文章目录
- 查询进阶
- 1.聚合查询
- 1.聚合函数
- 2.group by 子句
- 3.having
- 2.联合查询
- 笛卡尔积
- 1.内连接
- 查询许仙同学的成绩
- 查询所有同学的总成绩
- 查询所有同学的每门课程和分数：
- 2.外连接
- 1.内连接：
- 2.左外连接
- 3.右外链接
- 3.自连接
- 4.子查询
- 单行子查询
- 多行子查询
- 查询“语文”或“英文”课程的成绩信息
- in
- 5.合并查询
- union
查询进阶
1.聚合查询
表达式查询，是针对列和列之间进行运算。
聚合查询，是针对行和行之间进行运算
sql中提供了一些“聚合函数”,通过聚合函数来完成行与行之间的运算
1.聚合函数
- 函数名后面，不能乱加空格
-
count（） ：查询结果集的行数
count得到的结果可以参与各种运算，或者搭配其他sql语句使用
mysql> select *from exam_result;
+------+--------+---------+------+---------+
| id | name | chinese | math | english |
+------+--------+---------+------+---------+
| 1 | 唐三藏 | 67.0 | 98.0 | 56.0 |
| 2 | 孙悟空 | 87.5 | 78.0 | 77.0 |
| 3 | 猪悟能 | 88.0 | 98.5 | 90.0 |
| 4 | 曹孟德 | 82.0 | 84.0 | 67.0 |
| 5 | 刘玄德 | 55.5 | 85.0 | 45.0 |
| 6 | 孙权 | 70.0 | 73.0 | 78.5 |
| 7 | 宋公明 | 75.0 | 65.0 | 30.0 |
+------+--------+---------+------+---------+
7 rows in set (0.00 sec)
mysql> select count(*) from exam_result;
+----------+
| count(*) |
+----------+
| 7 |
+----------+
mysql> select count(name) from exam_result;
-- 先查询到name的结果集，再判断name有几行
+-------------+
| count(name) |
+-------------+
| 7 |
+-------------+
mysql> insert into exam_result values(null,null,null,null,null);
-- 插入一行空数据后
Query OK, 1 row affected (0.00 sec)
mysql> select count(*) from exam_result;
-- null也占一行，进行统计
+----------+
| count(*) |
+----------+
| 8 |
+----------+
1 row in set (0.00 sec)
mysql> select count(name) from exam_result;
-- 遇到null,不进行统计
+-------------+
| count(name) |
+-------------+
| 7 |
+-------------+
指定列进行去重
mysql> update exam_result set math = 98 where name = '猪悟能';
mysql> select * from exam_result;
+------+--------+---------+------+---------+
| id | name | chinese | math | english |
+------+--------+---------+------+---------+
| 1 | 唐三藏 | 67.0 | 98.0 | 56.0 |
| 2 | 孙悟空 | 87.5 | 78.0 | 77.0 |
| 3 | 猪悟能 | 88.0 | 98.0 | 90.0 |
| 4 | 曹孟德 | 82.0 | 84.0 | 67.0 |
| 5 | 刘玄德 | 55.5 | 85.0 | 45.0 |
| 6 | 孙权 | 70.0 | 73.0 | 78.5 |
| 7 | 宋公明 | 75.0 | 65.0 | 30.0 |
| NULL | NULL | NULL | NULL | NULL |
+------+--------+---------+------+---------+
mysql> select count(distinct math)from exam_result;
-- 统计数学成绩去重后的个数（null不算，重复的不算）
+----------------------+
| count(distinct math) |
+----------------------+
| 6 |
+----------------------+
-
sum()求和
把这一列的若干行，进行求和算数运算（只能针对数字类型使用 ）
mysql> select sum(chinese) from exam_result;
-- 计算所有人语文成绩之和。（会自动排除null）
+--------------+
| sum(chinese) |
+--------------+
| 525.0 |
+--------------+
mysql> select sum(name) from exam_result;
-- 如果是字符串来参与运算，会进行警告
+-----------+
| sum(name) |
+-----------+
| 0 |
+-----------+
1 row in set, 7 warnings (0.01 sec)
mysql> show warnings;
-- mysql会尝试把这一列转换成double,如果可以转成double,就可以进行运算。如果没有转成就会报错。
+---------+------+--------------------------------------------+
| Level | Code | Message |
+---------+------+--------------------------------------------+
