文章目录
聚合查询
聚合函数
COUNT
- 返回查询到的数量
mysql> select * from emp;
+----+---------+-----------+---------+
| id | name | role | salary |
+----+---------+-----------+---------+
| 1 | 丫丫1 | 老师 | 200.00 |
| 2 | 丫丫2 | 老师 | 300.00 |
| 3 | 丫丫3 | 老师 | 400.00 |
| 4 | 丫丫4 | 老师 | 500.00 |
| 5 | 丫丫5 | 老师 | 600.00 |
| 6 | 丫丫6 | 班主任 | 700.00 |
| 7 | 丫丫7 | 班主任 | 800.00 |
| 8 | 丫丫8 | 教导员 | 900.00 |
| 9 | 丫丫9 | 教导员 | 1000.00 |
+----+---------+-----------+---------+
9 rows in set (0.01 sec)
mysql> select count(id) from emp;
+-----------+
| count(id) |
+-----------+
| 9 |
+-----------+
1 row in set (0.01 sec)
SUM
- 返回查询到的数据的总和,不是数字没有意义
mysql> select sum(salary) from emp;
+-------------+
| sum(salary) |
+-------------+
| 5400.00 |
+-------------+
1 row in set (0.00 sec)
AVG
- 返回查询到的数据的平均值,不是数字没有意义
mysql> select avg(salary) from emp;
+-------------+
| avg(salary) |
+-------------+
| 600.000000 |
+-------------+
1 row in set (0.00 sec)
MAX
- 返回查询到的数据的最大值,不是数字没有意义
mysql> select max(salary) from emp;
+-------------+
| max(salary) |
+-------------+
| 1000.00 |
+-------------+
1 row in set (0.00 sec)
MIN
- 返回查询到的数据的最小追,不是数据没有意义
mysql> select min(salary) from emp;
+-------------+
| min(salary) |
+-------------+
| 200.00 |
+-------------+
1 row in set (0.00 sec)
GROUP BY 语句
- select中使用group by 语句 可以对指定列进行分组查询
- 注意: 使用group by进行分组查询时,SELECT 指定的字段必须是“分组依据字段”，其他字段若想出现在SELECT 中则必须包含在聚合函数中。
-- 查询每个角色的总工资
mysql> select role, sum(salary) from emp group by role;
+-----------+-------------+
| role | sum(salary) |
+-----------+-------------+
| 教导员 | 1900.00 |
| 班主任 | 1500.00 |
| 老师 | 2000.00 |
+-----------+-------------+
3 rows in set (0.00 sec)
HAVING 语句
- GROUP BY 子句进行分组以后，需要对分组结果再进行条件过滤时，不能使用 WHERE 语句，而需要用
HAVING
-- 查询每个角色总工资大于1500的
mysql> select role, sum(salary) as totle from emp group by role having totle > 1500;
+-----------+---------+
| role | totle |
+-----------+---------+
| 教导员 | 1900.00 |
| 老师 | 2000.00 |
+-----------+---------+
2 rows in set (0.00 sec)
联合查询
笛卡尔积
- 实际开发中往往数据来自不同的表，所以需要多表联合查询。多表查询是对多张表的数据取笛卡尔积.
