以validclick为分组，统计每个分组的记录数，和每个分组的totalclick总和
SELECT count(`id`),sum(`totalclick`), validclick FROM diary_log GROUP BY validclick;
count(`id`)
sum(`totalclick`)
validclick
配合if，可以模拟count()统计每个分组的记录数
SELECT count(`id`), sum(if(validclick = 1,1,0)) validclick_1,sum(if(validclick = 2,1,0)) validclick_2,validclick FROM diary_log GROUP BY validclick;
count(`id`)
validclick_1
validclick_2
validclick
对于count中应用if，则在第二第三个参数中只能赋值true和null：
SELECT COUNT(IF(`validclick`>2,TRUE,NULL)) AS `exceed_1_row_count` FROM diary_log;
错误的示范，这样做的话无论`validclick`是否大于2都会被计算：
SELECT COUNT(IF(`validclick`>2,1,0)) AS `exceed_1_row_count` FROM diary_log;