| Warning | 1292 | Truncated incorrect DOUBLE value: '唐三藏' |
| Warning | 1292 | Truncated incorrect DOUBLE value: '孙悟空' |
| Warning | 1292 | Truncated incorrect DOUBLE value: '猪悟能' |
| Warning | 1292 | Truncated incorrect DOUBLE value: '曹孟德' |
| Warning | 1292 | Truncated incorrect DOUBLE value: '刘玄德' |
| Warning | 1292 | Truncated incorrect DOUBLE value: '孙权' |
| Warning | 1292 | Truncated incorrect DOUBLE value: '宋公明' |
+---------+------+--------------------------------------------+
-- sum(表达式)
mysql> select sum(chinese+math+english) from exam_result;
-- 1，先把对于的列相加，得到一个临时表
-- 2.把这个临时表的结果，进行行与行之间的运算
+---------------------------+
| sum(chinese+math+english) |
+---------------------------+
| 1549.5 |
+---------------------------+
-
avg （）求平均值
mysql> select avg(chinese) from exam_result; +--------------+ | avg(chinese) | +--------------+ | 75.00000 | +--------------+ -
min() max() 求最大最小值
mysql> select min(chinese) from exam_result;
+--------------+
| min(chinese) |
+--------------+
| 55.5 |
+--------------+
1 row in set (0.01 sec)
mysql> select max(chinese) from exam_result;
+--------------+
| max(chinese) |
+--------------+
| 88.0 |
+--------------+
1 row in set (0.00 sec)
2.group by 子句
-
使用group by进行分组，再针对每个分组，分别进行聚合查询
-
针对指定的列进行分组，把这一列中值相同的行，分到一组中，得到若干个组。对这些组分别使用聚合函数。
mysql> select * from emp;
+----+------+----------+--------+
| id | name | role | salary |
+----+------+----------+--------+
| 1 | 张三 | 程序员 | 10000 |
| 2 | 李四 | 程序员 | 12000 |
| 3 | 王五 | 程序员 | 13000 |
| 4 | 赵六 | 产品经理 | 9000 |
| 5 | 李白 | 产品经理 | 9500 |
| 6 | 周八 | 老板 | 100000 |
+----+------+----------+--------+
6 rows in set (0.00 sec)
mysql> select avg(salary) from emp;
+-------------+
| avg(salary) |
+-------------+
| 25583.3333 |
+-------------+
1 row in set (0.00 sec)
mysql> select role,avg(salary) from emp group by role;
-- 1.先根据role进行分组
-- 2.再针对每组来求平均值
+----------+-------------+
| role | avg(salary) |
+----------+-------------+
| 产品经理 | 9250.0000 |
| 程序员 | 11666.6667 |
| 老板 | 100000.0000 |
+----------+-------------+
- 如果分组之后，不使用聚合函数。此时的结果就是查询出每一组中的某个代表数据。
搭配条件
要区分好是分组前的条件，还是分组后的条件。
mysql> select role,avg(salary)from emp where name!='张三' group by role;
-- 查询每个岗位的平均工资，但是排除张三
+----------+-------------+
| role | avg(salary) |
+----------+-------------+
| 产品经理 | 9250.0000 |
| 程序员 | 12500.0000 |
| 老板 | 100000.0000 |
+----------+-------------+
3 rows in set (0.01 sec)
3.having
- 在对分组后的结果再次进行条件过滤时，不能使用where语句，而是需要使用having
- having 一般写在 group by的后面
-- 查询每个岗位的平均工资，但是排除平均工资超过两万的结果
mysql> select role,avg(salary)from emp group by role having avg(salary)<20000;
+----------+-------------+
| role | avg(salary) |
+----------+-------------+
| 产品经理 | 9250.0000 |
| 程序员 | 11666.6667 |
+----------+-------------+
- 在group by 中，可以一个sql同时完成这两类条件的筛选
-- 查询每个岗位的平均薪资，不算张三，并且保留 平均值<2万的结果
