MySQL：只用一个sql select 语句查询有不及格科目的学生，不及格科目的数量，以及平均分（所有科目的平均分，分数分布在2张表中）。
要点：1、统计每个学生不及格可科目的个数。2、计算平均分。
create table students1(
id int,
name varchar(25),
course varchar(25),
score float
);
insert into students1 (id, name,course, score) values(1, 'zhangsan', 'math', 83);
insert into students1 (id, name,course, score) values(2, 'zhangsan', 'chinese', 54);
insert into students1 (id, name,course, score) values(3, 'zhangsan', 'eng', 59);
insert into students1 (id, name,course, score) values(4, 'lisi', 'math', 56);
insert into students1 (id, name,course, score) values(5, 'lisi', 'chinese', 57);
insert into students1 (id, name,course, score) values(6, 'lisi', 'eng', 50);
insert into students1 (id, name,course, score) values(7, 'wagnwu', 'math', 83);
insert into students1 (id, name,course, score) values(8, 'wagnwu', 'chinese', 100);
insert into students1 (id, name,course, score) values(9, 'wagnwu', 'chinese', 100);
create table students2(
id int,
name varchar(25),
course varchar(25),
score float
);
insert into students2 (id, name,course, score) values(1, 'zhangsan', 'physics', 60);
insert into students2 (id, name,course, score) values(2, 'zhangsan', 'chemistry', 49);
insert into students2 (id, name,course, score) values(3, 'zhangsan', 'biology', 90);
insert into students2 (id, name,course, score) values(4, 'lisi', 'physics', 59);
insert into students2 (id, name,course, score) values(5, 'lisi', 'biology', 60);
insert into students2 (id, name,course, score) values(6, 'lisi', 'biology', 100);
insert into students2 (id, name,course, score) values(7, 'wagnwu', 'physics', 90);
insert into students2 (id, name,course, score) values(8, 'wagnwu', 'biology', 45);
首先，将两个表的数据合并起来，使用union all，然后基于合并的数据根据学生名进行分组，使用聚合函数avg计算出平均分，然后使用case when来判断score不及格的数量（这里使用sum结合case when进行统计），使用having进行条件过滤，将全部含有不及格成绩的学生列出来。
SELECT name,avg(score), sum(case when score<60 then 1 else 0 end) FROM((SELECT * FROM students1) UNION ALL (SELECT * FROM students2 )) as a group by name having sum(case when score <60 then 1 else 0 end)>1;
+----------+--------------------+-------------------------------------------+
| name | avg(score) | sum(case when score<60 then 1 else 0 end) |
+----------+--------------------+-------------------------------------------+
| zhangsan | 65.83333333333333 | 3 |
| lisi | 63.666666666666664 | 4 |
+----------+--------------------+-------------------------------------------+