单表查询、多表查询、子查询
DML语句：
select语句
SELECT select-list FROM tb WHERE qualification;FROM子句：要查询的关系 表、多个表、其他的select语句
WHERE子句：布尔型关系表达式 ，还可以是算术运算表达式
| 符号 | 代表含义 |
|---|---|
| = | 等于 |
| <> 或者 != | 不等于 |
| <=> | 有NULL也能进行比较，等于 |
| < | 小于 |
| <= | 小于等于 |
= |
大于等于 |
| < | 小于 |
| > | 大于 |
| +、-、*、/、% | 加、减、乘、除、取余 |
| and 或者 && | 逻辑与 |
| or 或者 | |
| not 或者 ！ | 逻辑非 |
| xor | 异或 |
查询语句的类型：简单查询，多表查询，子查询
简单查询
投影：显示所有行的指定的字段（部分字段）
SELECT field1，field2 FROM tb_name;mysql> SELECT name,cid FROM student;
+------+-----+
| name | cid |
+------+-----+
| jim | 1 |
| tom | 2 |
| jack | 3 |
+------+-----+
3 rows in set (0.00 sec)
选择：显示表中与搜索码匹配的行（部分行）
SELECT * FROM tb_name WHERE qualification;mysql> SELECT * FROM student WHERE id=3;
+----+------+-----+
| id | name | cid |
+----+------+-----+
| 3 | jack | 3 |
+----+------+-----+
1 row in set (0.00 sec)
distinct 字段去重查询
查询的字段去重
mysql> select distinct gender from student;
+--------+
| gender |
+--------+
| M |
| F |
+--------+
2 rows in set (0.00 sec)
条件查询
筛选大于20岁的所有学生
mysql> select * from student where age>=20;
+----+-------+-----+--------+-----+
| id | name | cid | gender | age |
+----+-------+-----+--------+-----+
| 1 | jim | 1 | M | 21 |
| 3 | jack | 3 | M | 20 |
| 4 | lucy | 2 | F | 25 |
| 5 | neccy | 3 | F | 30 |
+----+-------+-----+--------+-----+
4 rows in set (0.00 sec)
逻辑运算符 and，or，not
筛选大于20的男性同学
mysql> select * from student where age>=20 and gender='M';
+----+------+-----+--------+-----+
| id | name | cid | gender | age |
+----+------+-----+--------+-----+
| 1 | jim | 1 | M | 21 |
| 3 | jack | 3 | M | 20 |
+----+------+-----+--------+-----+
2 rows in set (0.00 sec)
筛选年龄不大于等于20 并且为男性的同学
mysql> select * from student where not age>=20 and gender='M';
+----+------+-----+--------+-----+
| id | name | cid | gender | age |
+----+------+-----+--------+-----+
| 2 | tom | 2 | M | 19 |
+----+------+-----+--------+-----+
1 row in set (0.00 sec)between…and…. 字段
年龄在20-30 之间的同学（大于20小于等于30）
mysql> select * from student where age between 20 and 30;
+----+-------+-----+--------+-----+
| id | name | cid | gender | age |
+----+-------+-----+--------+-----+
| 1 | jim | 1 | M | 21 |
| 3 | jack | 3 | M | 20 |
| 4 | lucy | 2 | F | 25 |
| 5 | neccy | 3 | F | 30 |
+----+-------+-----+--------+-----+
4 rows in set (0.00 sec)
like操作符
| 通配符 | 含义 |
|---|---|
| % | 任意长度任意字符 |
| _ | 任意单个字符 |
查询名字以j开头的同学
mysql> select * from student where name like 'j%';
+----+------+-----+--------+-----+
| id | name | cid | gender | age |
+----+------+-----+--------+-----+
| 1 | jim | 1 | M | 21 |
| 3 | jack | 3 | M | 20 |
+----+------+-----+--------+-----+
2 rows in set (0.00 sec)
姓名中以 j 开头的三个单词组成的同学
mysql> select * from student where name like 'j__';
+----+------+-----+--------+-----+
| id | name | cid | gender | age |
+----+------+-----+--------+-----+
| 1 | jim | 1 | M | 21 |
+----+------+-----+--------+-----+
1 row in set (0.00 sec)
REGEXP、RLIKE 字段正则表达
查询姓名以 l、n、t 开头的所有同学
mysql> select * from student where name rlike '^[lnt].*';
+----+-------+-----+--------+-----+
| id | name | cid | gender | age |
+----+-------+-----+--------+-----+
| 2 | tom | 2 | M | 19 |
| 4 | lucy | 2 | F | 25 |
| 5 | neccy | 3 | F | 30 |
+----+-------+-----+--------+-----+
3 rows in set (0.00 sec)
IN 操作符
筛选年龄为19、20、30的学生
mysql> select * from student where age in (19,20,30);
+----+-------+-----+--------+-----+
| id | name | cid | gender | age |
+----+-------+-----+--------+-----+
| 2 | tom | 2 | M | 19 |
| 3 | jack | 3 | M | 20 |