mysql> select role,avg(salary) from emp where name !='张三' group by role having avg(salary)<20000;
+----------+-------------+
| role | avg(salary) |
+----------+-------------+
| 产品经理 | 9250.0000 |
| 程序员 | 12500.0000 |
+----------+-------------+
2.联合查询
一次需要在多张表中进行查询。
联合查询/多表查询 ：是对多张表的数据取笛卡尔积
笛卡尔积
- 笛卡尔积是通过排列组合的方式，得到一个更大的表
- 笛卡尔积的列数，是这两个表的列数相加。
- 笛卡尔积的行数，是这两个表的行数相乘。
笛卡尔积列举了所有可能的情况。其中，有些数据可能是非法的（无意义的数据）。在进行多表查询的时候，就需要把有意义的数据筛选出来，过滤掉无意义的数据。筛序的条件叫做连接条件。
mysql> create table classes(id int primary key auto_increment,name varchar(20), `desc` varchar(100));
Query OK, 0 rows affected (0.01 sec)
mysql> create table student(id int primary key auto_increment,sn varchar(20),name varchar(20), qq_mail varchar(20),classes_id int);
Query OK, 0 rows affected (0.00 sec)
mysql> create table course(id int primary key auto_increment,name varchar(20));
Query OK, 0 rows affected (0.01 sec)
mysql> create table score(score decimal(3,1),student_id int,course_id int);
Query OK, 0 rows affected (0.01 sec)
insert into classes(name, `desc`) values
('计算机系2019级1班', '学习了计算机原理、C和Java语言、数据结构和算法'),
('中文系2019级3班','学习了中国传统文学'),
('自动化2019级5班','学习了机械自动化');
insert into student(sn, name, qq_mail, classes_id) values
('09982','黑旋风李逵','xuanfeng@qq.com',1),
('00835','菩提老祖',null,1),
('00391','白素贞',null,1),
('00031','许仙','xuxian@qq.com',1),
('00054','不想毕业',null,1),
('51234','好好说话','say@qq.com',2),
('83223','tellme',null,2),
('09527','老外学中文','foreigner@qq.com',2);
insert into course(name) values
('Java'),('中国传统文化'),('计算机原理'),('语文'),('高阶数学'),('英文');
insert into score(score, student_id, course_id) values
-- 黑旋风李逵
(70.5, 1, 1),(98.5, 1, 3),(33, 1, 5),(98, 1, 6),
-- 菩提老祖
(60, 2, 1),(59.5, 2, 5),
-- 白素贞
(33, 3, 1),(68, 3, 3),(99, 3, 5),
-- 许仙
(67, 4, 1),(23, 4, 3),(56, 4, 5),(72, 4, 6),
-- 不想毕业
(81, 5, 1),(37, 5, 5),
-- 好好说话
(56, 6, 2),(43, 6, 4),(79, 6, 6),
-- tellme
(80, 7, 2),(92, 7, 6);
mysql> show tables;
+-------------------+
| Tables_in_day5_14 |
+-------------------+
| classes |
| course |
| score |
| student |
+-------------------+
-- 学生表、班级表、课程表
-- 分数表（学生和课程之间的联系表）
-- 学生表
mysql> select * from student;
+----+-------+------------+------------------+------------+
| id | sn | name | qq_mail | classes_id |
+----+-------+------------+------------------+------------+
| 1 | 09982 | 黑旋风李逵 | xuanfeng@qq.com | 1 |
| 2 | 00835 | 菩提老祖 | NULL | 1 |
| 3 | 00391 | 白素贞 | NULL | 1 |
| 4 | 00031 | 许仙 | xuxian@qq.com | 1 |
| 5 | 00054 | 不想毕业 | NULL | 1 |
| 6 | 51234 | 好好说话 | say@qq.com | 2 |
| 7 | 83223 | tellme | NULL | 2 |
| 8 | 09527 | 老外学中文 | foreigner@qq.com | 2 |
+----+-------+------------+------------------+------------+
-- 班级表
mysql> select * from classes;
+----+-------------------+-----------------------------------------------+
| id | name | desc |
+----+-------------------+-----------------------------------------------+
