一、sql要掌握
1、sum
select sum(score) from table
2、group
select name, sum(score) from table group by
3、avg()
SELECT AVG(OrderPrice) AS OrderAverage FROM Orders
一、查询是否存在
1、查询数据是否存在
SELECT COUNT(*) FROM Table_name WHERE ID=xxx;
2、查找重复数据
select * from people where peopleId in (select peopleId from people group by peopleId having count (peopleId) > 1)
3、查找表中多余的重复记录（多个字段）
比如名字重复：
select name, count(name) as count from test_table group by name having count>1;
例子：查下图重复的student_no, 输入：
输入：select student_no, count(student_no) as count from score GROUP BY student_no having count>1;
4、查询平均分大于80；字段（name、score1、score2）
select name from test_table group by name having avg(score)>80
例子，在以下两个表格找，平均分大于80的student name
运行：select student_name from student where student_no = (
select student_no from score a GROUP BY student_no having AVG(score) > 80)
5、查询三门课都大于80 的学生；
SELECT S.name FROM Student S GROUP BY S.name Having MIN(S.score)>=80