MySQL-计数和案例

Question

My google and stockoverflow searches failed me so need your help. I have a below table

+------+------+------+------------+
| id   | Loc  | MEL  | COUNT(MEL) |
+------+------+------+------------+
|    1 | AAA  | A    |          1 |
|    2 | BBB  | B    |          1 |
|    3 | CCC  | C    |          1 |
|    4 | AAA  | D    |          1 |
|    5 | AAA  | A    |          1 |
|    6 | BBB  | B    |          1 |
|    7 | BBB  | C    |          1 |
|    8 | AAA  | D    |          1 |
+------+------+------+------------+

I want to transform it to the following table:

+------+------+------+------+
| MEL  | AAA  | BBB  | CCC  |
+------+------+------+------+
| A    |  2   |      |      |
| B    |      |   2  |      |
| C    |      |   1  |   1  |
| D    |  2   |      |   1  |
+------+------+------+------+

All conbinations of COUNT and CASE WHEN did not work?

Code to get the below tables:

CREATE TABLE Orders
(
id INT,
Loc char(255),
MEL char (10))

insert into Orders values 
(1,   "AAA", "A"),
(2,   "BBB", "B"),
(3,   "CCC", "C"),
(4,   "AAA", "D"),
(5,   "AAA", "A"),
(6,   "BBB", "B"),
(7,   "BBB", "C"),
(8,   "AAA", "D");

Answer 1

select mel, 
sum(Loc="AAA") AS AAA, 
sum(Loc="BBB") AS BBB /* and so on */
from Orders group by mel;

You need to use sum instead of count , because the boolean expression returns 0 or 1. count doesn't care about that, it just counts, whether it's 0 or 1 doesn't matter.