MySQL 程序式写法

1. IF
if (condition, result1, result2)
IF(expr1 is not null, expr1, expr2) 等价于 ifnull(expr1, expr2)
   ifnull()效率胜过if()

2. Case
case
when day(MaxDate)<=10 then 1
when day(MaxDate)<=20 then 2
else 3
end