How to select data given several rows match? - mysql

Question

I have a configuration table and a users table.
users:
| id | name |
|----|----------|
| 0 | Bob |
| 1 | Ted |
| 2 | Sam |
config:
| user_id | name | value |
|---------|----------|-------|
| 0 | a | 11 |
| 0 | b | 2 |
| 0 | c | 54 |
| 1 | a | 5 |
| 1 | b | 3 |
| 1 | c | 0 |
| 2 | a | 1 |
| 2 | b | 74 |
| 2 | c | 54 |
I normalized the configuration this way since the config will be of unknown amount, but I will have to query users based on this config, so it couldn't be stored in a serialized form.
My issue is how do I find users based on multiple rows? For instance:
Select all users with a > 4 and b < 5
This should return Bob and Ted.

Answer 1

Using groups:
SELECT u.name
FROM users u
JOIN config c
ON c.user_id = u.id
GROUP BY u.name
HAVING MAX(c.name = 'a' AND c.value > 4)
AND MAX(c.name = 'b' AND c.value < 5)
Using joins:
SELECT u.name
FROM users u
JOIN config a
ON a.user_id = u.id
AND a.name = 'a'
AND a.value > 4
JOIN config b
ON b.user_id = u.id
AND b.name = 'b'
AND b.value < 5
I prefer the JOIN method, as you can name each JOIN after the property and collect the conditions in the JOIN. You also don't have to worry about the GROUPs which makes it more flexible for aggregates.
A bonus over EXISTS is that you can easily access the properties of the config if you require further joins/calculations.

Answer 2

Try this:
SELECT us.name
FROM USERS us
WHERE EXISTS (SELECT name FROM CONFIG WHERE name='a' AND value>4 AND user_id=us.id)
AND EXISTS (SELECT name FROM CONFIG WHERE name='b' AND value<5 AND user_id=us.id)
Alternatively, you can use two joins:
SELECT us.name
FROM USERS us, CONFIG c1, CONFIG c2
WHERE us.id=c1.user_id
AND c1.name='a'
AND c1.value<4
AND us.id=c2.user_id
AND c2.name='b'
AND c2.value>5

Answer 3

select u.name from users as u inner join config as c on u.id = c.user_id where (c.name = "a" and c.value > 4) or (c.name = "b" and c.value < 5);

Answer 4

you will need two exists statements for this:
SELECT * FROM users u
WHERE EXISTS ( SELECT 1
FROM config
WHERE user_id = u.id
AND name = 'a' AND value > 4)
AND EXISTS ( SELECT 1
FROM config
WHERE user_id = u.id
AND name = 'b' AND value < 5)

Answer 5

the following query achieves the result, as does not rely joining between the tables in any way.
to include more of "name" you could add another OR clause inside the HAVING.
NOTE: This has been tested on the W3Schools WebSQL Interface
SELECT
value,
name,
user_id
FROM config
GROUP BY value, name, user_id
HAVING
(
(value > 4 AND name = 'a')
OR (value < 5 AND name = 'b')
)
After ditching the grouping (thanks Arth)
SELECT
value,
name,
user_id
FROM config
WHERE
(
(value > 4 AND name = 'a')
OR (value < 5 AND name = 'b')
)