| 1 | 计算机系2019级1班 | 学习了计算机原理、C和Java语言、数据结构和算法 |
| 2 | 中文系2019级3班 | 学习了中国传统文学 |
| 3 | 自动化2019级5班 | 学习了机械自动化 |
+----+-------------------+-----------------------------------------------+
-- 课程表
mysql> select * from course;
+----+--------------+
| id | name |
+----+--------------+
| 1 | Java |
| 2 | 中国传统文化 |
| 3 | 计算机原理 |
| 4 | 语文 |
| 5 | 高阶数学 |
| 6 | 英文 |
+----+--------------+
-- 分数表
mysql> select * from score;
+-------+------------+-----------+
| score | student_id | course_id |
+-------+------------+-----------+
| 70.5 | 1 | 1 |
| 98.5 | 1 | 3 |
| 33.0 | 1 | 5 |
| 98.0 | 1 | 6 |
| 60.0 | 2 | 1 |
| 59.5 | 2 | 5 |
| 33.0 | 3 | 1 |
| 68.0 | 3 | 3 |
| 99.0 | 3 | 5 |
| 67.0 | 4 | 1 |
| 23.0 | 4 | 3 |
| 56.0 | 4 | 5 |
| 72.0 | 4 | 6 |
| 81.0 | 5 | 1 |
| 37.0 | 5 | 5 |
| 56.0 | 6 | 2 |
| 43.0 | 6 | 4 |
| 79.0 | 6 | 6 |
| 80.0 | 7 | 2 |
| 92.0 | 7 | 6 |
+-------+------------+-----------+
1.内连接
查询许仙同学的成绩
1.先把student和score两张表进行笛卡尔积
mysql> select * from student,score;
+----+-------+------------+------------------+------------+-------+------------+-----------+
| id | sn | name | qq_mail | classes_id | score | student_id | course_id |
+----+-------+------------+------------------+------------+-------+------------+-----------+
| 1 | 09982 | 黑旋风李逵 | xuanfeng@qq.com | 1 | 70.5 | 1 | 1 |
| 2 | 00835 | 菩提老祖 | NULL | 1 | 70.5 | 1 | 1 |
| 3 | 00391 | 白素贞 | NULL | 1 | 70.5 | 1 | 1 |
| 4 | 00031 | 许仙 | xuxian@qq.com | 1 | 70.5 | 1 | 1 |
-- -------- --------------------------------------- - - - - - - - - - - --- - - - -
| 6 | 51234 | 好好说话 | say@qq.com | 2 | 92.0 | 7 | 6 |
| 7 | 83223 | tellme | NULL | 2 | 92.0 | 7 | 6 |
| 8 | 09527 | 老外学中文 | foreigner@qq.com | 2 | 92.0 | 7 | 6 |
+----+-------+------------+------------------+------------+-------+------------+-----------+
160 rows in set (0.01 sec)
-- 一共160行（20*8）5列（2+3），中间省略
2.加上连接条件，筛选出有效数据
学生id在两个表中同时出现。筛选条件为： 学生表的id = 分数表的student_id
select * from student,score where id = student_id;
-- 但是如果两个表的列名一样，则无法进行区分
select * from student,score where student.id = score.student_id;
-- 所以用表名.列名进行区分。
+----+-------+------------+-----------------+------------+-------+------------+-----------+
| id | sn | name | qq_mail | classes_id | score | student_id | course_id |
+----+-------+------------+-----------------+------------+-------+------------+-----------+
| 1 | 09982 | 黑旋风李逵 | xuanfeng@qq.com | 1 | 70.5 | 1 | 1 |
| 1 | 09982 | 黑旋风李逵 | xuanfeng@qq.com | 1 | 98.5 | 1 | 3 |
| 1 | 09982 | 黑旋风李逵 | xuanfeng@qq.com | 1 | 33.0 | 1 | 5 |
| 1 | 09982 | 黑旋风李逵 | xuanfeng@qq.com | 1 | 98.0 | 1 | 6 |
| 2 | 00835 | 菩提老祖 | NULL | 1 | 60.0 | 2 | 1 |
| 2 | 00835 | 菩提老祖 | NULL | 1 | 59.5 | 2 | 5 |
| 3 | 00391 | 白素贞 | NULL | 1 | 33.0 | 3 | 1 |
| 3 | 00391 | 白素贞 | NULL | 1 | 68.0 | 3 | 3 |
| 3 | 00391 | 白素贞 | NULL | 1 | 99.0 | 3 | 5 |
| 4 | 00031 | 许仙 | xuxian@qq.com | 1 | 67.0 | 4 | 1 |
