在MySQL中枢转一张表格

Question

I'm currently running into a problem where I am required to pivot many rows into a single row, defined by a specific key. I know the GROUP_CONCAT functionality and am using that currently, but I would like to avoid having to use explode in PHP after fetching my results. My table looks like this:

----------------------------------
| primary_id | key_value | value |
----------------------------------
| 1          | my_key_1  | val_1 |
| 1          | my_key_2  | val_2 |
| 1          | my_key_3  | val_3 |
| 2          | my_key_4  | val_4 |
| 2          | my_key_5  | val_5 |
----------------------------------

And I would like to build a MySQL-Query that presents this exactly like this for primary id 1 and primary id 2:

-----------------------------------------------
| primary_id | my_key_1 | my_key_2 | my_key_3 |
-----------------------------------------------
| 1          | val_1    | val_2    | val_3    |
-----------------------------------------------

------------------------------------
| primary_id | my_key_4 | my_key_5 |
------------------------------------
| 2          | val_4    | val_5    |
------------------------------------

So I can retrieve the output as an array in PHP, with the form:

$output[1][my_key_1] = val_1
$output[1][my_key_2] = val_2
...

The GROUP_CONCAT functionality works, but it would be much nicer to just have the value in the form I needd it only using SQL.

Would appreciate any pointers, best regards.

Answer 1

Use a Pivot

select max(case (when key_value=my_key_1 then val_1 end)), max(case (when key_value=my_key_2 then val_2 end)), from... etc...