| 5 | neccy | 3 | F | 30 |
+----+-------+-----+--------+-----+
3 rows in set (0.00 sec)
查询null
筛选课程二字段为空的学生
mysql> select * from student where cid2 is null;
+----+-------+-----+--------+-----+------+
| id | name | cid | gender | age | cid2 |
+----+-------+-----+--------+-----+------+
| 2 | tom | 2 | M | 19 | NULL |
| 5 | neccy | 3 | F | 30 | NULL |
+----+-------+-----+--------+-----+------+
2 rows in set (0.00 sec)
mysql> select * from student where cid2 is not null;
+----+------+-----+--------+-----+------+
| id | name | cid | gender | age | cid2 |
+----+------+-----+--------+-----+------+
| 1 | jim | 1 | M | 21 | 1 |
| 3 | jack | 3 | M | 20 | 4 |
| 4 | lucy | 2 | F | 25 | 4 |
+----+------+-----+--------+-----+------+
3 rows in set (0.00 sec)
ORDER BY filed_name [ASC|DESC]字段
将查询结果通过姓名排序，正序和反序
mysql> select * from student where cid2 is not null order by name;
+----+------+-----+--------+-----+------+
| id | name | cid | gender | age | cid2 |
+----+------+-----+--------+-----+------+
| 3 | jack | 3 | M | 20 | 4 |
| 1 | jim | 1 | M | 21 | 1 |
| 4 | lucy | 2 | F | 25 | 4 |
+----+------+-----+--------+-----+------+
3 rows in set (0.00 sec)
mysql> select * from student where cid2 is not null order by name DESC;
+----+------+-----+--------+-----+------+
| id | name | cid | gender | age | cid2 |
+----+------+-----+--------+-----+------+
| 4 | lucy | 2 | F | 25 | 4 |
| 1 | jim | 1 | M | 21 | 1 |
| 3 | jack | 3 | M | 20 | 4 |
+----+------+-----+--------+-----+------+
3 rows in set (0.00 sec)
AS ，显示结果字段重命名
mysql> select name as stu_name from student where age >=20;
+----------+
| stu_name |
+----------+
| jim |
| jack |
| lucy |
| neccy |
+----------+
4 rows in set (0.00 sec)
LIMIT ，显示指定行数
LIMIT [offiset,]count
offset:偏移量
count：显示的总个数。
mysql> select name from student where age >=20;
+-------+
| name |
+-------+
| jim |
| jack |
| lucy |
| neccy |
+-------+
4 rows in set (0.00 sec)
mysql> select name from student where age >=20 limit 2;
+------+
| name |
+------+
| jim |
| jack |
+------+
2 rows in set (0.00 sec)
mysql> select name from student where age >=20 limit 2,2;
+-------+
| name |
+-------+
| lucy |
| neccy |
+-------+
2 rows in set (0.00 sec)
聚合函数
MAX()、MIN()、SUM()、AVG()、COUNT()
显示男同学和女同学的平均年龄
mysql> select avg(age) from student where gender='M';
+----------+
| avg(age) |
+----------+
| 20.0000 |
+----------+
1 row in set (0.00 sec)
mysql> select avg(age) from student where gender='F';
+----------+
| avg(age) |
+----------+
| 27.5000 |
+----------+
1 row in set (0.00 sec)
group 分组
根据相同规则的分组，利用聚合函数
求男、女同学的平均年龄
mysql> select avg(age),gender from student group by gender;
+----------+--------+
| avg(age) | gender |
+----------+--------+
| 20.0000 | M |
| 27.5000 | F |
+----------+--------+
2 rows in set (0.00 sec)
HAVING 和GROUP BY 连用
HAVING 和GROUP BY 连用，用于过滤满足条件的内容。
打印同一门课，选修人数超过2个的选修人数以及课程ID。
mysql> select count(name) as num_person,cid from student group by cid having num_person >= 2;
+------------+-----+
| num_person | cid |
+------------+-----+
| 2 | 2 |
| 2 | 3 |
+------------+-----+
2 rows in set (0.00 sec)
多表查询
| 连接方式 | 举例 |
|---|---|
| 交叉连接 | 笛卡尔乘积（a+b）(c+d)= ac+ad + bc+bd,表A和表B的每一行都做匹配 |
| 自然连接（内连接） | 两张表中对应字段存在等值关系 |
| 外连接 | 两张表中，不是完全的等值关系，表A和表B，以某字段作为连接关系，包括左外连接和右外连接。 |
| 左外连接 | 两张表中，不是完全的等值关系，显示所有学生的选修情况，如果还没有选择，选修课为空。 … LEFT JOIN … ON … |
| 右外连接 | 两张表中，不是完全的等值关系，显示所有学生的选修情况，如果有一门课没有人选择，学生姓名显示为空 … RIGHT JOIN … ON … |
| 自连接 | 一样表，不同的字段确立等值关系。 |
mysql> select * from student;