| 4 | 00031 | 许仙 | xuxian@qq.com | 1 | 23.0 | 4 | 3 |
| 4 | 00031 | 许仙 | xuxian@qq.com | 1 | 56.0 | 4 | 5 |
| 4 | 00031 | 许仙 | xuxian@qq.com | 1 | 72.0 | 4 | 6 |
| 5 | 00054 | 不想毕业 | NULL | 1 | 81.0 | 5 | 1 |
| 5 | 00054 | 不想毕业 | NULL | 1 | 37.0 | 5 | 5 |
| 6 | 51234 | 好好说话 | say@qq.com | 2 | 56.0 | 6 | 2 |
| 6 | 51234 | 好好说话 | say@qq.com | 2 | 43.0 | 6 | 4 |
| 6 | 51234 | 好好说话 | say@qq.com | 2 | 79.0 | 6 | 6 |
| 7 | 83223 | tellme | NULL | 2 | 80.0 | 7 | 2 |
| 7 | 83223 | tellme | NULL | 2 | 92.0 | 7 | 6 |
+----+-------+------------+-----------------+------------+-------+------------+-----------+
3.结合需求，进一步添加条件，针对结果进行筛选。
mysql> select * from student,score where student.id = score.student_id and student.name = '许仙';
+----+-------+------+---------------+------------+-------+------------+-----------+
| id | sn | name | qq_mail | classes_id | score | student_id | course_id |
+----+-------+------+---------------+------------+-------+------------+-----------+
| 4 | 00031 | 许仙 | xuxian@qq.com | 1 | 67.0 | 4 | 1 |
| 4 | 00031 | 许仙 | xuxian@qq.com | 1 | 23.0 | 4 | 3 |
| 4 | 00031 | 许仙 | xuxian@qq.com | 1 | 56.0 | 4 | 5 |
| 4 | 00031 | 许仙 | xuxian@qq.com | 1 | 72.0 | 4 | 6 |
+----+-------+------+---------------+------------+-------+------------+-----------+
4.针对查询到的列，进行精简。只保留需求。
mysql> select student.name,score.score from student,score where student.id = score.student_id and student.name = '许仙';
+------+-------+
| name | score |
+------+-------+
| 许仙 | 67.0 |
| 许仙 | 23.0 |
| 许仙 | 56.0 |
| 许仙 | 72.0 |
+------+-------+
查询所有同学的总成绩
用聚合函数sum对多行数据进行加和，还需要按照同学来进行分组
1.先将学生表和分数表进行笛卡尔积
mysql> select * from student,score;
2.指定连接条件 student.id = score.student_id
mysql> select * from student,score where student.id = score.student_id;
+----+-------+------------+-----------------+------------+-------+------------+-----------+
| id | sn | name | qq_mail | classes_id | score | student_id | course_id |
+----+-------+------------+-----------------+------------+-------+------------+-----------+
| 1 | 09982 | 黑旋风李逵 | xuanfeng@qq.com | 1 | 70.5 | 1 | 1 |
| 1 | 09982 | 黑旋风李逵 | xuanfeng@qq.com | 1 | 98.5 | 1 | 3 |
| 1 | 09982 | 黑旋风李逵 | xuanfeng@qq.com | 1 | 33.0 | 1 | 5 |
| 1 | 09982 | 黑旋风李逵 | xuanfeng@qq.com | 1 | 98.0 | 1 | 6 |
| 2 | 00835 | 菩提老祖 | NULL | 1 | 60.0 | 2 | 1 |
| 2 | 00835 | 菩提老祖 | NULL | 1 | 59.5 | 2 | 5 |
| 3 | 00391 | 白素贞 | NULL | 1 | 33.0 | 3 | 1 |
| 3 | 00391 | 白素贞 | NULL | 1 | 68.0 | 3 | 3 |
| 3 | 00391 | 白素贞 | NULL | 1 | 99.0 | 3 | 5 |
| 4 | 00031 | 许仙 | xuxian@qq.com | 1 | 67.0 | 4 | 1 |
| 4 | 00031 | 许仙 | xuxian@qq.com | 1 | 23.0 | 4 | 3 |
| 4 | 00031 | 许仙 | xuxian@qq.com | 1 | 56.0 | 4 | 5 |
| 4 | 00031 | 许仙 | xuxian@qq.com | 1 | 72.0 | 4 | 6 |
| 5 | 00054 | 不想毕业 | NULL | 1 | 81.0 | 5 | 1 |
| 5 | 00054 | 不想毕业 | NULL | 1 | 37.0 | 5 | 5 |
