MySQL SELECT IF区分SUM

Question

My database structure looks as follows:

id    |    valuefor   |    quantity
1          alpha           500
2          alpha           100
3          beta            200

I am using following mysql command

SELECT IF(valuefor=alpha, SUM(quantity),0) as alpha
     , IF(valuefor=beta, SUM(quantity),0) as beta 
  FROM mytable

Current Output:

alpha   |   beta
800         0

The obvious issue with the above is that it SUMS all the rows since I have used SUM. How can I overcome this dilemma? I want to get quantity of alpha and beta separately.

Secondly I am confused as to why did the beta not have value=800 in the output and why does it say 0. This is just for my better understanding of the issue.

I want to result to look something as follows:

alpha    |    beta
600           200

Answer 1

Try this:

SELECT SUM(CASE WHEN valuefor=alpha THEN quantity ELSE 0 END) AS alpha,
       SUM(CASE WHEN valuefor=beta THEN quantity ELSE 0 END) AS beta
FROM mytable

Answer 2

You can use the case statement for that like so:

select
  sum(case valuefor when 'alpha' then quantity else 0 end) as alpha,
  sum(case valuefor when 'beta' then quantity else 0 end) as beta
from test;

Example:

http://sqlfiddle.com/#!9/f07bbf

create table test (
  id int,
  valuefor varchar(20),
  quantity int
);

insert into test values
(1, 'alpha', 500),
(2, 'alpha', 100),
(3, 'beta', 200);

Result

| alpha | beta |
|-------|------|
|   600 |  200 |