在 mysql 中if()函数的用法类似于 java 中的三目表达式，其用处也比较多，具体语法如下：
IF(expr1,expr2,expr3)，如果expr1的值为true，则返回expr2的值，如果expr1的值为false，
则返回expr3的值。
其经常判断查询出来的值，示例:
mysql> select name,if(sex=0,'女','男') as sex from student;
+-------+-----+
| name | sex |
+-------+-----+
| name1 | 女 |
| name2 | 女 |
| name3 | 男 |
| name4 | 女 |
+-------+-----+
4 rows in set (0.00 sec)
其也经常用到判断的关联条件中，其示例如下：
SELECT s.SCHOOL_CITY as schoolCity,
s.SCHOOL_COUNTY as schoolCounty,
count(DISTINCT `s`.`SCHOOL_ID`) as schoolNum,
sum(m.duration) as sumDuration,
sum(`m`.`VIEWERCOUNT`) as viewLiveSum,
sum(m.replayViewerCount)as reViewSum,
sum(m.praisecount) as sumpraise,
sum(`m`.`VIEWERCOUNT`+m.replayViewerCount) as viewSum
from ((tbl_hbb_mobile_live_statistics m join tbl_hbb_resource_visit_map v) join tbl_school_info s)
where`m`.`RESOURCEID`= `v`.`RESOURCEID`
and if((`v`.`rangeTYPE`= '2'),(`v`.`rangeID`= `s`.`SCHOOL_ID`),(`v`.`parentId`= `s`.`SCHOOL_ID`))
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