| 6 | 51234 | 好好说话 | say@qq.com | 2 | 56.0 | 6 | 2 |
| 6 | 51234 | 好好说话 | say@qq.com | 2 | 43.0 | 6 | 4 |
| 6 | 51234 | 好好说话 | say@qq.com | 2 | 79.0 | 6 | 6 |
| 7 | 83223 | tellme | NULL | 2 | 80.0 | 7 | 2 |
| 7 | 83223 | tellme | NULL | 2 | 92.0 | 7 | 6 |
+----+-------+------------+-----------------+------------+-------+------------+-----------+
3.先精简列
mysql> select student.name,score.score from student,score where student.id = score.student_id;
+------------+-------+
| name | score |
+------------+-------+
| 黑旋风李逵 | 70.5 |
| 黑旋风李逵 | 98.5 |
| 黑旋风李逵 | 33.0 |
| 黑旋风李逵 | 98.0 |
| 菩提老祖 | 60.0 |
| 菩提老祖 | 59.5 |
| 白素贞 | 33.0 |
| 白素贞 | 68.0 |
| 白素贞 | 99.0 |
| 许仙 | 67.0 |
| 许仙 | 23.0 |
| 许仙 | 56.0 |
| 许仙 | 72.0 |
| 不想毕业 | 81.0 |
| 不想毕业 | 37.0 |
| 好好说话 | 56.0 |
| 好好说话 | 43.0 |
| 好好说话 | 79.0 |
| tellme | 80.0 |
| tellme | 92.0 |
+------------+-------+
4.对上述结果，进行group by聚合查询
mysql> select student.name,sum(score.score) from student,score where student.id = score.student_id group by student.name ;
+------------+------------------+
| name | sum(score.score) |
+------------+------------------+
| tellme | 172.0 |
| 不想毕业 | 118.0 |
| 好好说话 | 178.0 |
| 白素贞 | 200.0 |
| 菩提老祖 | 119.5 |
| 许仙 | 218.0 |
| 黑旋风李逵 | 300.0 |
+------------+------------------+
查询所有同学的每门课程和分数：
1.学生表、课程表、分数表三张表进行笛卡尔积
mysql> select * from student,course,score;
2.指定连接条件筛选数据。三个表涉及到两个连接条件。
score.student_id = student,id
score.course_id = course.id
mysql> select * from student,course,score where student.id = score.student_id and score.course_id = course.id ;
3.精简列
mysql> select student.name,course.name,score.score from student,course,score where student.id = score.student_id and score.course_id = course.id ;
-- 进行取别名
mysql> select student.name as studentName,course.name as courseName ,score.score from student,course,score where student.id = score.student_id and score.course_id = course.id ;
+-------------+--------------+-------+
| studentName | courseName | score |
2.外连接
左外连接 ：左侧的表完全显示 (left join)
右外连接 ：右侧的表完全显示 (right jion)
如果两个表中，里面的记录都是存在对应关系，内连接和外连接的结果就是一致的
如果存在不对于的记录，内连接和外连接就会出现差别。
mysql> insert into student2 values(1,'张三'),(2,'李四'),(3,'王五');
Query OK, 3 rows affected (0.01 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> insert into score2 value(1,90),(2,80),(4,70);
Query OK, 3 rows affected (0.01 sec)
mysql> select * from student2;
+------+------+
| id | name |
+------+------+
| 1 | 张三 |
| 2 | 李四 |
| 3 | 王五 |
+------+------+
3 rows in set (0.00 sec)
mysql> select * from score2;
+------+-------+
| id | score |
+------+-------+
| 1 | 90 |
| 2 | 80 |
| 4 | 70 |
+------+-------+
1.内连接：
mysql> select * from student2,score2 where student2.id = score2.id;
-- 内连接写法一
+------+------+------+-------+
| id | name | id | score |
+------+------+------+-------+
| 1 | 张三 | 1 | 90 |
| 2 | 李四 | 2 | 80 |
