5、MySQL多表查询

上一章内容简单介绍了对单个数据库表的操作，本章内容将详细介绍关于多表查询的方法和技乔，以及关于MySQL查询语句中关于的一些函数的用法。
一、多表组合查询
1.1 外连接
SQL语句可以通过外连接的方式对多表进行联合查找，外连接右分为做链接和有链接之分，其使用方法如下；
左外连接

语法：select 字段 from tb_a left join tb_b on 链接条件

注：tb_a表是主表，tb_b是从表，其中tb_a作为主表显示全部内容，若表中无数据显示则显示为null。
右链接
语法：select 字段 from tb_a right join tb_b on 条件
注：tb_a表是从表，tb_b则为主表，都显示。
例：首先创建数据库jiaowu，并创建学生表和成绩表

mysql> create database jiaowu;
mysql> create table student(sid int(10),name varchar(48),id int(11));
mysql> create table grade(sid int(10),score int(5));
mysql> insert into student values(1,'孙悟空',1),(2,'猪八戒',2),(3,'沙悟净',3),(4,'小白龙',4),(5,'唐三藏',5),(6,'红孩儿'，6),(7,'哪吒',7););
mysql>  insert into  grade(sid,score) values(1,1234),(2,1235),(4,1423),(5,1120),(6,1354),(6,1367);

mysql> select * from student  left join grade on student.sid=grade.sid;
+------+-----------+------+------+-------+
| sid  | name      | id   | sid  | score |
+------+-----------+------+------+-------+
|    1 | 孙悟空    |    1 |    1 |  1234 |
|    2 | 猪八戒    |    2 |    2 |  1235 |
|    4 | 小白龙    |    4 |    4 |  1423 |
|    5 | 唐三藏    |    5 |    5 |  1120 |
|    6 | 红孩儿    |    6 |    6 |  1354 |
|    6 | 红孩儿    |    6 |    6 |  1367 |
|    3 | 沙悟净    |    3 | NULL |  NULL |
|    7 | 哪吒      |    7 | NULL |  NULL |
+------+-----------+------+------+-------+
8 rows in set (0.00 sec)

使用别名查询

mysql> select * from student as s left join grade as g on s.sid=g.sid;
mysql> select * from grade as g right join student as s on g.sid=s.sid;
mysql> select * from grade as g left join student as s on g.sid=s.sid;
+------+-------+------+-----------+------+
| sid  | score | sid  | name      | id   |
+------+-------+------+-----------+------+
|    1 |  1234 |    1 | 孙悟空    |    1 |
|    2 |  1235 |    2 | 猪八戒    |    2 |
|    4 |  1423 |    4 | 小白龙    |    4 |
|    5 |  1120 |    5 | 唐三藏    |    5 |
|    6 |  1354 |    6 | 红孩儿    |    6 |
|    6 |  1367 |    6 | 红孩儿    |    6 |
+------+-------+------+-----------+------+
6 rows in set (0.00 sec)

三张表如何链接
创建成绩表grade2

mysql> create table grade2(sid int(10),score int(5));
Query OK, 0 rows affected (0.03 sec)
mysql> insert into  grade2(sid,score) values(1,1234),(2,1235),(4,1423),(5,1120),(6,1354),(6,1367);
Query OK, 6 rows affected (0.00 sec)
Records: 6  Duplicates: 0  Warnings: 0
mysql> show tables;
+----------------+
| Tables_in_book |
+----------------+
| books          |
| category       |
| grade          |
| grade2         |
| student        |
| tbdate         |
+----------------+
6 rows in set (0.00 sec)

mysql> select s.*,g1.*,g2.* from student as s right join grade as g1 on s.sid=g1.sid right join grade2 as g2 on s.sid=g2.sid;
+------+-----------+------+------+-------+------+-------+
| sid  | name      | id   | sid  | score | sid  | score |
+------+-----------+------+------+-------+------+-------+
|    1 | 孙悟空    |    1 |    1 |  1011 |    1 |  1234 |
|    2 | 猪八戒    |    2 |    2 |  1012 |    2 |  1235 |
|    4 | 小白龙    |    4 |    4 |  1162 |    4 |  1423 |
|    5 | 唐三藏    |    5 |    5 |   920 |    5 |  1120 |
|    6 | 红孩儿    |    6 |    6 |  1107 |    6 |  1354 |
|    6 | 红孩儿    |    6 |    6 |  1107 |    6 |  1367 |
|    6 | 红孩儿    |    6 |    6 |  1118 |    6 |  1354 |
|    6 | 红孩儿    |    6 |    6 |  1118 |    6 |  1367 |
+------+-----------+------+------+-------+------+-------+
或者：
mysql> select s.*,g1.*,g2.* from student s,grade g1,grade2 g2 where s.sid=g1.sid and g1.sid=g2.sid;
mysql> select s.*,g1.score,g2.score from student s,grade g1,grade2 g2 where s.sid=g1.sid=g2.sid;
mysql> select s.*,g1.score,g2.score from student s,grade g1,grade2 g2 where s.sid=g1.sid=g2.sid;
+------+-----------+------+-------+-------+
| sid  | name      | id   | score | score |
+------+-----------+------+-------+-------+
|    1 | 孙悟空    |    1 |  1011 |  1234 |
|    2 | 猪八戒    |    2 |  1012 |  1234 |
|    4 | 小白龙    |    4 |  1162 |  1234 |
|    5 | 唐三藏    |    5 |   920 |  1234 |
|    6 | 红孩儿    |    6 |  1107 |  1234 |
|    6 | 红孩儿    |    6 |  1118 |  1234 |
+------+-----------+------+-------+-------+
6 rows in set (0.00 sec)

