SQL語句中case、when、then的使用
使用語法為：select `pro`.`id` AS `id`,`pro`.`title` AS `title`,`pro`.`name` AS `name`,`pro`.`ltime` AS `ltime`,`pro`.`place` AS `place`,`pro`.`linkman` AS `linkman`,`pro`.`address` AS `address`,`pro`.`tel` AS `tel`,`pro`.`email` AS `email`,`pro`.`adder` AS `adder`,`pro`.`addtime` AS `addtime`,`pro`.`remark` AS `remark`,`prep`.`addtime` AS `addtime1`,`preplog`.`addtime` AS `addtime2`,
(case
when (`prep`.`addtime` is not null) then '1'
when ((`preplog`.`addtime` is not null) and isnull(`prep`.`addtime`)) then '2'
else '0' end) AS `status`
from ((`pro` left join `prep` on((`pro`.`id` = `prep`.`pid`))) left join `preplog` on((`pro`.`id` = `preplog`.`pid`))) where (`pro`.`type` = '認領資訊')；
這裡主要注意的問題：
（1）as關鍵字主要是對欄位進行重新命名，可以忽略不寫；
（2）is not null 、is null、 isnull （）判斷欄位是否為空；
（3）(case
when then 1
when then 2
else 0 end ) as '';
end 關鍵字不能缺少，as後面是對條件查詢結果的重新命名，也可以忽略；
（4）left join on 實現多表的連線，where 關鍵字是對多表連結結果的條件篩選。