+------+------+------+-------+
mysql> select * from student2 [inner] join score2 on student2.id = score2.id;
-- 内连接写法二
+------+------+------+-------+
| id | name | id | score |
+------+------+------+-------+
| 1 | 张三 | 1 | 90 |
| 2 | 李四 | 2 | 80 |
+------+------+------+-------+
2.左外连接
mysql> select * from student2 left join score2 on student2.id = score2.id;
+------+------+------+-------+
| id | name | id | score |
+------+------+------+-------+
| 1 | 张三 | 1 | 90 |
| 2 | 李四 | 2 | 80 |
| 3 | 王五 | NULL | NULL |
+------+------+------+-------+
- 王五的数据只在左侧表（student2）存在
- 以左侧表为基准，保证左侧表的每个记录都体现在结果中。如果在右侧表中不存在，就填成null
3.右外链接
mysql> select * from student2 right join score2 on student2.id = score2.id;
+------+------+------+-------+
| id | name | id | score |
+------+------+------+-------+
| 1 | 张三 | 1 | 90 |
| 2 | 李四 | 2 | 80 |
| NULL | NULL | 4 | 70 |
+------+------+------+-------+
- 以右侧表为基准，保证右侧表的每个数据，都会出现在最终结果里。如果左侧表中不存在，对于的列填null
3.自连接
- 一张表，自己和自己进行笛卡尔积。
有的时候，需要进行与行之间的比较。而sql只能进行列和列之间的比较。通过自连接，把行关系转换成列关系
显示所有“计算机原理”成绩比“Java”成绩高的成绩信息
mysql> select * from score;
+-------+------------+-----------+
| score | student_id | course_id |
+-------+------------+-----------+
| 70.5 | 1 | 1 |
| 98.5 | 1 | 3 |
| 33.0 | 1 | 5 |
| 98.0 | 1 | 6 |
| 60.0 | 2 | 1 |
| 59.5 | 2 | 5 |
| 33.0 | 3 | 1 |
| 68.0 | 3 | 3 |
| 99.0 | 3 | 5 |
| 67.0 | 4 | 1 |
| 23.0 | 4 | 3 |
| 56.0 | 4 | 5 |
| 72.0 | 4 | 6 |
| 81.0 | 5 | 1 |
| 37.0 | 5 | 5 |
| 56.0 | 6 | 2 |
| 43.0 | 6 | 4 |
| 79.0 | 6 | 6 |
| 80.0 | 7 | 2 |
| 92.0 | 7 | 6 |
+-------+------------+-----------+
mysql> select * from course;
+----+--------------+
| id | name |
+----+--------------+
| 1 | Java |
| 2 | 中国传统文化 |
| 3 | 计算机原理 |
| 4 | 语文 |
| 5 | 高阶数学 |
| 6 | 英文 |
+----+--------------+
-- 看3比1高的成绩
要将行关系，转换成列关系，
mysql> select * from score,score;
ERROR 1066 (42000): Not unique table/alias: 'score'
-- 不能直接用，需要设置别名
mysql> select * from score as s1,score as s2;
mysql> select * from score as s1,score as s2 where s1.student_id = s2.student_id and s1.course_id=3 and s2.course_id = 1;
+-------+------------+-----------+-------+------------+-----------+
| score | student_id | course_id | score | student_id | course_id |
+-------+------------+-----------+-------+------------+-----------+
| 98.5 | 1 | 3 | 70.5 | 1 | 1 |
| 68.0 | 3 | 3 | 33.0 | 3 | 1 |
| 23.0 | 4 | 3 | 67.0 | 4 | 1 |
+-------+------------+-----------+-------+------------+-----------+
3 rows in set (0.00 sec)
mysql> select * from score as s1,score as s2 where s1.student_id = s2.student_id and s1.course_id=3 and s2.course_id = 1 and s1.score>s2.score;
+-------+------------+-----------+-------+------------+-----------+
| score | student_id | course_id | score | student_id | course_id |
+-------+------------+-----------+-------+------------+-----------+
| 98.5 | 1 | 3 | 70.5 | 1 | 1 |