- 关联查询的时候可以对关联表使用别名
内连接
- 使用内连接的思路:
- 观察要查询的数据涉及到几张表
- 多张表连接时使用的是笛卡尔积运算,所以会产生许多无用的数据,此时我们就应该加上表的连接条件去除掉无用的数据
- 再用选择条件和字段去控制得到的数据表为我们所需要的
-- 查询张飞的成绩,其涉及到学生表和成绩表
mysql> select student.id, student.name, score from student, score where student.id = score.student_id and student.name = '张飞';
+----+-----------+-------+
| id | name | score |
+----+-----------+-------+
| 3 | 张飞 | 33.0 |
| 3 | 张飞 | 68.0 |
| 3 | 张飞 | 99.0 |
+----+-----------+-------+
3 rows in set (0.00 sec)
-- 还有这种写法
mysql> select student.id, student.name, score from student join score on student.id = score.student_id and student.name = '张飞';
+----+-----------+-------+
| id | name | score |
+----+-----------+-------+
| 3 | 张飞 | 33.0 |
| 3 | 张飞 | 68.0 |
| 3 | 张飞 | 99.0 |
+----+-----------+-------+
3 rows in set (0.00 sec)
– 结合分组查询
-- 查询所有同学的成绩
mysql> select student.id, student.name, score from student, score where student.id = score.student_id;
+----+-----------+-------+
| id | name | score |
+----+-----------+-------+
| 1 | 诸葛亮 | 70.5 |
| 1 | 诸葛亮 | 98.5 |
| 1 | 诸葛亮 | 33.0 |
| 1 | 诸葛亮 | 98.0 |
| 2 | 刘备 | 60.0 |
| 2 | 刘备 | 59.5 |
| 3 | 张飞 | 33.0 |
| 3 | 张飞 | 68.0 |
| 3 | 张飞 | 99.0 |
| 4 | 关羽 | 67.0 |
| 4 | 关羽 | 23.0 |
| 4 | 关羽 | 56.0 |
| 4 | 关羽 | 72.0 |
| 5 | 孙权 | 81.0 |
| 5 | 孙权 | 37.0 |
| 6 | 吕布 | 56.0 |
| 6 | 吕布 | 43.0 |
| 6 | 吕布 | 79.0 |
| 7 | 曹操 | 80.0 |
| 7 | 曹操 | 92.0 |
+----+-----------+-------+
20 rows in set (0.00 sec)
-- 查询所有同学的总成绩
mysql> select student.id, student.name, sum(score) from student, score where student.id = score.student_id group by student.id;
+----+-----------+------------+
| id | name | sum(score) |
+----+-----------+------------+
| 1 | 诸葛亮 | 300.0 |
| 2 | 刘备 | 119.5 |
| 3 | 张飞 | 200.0 |
| 4 | 关羽 | 218.0 |
| 5 | 孙权 | 118.0 |
| 6 | 吕布 | 178.0 |
| 7 | 曹操 | 172.0 |
+----+-----------+------------+
7 rows in set (0.00 sec)
外连接
左外连接
- 联合查询,左侧的表完全显示我们就说是左外连接
-- 查成绩,没有成绩也要体现
mysql> select student.id, student.name, score from student left join score on student.id = score.student_id;
右外连接
- 联合查询,右侧的表完全显示我们就说是右外连接
mysql> select student.id, student.name, score from student right join score on student.id = score.student_id;
-- 三张表的关联查询
mysql> select student.id, student.name, score from
-> student
-> left join score on student.id = score.student_id
-> left join course on score.course_id = course.id
-> where score < 60 or score <=> null
-> order by score desc;
+----+-----------+-------+
| id | name | score |
+----+-----------+-------+
| 2 | 刘备 | 59.5 |
| 4 | 关羽 | 56.0 |
| 6 | 吕布 | 56.0 |
| 6 | 吕布 | 43.0 |
| 5 | 孙权 | 37.0 |
| 1 | 诸葛亮 | 33.0 |
| 3 | 张飞 | 33.0 |
| 4 | 关羽 | 23.0 |
| 8 | 赵云 | NULL |
+----+-----------+-------+
9 rows in set (0.00 sec)
自连接
- 自连接是指在同一张表连接自身进行查询
-- 查 计算机原理 比 java高的成绩信息
mysql> select s1.course_id, s1.score, s2.course_id, s2.score
-> from
-> score s1, score s2
-> where
-> s1.student_id = s2.student_id
-> and s1.course_id = (select id from course where name = '计算机原理')
-> and s2.course_id = (select id from course where name = 'java')
-> and s1.score > s2.score;
+-----------+-------+-----------+-------+
| course_id | score | course_id | score |
+-----------+-------+-----------+-------+
| 3 | 98.5 | 1 | 70.5 |
| 3 | 68.0 | 1 | 33.0 |
+-----------+-------+-----------+-------+
2 rows in set (0.00 sec)
子查询
- 在一个查询语句里套用查询语句称为子查询
单行子查询
- 子查询返回一条结果
-- 查询与刘备同班同学的总成绩
mysql> select student.id, student.name, sum(score)
-> from
