Mysql之case语句（附带实例）

这段时间，做项目做累了，好不容易有点个人的学习时间，利用这个小时，总结一下，最近做统计的时候常用的case语句吧。

结构：case  when… then …end

1.判断的同时改变其值

eg:

                    select OperatorAccount,

                            case

                                         when CreateTime>‘2016-02-14 16:24:42‘ then ‘after‘

                                         when CreateTime<‘2016-02-14 16:24:42‘ then ‘before‘

                                         else ‘now‘ end stage

from log_login order by CreateTime DESC

第二种写法

                    SELECT CallerNumber, CASE IsLocal

                                         WHEN 0 THEN ‘外线‘

                                         WHEN 1 THEN ‘内线‘ END

                    FROM cdr

2.拆分一行为多列

eg:

                    SELECT SipAccount, COUNT(1) AS number,IsCheck

                    FROM cdr

                    GROUP BY SipAccount,IsCheck

针对这个统计结果进行拆分（0表示未打分，1代表优秀，2代表合格，3代表不合格）

最终结果如下：

所以最终要用到行拆分成三列，语句如下

                    SELECT SipAccount,

                    (CASE IsCheck WHEN 1 THEN number END) youxiu,

                    (CASE IsCheck WHEN 2 THEN number END) hege,

                    (CASE IsCheck WHEN 3 THEN number END) buhege

                    FROM

                    (SELECT SipAccount, COUNT(1) AS number,IsCheck

                    FROM cdr

                    GROUP BY SipAccount,IsCheck) AS a

现在结果是这样的，你会发现虽然拆成了三列，但是最终结果还不是自己需要的，接下来就需要根据sipaccount来分组的同时对结果进行处理了。语句如下：

                    SELECT sipaccount,

                                         IF(MAX(youxiu) IS NULL,0, MAX(youxiu)) youxiu,

                                         IF(MAX(hege) IS NULL,0, MAX(hege)) hege,

                                         IF(MAX(buhege) IS NULL,0, MAX(buhege)) buhege

                    FROM

                    (SELECT SipAccount,

                                         (CASE IsCheck WHEN 1 THEN number END) youxiu,

                                         (CASE IsCheck WHEN 2 THEN number END) hege,

                                         (CASE IsCheck WHEN 3 THEN number END) buhege

                    FROM

  (SELECT SipAccount, COUNT(1) AS number,IsCheck  FROM cdr  GROUP BY SipAccount,IsCheck) AS a) AS b

                    GROUP BY sipaccount

最终得到了这个结果。正式我们需要的格式