| 68.0 | 3 | 3 | 33.0 | 3 | 1 |
+-------+------------+-----------+-------+------------+-----------+
2 rows in set (0.00 sec)
mysql> select s1.student_id,s1.score,s2.score from score as s1,score as s2 where s1.student_id = s2.student_id and s1.course_id=3 and s2.course_id = 1 and s1.score>s2.score;
+------------+-------+-------+
| student_id | score | score |
+------------+-------+-------+
| 1 | 98.5 | 70.5 |
| 3 | 68.0 | 33.0 |
+------------+-------+-------+
4.子查询
也叫嵌套查询：嵌入到其他sql语句中select语句。（套娃）
单行子查询
- 返回一行记录的子查询
查询与“不想毕业” 同学的同班同学：
mysql> select * from student;
+----+-------+------------+------------------+------------+
| id | sn | name | qq_mail | classes_id |
+----+-------+------------+------------------+------------+
| 1 | 09982 | 黑旋风李逵 | xuanfeng@qq.com | 1 |
| 2 | 00835 | 菩提老祖 | NULL | 1 |
| 3 | 00391 | 白素贞 | NULL | 1 |
| 4 | 00031 | 许仙 | xuxian@qq.com | 1 |
| 5 | 00054 | 不想毕业 | NULL | 1 |
| 6 | 51234 | 好好说话 | say@qq.com | 2 |
| 7 | 83223 | tellme | NULL | 2 |
| 8 | 09527 | 老外学中文 | foreigner@qq.com | 2 |
+----+-------+------------+------------------+------------+
8 rows in set (0.01 sec)
mysql> select * from student where classes_id = (select classes_id from student where name='不想毕业');
-- 第二个select 返回的结果是必须是一行记录
+----+-------+------------+-----------------+------------+
| id | sn | name | qq_mail | classes_id |
+----+-------+------------+-----------------+------------+
| 1 | 09982 | 黑旋风李逵 | xuanfeng@qq.com | 1 |
| 2 | 00835 | 菩提老祖 | NULL | 1 |
| 3 | 00391 | 白素贞 | NULL | 1 |
| 4 | 00031 | 许仙 | xuxian@qq.com | 1 |
| 5 | 00054 | 不想毕业 | NULL | 1 |
+----+-------+------------+-----------------+------------+
多行子查询
- 返回多行记录的子查询
查询“语文”或“英文”课程的成绩信息
1.先通过课程的名字，找到课程的id
2.通过课程id在分数表中进行查询
in
mysql> select * from score where course_id in (select id from course where name = '语文' or name = '英文');
mysql> select score.student_id,score.course_id,score.score from score where score.course_id in (select id from course where name = '语文' or name = '英文');
+------------+-----------+-------+
| student_id | course_id | score |
+------------+-----------+-------+
| 1 | 6 | 98.0 |
| 4 | 6 | 72.0 |
| 6 | 4 | 43.0 |
| 6 | 6 | 79.0 |
| 7 | 6 | 92.0 |
+------------+-----------+-------+
5.合并查询
- 把多个sql的结果集，合并到一起。
union
- 前后查询的结果集，字段必须一致
- 用于取两个不同表的结果集的并集。会自动去掉重复行。
查询id小于3，或者名字为“英文”的课程
mysql> select * from course where id<3 or name = '英文';
-- or只能一个表
+----+--------------+
| id | name |
+----+--------------+
| 1 | Java |
| 2 | 中国传统文化 |
| 6 | 英文 |
+----+--------------+
mysql> select * from course where id<3 union select * from course where name = '英文';
-- union 可以合并两个不同表的结果集
-- 两个表列的个数和类型必须一致。
+----+--------------+
| id | name |
+----+--------------+
| 1 | Java |
| 2 | 中国传统文化 |
| 6 | 英文 |
+----+--------------+
- 如果不需要去重，使用union all
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