-> student, score
-> where
-> student.id = score.student_id
-> and student.classes_id = (select student.classes_id from student where name = '刘备')
-> group by student.id;
+----+-----------+------------+
| id | name | sum(score) |
+----+-----------+------------+
| 1 | 诸葛亮 | 300.0 |
| 2 | 刘备 | 119.5 |
| 3 | 张飞 | 200.0 |
| 4 | 关羽 | 218.0 |
| 5 | 孙权 | 118.0 |
+----+-----------+------------+
5 rows in set (0.00 sec)
多行子查询
- 子查询返回多行结果
IN 处理多行
-- 查询语文和高阶数学的成绩
mysql> select score.course_id, score
-> from
-> score
-> where
-> score.course_id
-> in
-> (select id from course where name = '语文' or name = '高阶数学');
+-----------+-------+
| course_id | score |
+-----------+-------+
| 5 | 33.0 |
| 5 | 59.5 |
| 5 | 99.0 |
| 5 | 56.0 |
| 5 | 37.0 |
| 4 | 43.0 |
+-----------+-------+
6 rows in set (0.00 sec)
- 还有not in
EXISTS 处理多行
- 括号不能省
mysql> select
-> score.course_id, score
-> from
-> score
-> where exists
-> (select score.course_id from course where (name = '语文' or name = '高阶数 学')
-> and course.id = score.course_id);
+-----------+-------+
| course_id | score |
+-----------+-------+
| 5 | 33.0 |
| 5 | 59.5 |
| 5 | 99.0 |
| 5 | 56.0 |
| 5 | 37.0 |
| 4 | 43.0 |
+-----------+-------+
6 rows in set (0.00 sec)
IN 与 EXISTS 的区别
- in 的子查询是 先执行子查询,把结果保存到内存中,再进行主查询,最后结合子查询的结果进行筛选
- exists 的子查询是 先执行主查询,再触发子查询,最后子查询的结果会对主表查询结果的每一条进行筛选.
- 如果子查询的结果小,就用in
- 如果子查询大,而主查询小,就用 exists
合并查询
- 合并多个select的查询结果,但必须保证合并表的字段需一致
UNION
- 将俩个结果集进行并集, 会去掉集中的重复行
mysql> select * from student where id > 3 or id < 7;
+----+-------+-----------+------------------+------------+
| id | sn | name | qq_mail | classes_id |
+----+-------+-----------+------------------+------------+
| 1 | 09982 | 诸葛亮 | xuanfeng@qq.com | 1 |
| 2 | 00835 | 刘备 | NULL | 1 |
| 3 | 00391 | 张飞 | NULL | 1 |
| 4 | 00031 | 关羽 | xuxian@qq.com | 1 |
| 5 | 00054 | 孙权 | NULL | 1 |
| 6 | 51234 | 吕布 | say@qq.com | 2 |
| 7 | 83223 | 曹操 | NULL | 2 |
| 8 | 09527 | 赵云 | foreigner@qq.com | 2 |
+----+-------+-----------+------------------+------------+
8 rows in set (0.00 sec)
mysql> select * from student where id > 3 union select * from student where id < 7;
+----+-------+-----------+------------------+------------+
| id | sn | name | qq_mail | classes_id |
+----+-------+-----------+------------------+------------+
| 4 | 00031 | 关羽 | xuxian@qq.com | 1 |
| 5 | 00054 | 孙权 | NULL | 1 |
| 6 | 51234 | 吕布 | say@qq.com | 2 |
| 7 | 83223 | 曹操 | NULL | 2 |
| 8 | 09527 | 赵云 | foreigner@qq.com | 2 |
| 1 | 09982 | 诸葛亮 | xuanfeng@qq.com | 1 |
| 2 | 00835 | 刘备 | NULL | 1 |
| 3 | 00391 | 张飞 | NULL | 1 |
+----+-------+-----------+------------------+------------+
8 rows in set (0.00 sec)
UNION ALL
- 将俩个结果集进行并集, 不会去掉集中的重复行
mysql> select * from student where id > 3 union all select * from student where id < 7;
+----+-------+-----------+------------------+------------+
| id | sn | name | qq_mail | classes_id |
+----+-------+-----------+------------------+------------+
| 4 | 00031 | 关羽 | xuxian@qq.com | 1 |
| 5 | 00054 | 孙权 | NULL | 1 |
| 6 | 51234 | 吕布 | say@qq.com | 2 |
| 7 | 83223 | 曹操 | NULL | 2 |
| 8 | 09527 | 赵云 | foreigner@qq.com | 2 |
| 1 | 09982 | 诸葛亮 | xuanfeng@qq.com | 1 |
| 2 | 00835 | 刘备 | NULL | 1 |
| 3 | 00391 | 张飞 | NULL | 1 |
| 4 | 00031 | 关羽 | xuxian@qq.com | 1 |
| 5 | 00054 | 孙权 | NULL | 1 |
| 6 | 51234 | 吕布 | say@qq.com | 2 |
+----+-------+-----------+------------------+------------+
11 rows in set (0.00 sec)