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)
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