二、MySQL下的聚合函数
函数：被封装成特定功能的代码块
2.1 求和函数
查看总分数

mysql> select sum(score) from grade;
+------------+
| sum(score) |
+------------+
|       7733 |
+------------+
1 row in set (0.01 sec)

2.2 查看平均分

mysql> select avg(score) from grade;
+------------+
| avg(score) |
+------------+
|  1288.8333 |
+------------+
1 row in set (0.00 sec)

2.3 查看最高分数

mysql> select max(score) from grade;
+------------+
| max(score) |
+------------+
|       1423 |
+------------+
1 row in set (0.00 sec)

2.4 查看最高分数及其对应的sid

mysql> select sid,score from grade where score=(select max(score) from grade);
+------+-------+
| sid  | score |
+------+-------+
|    4 |  1423 |
+------+-------+

2.5 查看最低分及对应的sid；

mysql> select sid,score from grade where score=(select min(score) from grade);
+------+-------+
| sid  | score |
+------+-------+
|    5 |  1120 |
+------+-------+
1 row in set (0.00 sec)

2.6 统计分数大于1300的人数

mysql> select count(*) from grade where score > 1300;
+--------------+
| count(score) |
+--------------+
|            3 |
+--------------+
1 row in set (0.00 sec)

2.7 算术运算
由于算法不同，没人在原本基础上各加30分；

mysql> update grade set score=score+30;
Query OK, 6 rows affected (0.01 sec)
Rows matched: 6  Changed: 6  Warnings: 0
mysql> select * from grade;
+------+-------+
| sid  | score |
+------+-------+
|    1 |  1264 |
|    2 |  1265 |
|    4 |  1453 |
|    5 |  1150 |
|    6 |  1384 |
|    6 |  1397 |
+------+-------+
6 rows in set (0.00 sec)

分数设置为原来的80%

mysql> update grade set score=score*0.8;

2.8 字符串函数

substr(string,start,len)截取：从start开始，长度为len，。start从1开始算。
mysql> select substr(name,1,2) from student where sid=1;
+------------------+
| substr(name,1,2) |
+------------------+
| 孙悟             |
+------------------+
1 row in set (0.00 sec)
concat(str1,str2,str3,...)拼接
mysql> select concat(sid,name,id) from student;
+---------------------+
| concat(sid,name,id) |
+---------------------+
| 1孙悟空1            |
| 2猪八戒2            |
| 3沙悟净3            |
| 4小白龙4            |
| 5唐三藏5            |
| 6红孩儿6            |
| 7哪吒7              |
+---------------------+
7 rows in set (0.00 sec)

大小写切换

mysql> select upper(name) from student where sid=10;
+------------------+
| upper(name)      |
+------------------+
| CAPTAION AMERICA |
+------------------+
1 row in set (0.00 sec)

将大写字母改成小写

mysql> select lower(name) from student where sid=9;
+-------------+
| lower(name) |
+-------------+
| green giant |
+-------------+
1 row in set (0.00 sec)

日期查询

mysql> select curdate(),now(),curtime();
+------------+---------------------+-----------+
| curdate()  | now()               | curtime() |
+------------+---------------------+-----------+
| 2017-11-17 | 2017-11-17 00:12:42 | 00:12:42  |
+------------+---------------------+-----------+
1 row in set (0.00 sec)
mysql> create table tbdate(name char(13),birthday date);
Query OK, 0 rows affected (0.04 sec)

创建日期及使用

mysql> insert into tbdate values('HA',now());
Query OK, 1 row affected, 1 warning (0.00 sec)
mysql> select * from tbdae;
ERROR 1146 (42S02): Table 'book.tbdae' doesn't exist
mysql> select * from tbdate;
+------+------------+
| name | birthday   |
+------+------------+
| HA   | 2017-11-17 |
+------+------------+
1 row in set (0.00 sec)