Question

I have trouble explaining my problem in words. so thats why my table examples:

let say i want a result table like:

user_id |data_a |data_b
1       |hello  |world

and i need to extract it out of the table “dataset” in this form

data_id |user_id|data_type  |data_data
1       |1      |data_a     |hello
2       |1      |data_c     |xxx
3       |2      |data_a     |xxx
4       |1      |data_b     |world

I do somehting like

SELECT user_id , data_a, data_b
FROM dataset
WHERE ( data_type = data_a OR data_type = data_b )
AND user_id = 1

AND a = ( SELECT data_data WHERE data_type = data_a )
AND b = ( SELECT data_data WHERE data_type = data_b )

however it is not working. i know i am doing it wrong but i dont know how to do it and how it is called. I think a privot table but i am not sure
Thats why the description is so vague i hope someone understands.

Answer 1

This is basically a PIVOT but MySQL does not have a PIVOT function so you can replicate it using an aggregate function and a CASE statement. So your SQL will be something like this:

select user_id,
  max(case when data_type = 'data_a' then data_data end) data_a,
  max(case when data_type = 'data_b' then data_data end) data_b
from dataset
group by user_id

See SQL Fiddle with Demo

Result:

| USER_ID | DATA_A | DATA_B |
-----------------------------
|       1 |  hello |  world |
|       2 |    xxx | (null) |