mysql----case when then

mysql中的case when then 語句類似於其他編程中的if/else if/else。基本語法如下：

case
	when...then...
	when...then...
	when...then...
	else
end

比賽年份和結果表
create table scores
(
	date_year int,
	win_loss varchar(5)
);


數據
insert into scores
value
(2005,'loss'),(2006,'win'),(2007,'win'),(2008,'loss'),(2009,'loss'),(2010,'tie'),
(2005,'tie'),(2006,'win'),(2007,'loss'),(2008,'loss'),(2009,'win'),(2010,'tie');


輸出結果
select date_year,
	case
		when win_loss ='loss' then '輸'
		when win_loss ='win' then '贏'
		when win_loss ='tie' then '平'
	end as result
from scores;




mysql> select date_year,
    -> case
    -> when win_loss ='loss' then '輸'
    -> when win_loss ='win' then '贏'
    -> when win_loss ='tie' then '平'
    -> end as result
    -> from scores;
+-----------+--------+
| date_year | result |
+-----------+--------+
|      2005 | 輸     |
|      2006 | 贏     |
|      2007 | 贏     |
|      2008 | 輸     |
|      2009 | 輸     |
|      2010 | 平     |
|      2005 | 平     |
|      2006 | 贏     |
|      2007 | 輸     |
|      2008 | 輸     |
|      2009 | 贏     |
|      2010 | 平     |
+-----------+--------+
12 rows in set (0.00 sec)



case when then 中使用聚合函數。
假如贏一場得3分，輸一場-1分，平一場0分。如何得到每年的積分。
select date_year,
	sum(case
		when win_loss ='loss' then -1
		when win_loss ='win' then 3
		when win_loss ='tie' then 0
	end) all_score
from scores
group by date_year;


mysql> select date_year,
    -> sum(case
    -> when win_loss ='loss' then -1
    -> when win_loss ='win' then 3
    -> when win_loss ='tie' then 0
    -> end) all_score
    -> from scores
    -> group by date_year;
+-----------+-----------+
| date_year | all_score |
+-----------+-----------+
|      2005 |        -1 |
|      2006 |         6 |
|      2007 |         2 |
|      2008 |        -2 |
|      2009 |         2 |
|      2010 |         0 |
+-----------+-----------+
6 rows in set (0.00 sec)