SQL??? ?? ??? "Case When"?? ?????. ?? ??? ??? ?? ??? ? ?? ?? ??? "CASE ?? WHEN ?? 1 THEN ?? 1 WHEN"? ?? ?? ??? ?????. ?? 2 THEN ?? 2...ELSE ?? n END;".

? ????? ?? ??: Windows 7 ???, Microsoft SQL Server 2016 ??, Dell G3 ???.

SQL

case when? ???? ????? ??? if else ?? ? switch case ?? ?????. ? ???? ???? ?? ??? ??? ? ?? ??? ?? ?? ??? ?????.

Case?? ?? ?? ??? ?? ?? ??? ? ?? ??? ????.

Simple Case ??:

CASE sex
WHEN ‘1’ THEN ‘男’
WHEN ‘0’ THEN ‘女’
ELSE ‘其他’ END

Case ?? ??:

CASE WHEN sex = ‘1’ THEN ‘男’
WHEN sex = ‘0’ THEN ‘女’
ELSE ‘其他’ END

?? ?? Case ??? ???? ? ??? ?? ??? ?? ?? ?? ? ???? ????, ?? ?? ??? ?? ?? ??? ????? ????.

? ???? ? ??? ????. Case ??? ?? ??? ???? ??? ??? ???? ?????. ??? ?? ??? ?????? ?? ??? ? ?? ??? ?????.

(PHP ??? ?????? ?? SQL ????? ?? ????. ??? ?? ? ????!)

CASE WHEN? ???? ??? ?? ?? ?? ???? ??? ?? ??? ??? ? ????. ??? ? ?????. ?? ??? ??? ? ??? ???? ? as? ??? ? ????. ?? ??? ?? ??????. ?? CASE WHEN? ?? ??? ?????.

?? ????

1. ??? ???? ?? ???? ????? ??? ? ????.

? ??? ?? ??? ??? ???? ??? ??? ?????. ?? SQL? ??????.

SELECT  CASE country
WHEN '中國'     THEN '亞洲'
WHEN '印度'     THEN '亞洲'
WHEN '日本'     THEN '亞洲'
WHEN '美國'     THEN '北美洲'
WHEN '加拿大'  THEN '北美洲'
WHEN '墨西哥'  THEN '北美洲'
ELSE '其他' END as '洲' , SUM(population) as '人口'
FROM test
GROUP BY CASE country
WHEN '中國'     THEN '亞洲'
WHEN '印度'     THEN '亞洲'
WHEN '日本'     THEN '亞洲'
WHEN '美國'     THEN '北美洲'
WHEN '加拿大'  THEN '北美洲'
WHEN '墨西哥'  THEN '北美洲'
ELSE '其他' END;

??? ? CASE WHEN? ??? ??? ????? ? ?? CASE WHEN? THEN ?? ??? ?? ???? ??? ??? ??? ?? ??? ???? ?????? THEN ?? ?? ? ? ?? ??? ???? ??? ? ????. GROUP BY? ??? ?? ? ?? ????.

GROUP BY CASE country
WHEN '中國'     THEN 0
WHEN '印度'     THEN 0
WHEN '日本'     THEN 0
WHEN '美國'     THEN 1
WHEN '加拿大'  THEN 1
WHEN '墨西哥'  THEN 1
ELSE 2 END;

2. ??? SQL ?? ???? ??? ??? ???? ?????.

?? ???? ????.

??? ??? ???? ?? ? ???? ???? ?????. ?? SQL? ??????.

SELECT country,
SUM( CASE WHEN sex = '1' THEN population ELSE 0 END  ),  --男性人口
SUM( CASE WHEN sex = '2' THEN population ELSE 0 END )   --女性人口
FROM  Table_A
GROUP BY country;

?? ??? ?????.

? ?? CASE WHEN? ??????.

CASE WHEN sex = '1' THEN
population ELSE 0 END

??? ??? 1? ?? ? ??? ?? ?? ??? ?? ????. ??? ??? 0??? ? ??? ?? ??? ??? ? ????.

? ?? SQL ? ?? ????? ?????? ??? PHP ??? ????? ?? ??? ?????! !? ?

? ??? SQL?? Case ??? ?? ?????. ??? ??? PHP ??? ????? ?? ?? ??? ?????!