SQL语句高级（七）——复制表a、b

一、复制表数据做条件查询
mysql> select cno,avg(degree) from score group by cno;
+--------+-------------+
| cno    | avg(degree) |
+--------+-------------+
| 3-105  |     71.6667 |
| 3-245  |     89.2000 |
| 6-1166 |     74.0000 |
+--------+-------------+
3 rows in set (0.21 sec)

mysql> select * from score;
		  a表                                  b表
+-----+--------+--------+           +-----+--------+--------+
| sno | cno    | degree |           | sno | cno    | degree |
+-----+--------+--------+           +-----+--------+--------+
| 100 | 3-105  |     75 |           | 100 | 3-105  |     75 |
| 100 | 3-245  |     75 |           | 100 | 3-245  |     75 |
| 101 | 6-1166 |     59 |           | 101 | 6-1166 |     59 |
| 102 | 3-245  |     95 |           | 102 | 3-245  |     95 |
| 103 | 3-105  |     81 |           | 103 | 3-105  |     81 |
| 103 | 3-245  |     89 |           | 103 | 3-245  |     89 |
| 103 | 6-1166 |     59 |           | 103 | 6-1166 |     59 |
| 104 | 3-105  |     59 |           | 104 | 3-105  |     59 |
| 104 | 3-245  |     98 |           | 104 | 3-245  |     98 |
| 104 | 6-1166 |     89 |           | 104 | 6-1166 |     89 |
| 105 | 3-245  |     89 |           | 105 | 3-245  |     89 |
| 105 | 6-1166 |     89 |           | 105 | 6-1166 |     89 |
+-----+--------+--------+           +-----+--------+--------+
12 rows in set (0.00 sec)

mysql> select * from score a --这里的a表和b表都是score表！！！！ 
	-> where degree < (select avg(degree) from score b where a.cno = b.cno);
+-----+--------+--------+
| sno | cno    | degree |
+-----+--------+--------+
| 100 | 3-245  |     75 |
| 101 | 6-1166 |     59 |
| 103 | 3-245  |     89 |
| 103 | 6-1166 |     59 |
| 104 | 3-105  |     59 |
| 105 | 3-245  |     89 |
+-----+--------+--------+

二、查询所有任课教师的tname和depart。
即课程表中要有对应课程！
mysql> select * from teacher;
+-----+-------+-------+---------------------+------------+--------+
| tno | tname | tsex  | tbirthday           | prof       | depart |
+-----+-------+-------+---------------------+------------+--------+
| 804 | aaa   | man   | 1986-12-10 00:00:00 | fujiaoshou | cs     |
| 811 | ddd   | man   | 1984-05-05 00:00:00 | zhujiao    | dianzi |
| 825 | ccc   | man   | 1966-10-04 00:00:00 | zhujiao    | cs     |
| 856 | bbb   | woman | 1945-02-09 00:00:00 | jiaoshou   | dianzi |
+-----+-------+-------+---------------------+------------+--------+
4 rows in set (0.00 sec)

mysql> select * from course;
+--------+----------+-----+
| cno    | cname    | tno |
+--------+----------+-----+
| 3-105  | csdaolun | 825 |
| 3-245  | os       | 856 |
| 6-1166 | math     | 804 |
+--------+----------+-----+
3 rows in set (0.00 sec)

mysql> select tname,depart from teacher 
	-> where tno in (select tno from course); --就是经典的子查询！！！
+-------+--------+
| tname | depart |
+-------+--------+
| aaa   | cs     |
| ccc   | cs     |
| bbb   | dianzi |
+-------+--------+
3 rows in set (0.03 sec)

三、条件+分组筛选！
查询至少有两名男生的班级编号：
mysql> select * from student;
+-----+--------+---------------------+-------+-------+
| sno | sname  | sbirthday           | class | ssex  |
+-----+--------+---------------------+-------+-------+
| 100 | Java   | 1977-09-01 00:00:00 | 95033 | man   |
| 101 | C      | 1975-08-21 00:00:00 | 95034 | woman |
| 102 | C++    | 1976-02-11 00:00:00 | 95035 | woman |
| 103 | C#     | 1974-12-01 00:00:00 | 95034 | woman |
| 104 | Python | 1977-10-11 00:00:00 | 95033 | man   |
| 105 | JS     | 1974-11-11 00:00:00 | 95033 | woman |
+-----+--------+---------------------+-------+-------+
6 rows in set (0.00 sec)

mysql> select class from student where ssex='man'
	-> group by class having count(*)>1; --以班级分组！注意这个写法！！！
+-------+
| class |
+-------+
| 95033 |
+-------+
1 row in set (0.00 sec)

mysql> select class from student where ssex='woman'
	-> group by class having count(*)>1;
+-------+
| class |
+-------+
| 95034 |
+-------+
1 row in set (0.00 sec)