`
FROM your_table_name
WHERE name LIKE '%al%'
) AS dt
WHERE dt.rn <= 3
ORDER BY dt.id
Explanation:
In a Derived table (subquery), determine Row_Number() within a partition (group) of specific name, ordered by id in ascending order. We will consider only names matching %al% condition.
Now, use the subquery result to SELECT only the rows having row number upto 3 (basically limiting to 3 rows per name).
By the way, key is a Reserved Keyword in MySQL. You should consider renaming column to something else; otherwise you will need to use backticks around it.
Result
| id | name | key |
| --- | ---- | --- |
| 1 | alfa | a |
| 2 | alfa | b |
| 3 | alfa | c |
| 4 | beal | a |
| 5 | beal | b |
| 6 | gala | c |
| 7 | gala | d |
View on DB Fiddle