+----+-------+-----+--------+-----+------+
| id | name | cid | gender | age | cid2 |
+----+-------+-----+--------+-----+------+
| 1 | jim | 1 | M | 21 | 1 |
| 2 | tom | 2 | M | 19 | NULL |
| 3 | jack | 3 | M | 20 | 4 |
| 4 | lucy | 2 | F | 25 | 4 |
| 5 | neccy | 3 | F | 30 | NULL |
| 6 | mary | 4 | F | 21 | 5 |
| 7 | kaka | 5 | M | 21 | 3 |
| 8 | suke | 3 | M | 20 | 5 |
+----+-------+-----+--------+-----+------+
8 rows in set (0.00 sec)
mysql> select * from courses;
+-----+------------+
| cid | course |
+-----+------------+
| 1 | Chinese |
| 2 | English |
| 3 | math |
| 4 | computer |
| 5 | biological |
+-----+------------+
5 rows in set (0.00 sec)
自然连接
显示所有同学的选修的第一门课程。
mysql> select student.name,courses.course from student,courses where student.cid = courses.cid;
+-------+------------+
| name | course |
+-------+------------+
| jim | Chinese |
| tom | English |
| lucy | English |
| jack | math |
| neccy | math |
| suke | math |
| mary | computer |
| kaka | biological |
+-------+------------+
8 rows in set (0.00 sec)
mysql> select s.name,c.course from student as s,courses as c where s.cid = c.cid;
+-------+------------+
| name | course |
+-------+------------+
| jim | Chinese |
| tom | English |
| lucy | English |
| jack | math |
| neccy | math |
| suke | math |
| mary | computer |
| kaka | biological |
+-------+------------+
8 rows in set (0.00 sec)
左外连接
表A... LEFT JOIN 表B... ON ...匹配条件显示所有学生的第二门选修课程
mysql> select s.name,c.course from student as s left join courses as c on s.cid2 = c.cid;
+-------+------------+
| name | course |
+-------+------------+
| jim | Chinese |
| tom | NULL |
| jack | computer |
| lucy | computer |
| neccy | NULL |
| mary | biological |
| kaka | math |
| suke | biological |
+-------+------------+
8 rows in set (0.00 sec)
右外连接
表A... RIGHT JOIN 表B... ON ...匹配条件显示第二门选修课程对应的人
mysql> select s.name,c.course from student as s right join courses as c on s.cid2 = c.cid;
+------+------------+
| name | course |
+------+------------+
| jim | Chinese |
| NULL | English |
| kaka | math |
| jack | computer |
| lucy | computer |
| mary | biological |
| suke | biological |
+------+------------+
7 rows in set (0.00 sec)
自连接
cid代表选修科目，cid2代表补考科目，显示选修课挂了的同学。（选修课和补考一样）
mysql> select s.id,s.name,s.cid,c.name,c.cid2 from student as s,student as c where s.cid=c.cid2 and s.name=c.name;
+----+------+-----+------+------+
| id | name | cid | name | cid2 |
+----+------+-----+------+------+
| 1 | jim | 1 | jim | 1 |
+----+------+-----+------+------+
1 row in set (0.00 sec)子查询
一个查询中嵌套另外一个查询语句
- 比较查询中使用子查询，子查询只能返回单值
- in()中的子查询的方法
- from中使用子查询
查询大于平均年龄的同学
mysql> select name,age from student where age > (select avg(age) from student);
+-------+-----+
| name | age |
+-------+-----+
| lucy | 25 |
| neccy | 30 |
+-------+-----+
2 rows in set (0.00 sec)
显示学生年龄和老师年龄相同的学生
mysql> select name from student where age in (select age from teacher);
+-------+
| name |
+-------+
| lucy |
| neccy |
+-------+
2 rows in set (0.00 sec)
查询学生中大于30岁的人
mysql> select name,age from (select name,age from student) as t where t.age >=30;
+-------+-----+
| name | age |
+-------+-----+
| neccy | 30 |
+-------+-----+
1 row in set (0.00 sec)
联合查询union
mysql> (select name,age from student) union (select name,age from teacher);
+------------+------+
| name | age |
+------------+------+
| jim | 21 |
| tom | 19 |
| jack | 20 |
| lucy | 25 |
| neccy | 30 |
| mary | 21 |
| kaka | 21 |
| suke | 20 |
| yuehan | 25 |
| tomxu | 30 |
| tangyouyou | 50 |
| caiyuanpei | 100 |
+------------+------+
12 rows in set (0.00 sec)
实例
mysql> select * from student;
+----+-------+-----+--------+-----+------+
| id | name | cid | gender | age | cid2 |
+----+-------+-----+--------+-----+------+
| 1 | jim | 1 | M | 21 | 1 |
| 2 | tom | 2 | M | 19 | NULL |
| 3 | jack | 3 | M | 20 | 4 |
| 4 | lucy | 2 | F | 25 | 4 |
| 5 | neccy | 3 | F | 30 | NULL |
| 6 | mary | 4 | F | 21 | 5 |
| 7 | kaka | 5 | M | 21 | 3 |
| 8 | suke | 3 | M | 20 | 5 |
+----+-------+-----+--------+-----+------+
8 rows in set (0.00 sec)
mysql> select * from courses;
+-----+------------+-----+
| cid | course | tid |
+-----+------------+-----+
| 1 | Chinese | 3 |
| 2 | English | 2 |
| 3 | math | 1 |
| 4 | computer | 3 |
| 5 | biological | 4 |
+-----+------------+-----+
5 rows in set (0.00 sec)
mysql> select * from teacher;
+-----+------------+------+
| tid | name | age |
+-----+------------+------+
| 1 | yuehan | 25 |
| 2 | tomxu | 30 |
| 3 | tangyouyou | 50 |
| 4 | caiyuanpei | 100 |
| 5 | hongqigong | 88 |
| 6 | huangrong | 45 |
+-----+------------+------+
6 rows in set (0.00 sec)
1.挑选出courses表中没有被students中的CID2学习的课程的课程名字
mysql> select * from courses where cid not in (select distinct cid2 from student where cid2 is not null);
+-----+---------+
| cid | course |
+-----+---------+
| 2 | English |
+-----+---------+
1 row in set (0.00 sec)
显示CID选择相同的学生大于2的学科
mysql> select course from courses where cid in (select cid from student group by cid having count(cid)>=2);
+---------+
| course |
+---------+
| English |
| math |
+---------+
2 rows in set (0.00 sec)
2.显示每一位老师以及其所有教授的课程，没有教授的课程的保持null
mysql> select t.name,c.course from teacher as t left join courses as c on c.tid=t.tid;
+------------+------------+
| name | course |
+------------+------------+
| yuehan | math |
| tomxu | English |
| tangyouyou | Chinese |
| tangyouyou | computer |
| caiyuanpei | biological |
| hongqigong | NULL |
| huangrong | NULL |
+------------+------------+
7 rows in set (0.00 sec)
3.显示每一个课程及其相关的老师，没有老师教授的课程将其老师显示为null
mysql> select t.name,c.course from teacher as t right join courses as c on c.tid=t.tid;
+------------+------------+
| name | course |
+------------+------------+
| tangyouyou | Chinese |
| tomxu | English |
| yuehan | math |
| tangyouyou | computer |
| caiyuanpei | biological |
+------------+------------+
5 rows in set (0.00 sec)
4.显示每位同学CID1课程的课程名称及其讲授相关课程的老师的名称
mysql> select obj.name,obj.course,t.name from (select s.name,c.course,c.tid from student as s,courses as c where s.cid=c.cid) as obj,teacher as t where obj.tid = t.tid;
+-------+------------+------------+
| name | course | name |
+-------+------------+------------+
| jack | math | yuehan |
| neccy | math | yuehan |
| suke | math | yuehan |
| tom | English | tomxu |
| lucy | English | tomxu |
| jim | Chinese | tangyouyou |
| mary | computer | tangyouyou |
| kaka | biological | caiyuanpei |
+-------+------------+------------+
8 rows in set (0.00 sec)
mysql> select s.name,c.course,t.name from student as s,courses as c,teacher as t where s.cid=c.cid and c.tid=t.tid;
+-------+------------+------------+
| name | course | name |
+-------+------------+------------+
| jim | Chinese | tangyouyou |
| tom | English | tomxu |
| lucy | English | tomxu |
| jack | math | yuehan |
| neccy | math | yuehan |
| suke | math | yuehan |
| mary | computer | tangyouyou |
| kaka | biological | caiyuanpei |
+-------+------------+------------+
8 rows in set (0.00 sec)