单表查询多表查询和子查询

Posted 2020-08-12

查询语句：

select SELECT-LIST from  TB where QYAKUDUCATION;

简单查询：

select * from TB_NAME;

select FIED1,FIED2 from TB_NAME; 投影

select [distinct] * from TB_NAME where QYAKUDUCATION;选择  #distinct重复的值只显示一次

from子句：要查询的关系   表，多个表，其他的select语句

where子句：指定布尔关系表达式，

= ， >   , >=  ,  <=  , <

<> 不等于

<=> 完全不等于，NULL值也可以进行比较

也可以使用算数表达式

mysql> select NAME  from students where AGE+1>20

与或非关系: 

and   &&

or    || 

not    !

xor 异或

between ... and ...  在谁和谁之间

like ‘‘ 比较操作

    %：任意长度。任意字符

    _：任意单个字符

regexp或者 rlike支持正则表达式

in (  ) 列表

is null 判断是否为空

is not null 判断是否不空

where条件应用：

mysql> select NAME  from students where AGE>20 and GENDER=‘M‘;

mysql> select NAME  from students where AGE>20 or GENDER=‘M‘;

mysql> select NAME  from students where not AGE>20;

查询年龄不大于20并且不为男性的人

mysql> select NAME  from students where not AGE>20 and not GENDER=‘M‘;

mysql> select NAME  from students where not (AGE>20 or  GENDER=‘M‘);

查询年龄20-25之间的所有人

mysql> select NAME  from students where AGE>=20 and AGE<=25;

mysql> select NAME  from students where  AGE between 20 and 25;  

  

查询名字为Y开头的所有人

mysql> select NAME  from students where  NAME like ‘Y%‘;

查询Y开头。后面跟四个字符的人名

mysql> select NAME  from students where  NAME like ‘Y____‘;

查询整个名字中 包含 ing字符串的人名

mysql> select NAME  from students where  NAME like ‘%ing%‘;

查询用户名称中以大写字母M N  Y开头的

mysql> select NAME  from students where  NAME like ‘M%‘ or  NAME like ‘N%‘ or NAME like ‘Y%‘;

mysql> select NAME  from students where  NAME rlike ‘^[MNY].*$‘;

查询年龄为18 20 25的姓名

mysql> select NAME  from students where  AGE=18  or AGE=20 or AGE=25;

mysql> select NAME  from students where  AGE in (18,20,25);

查询CID2为空的

mysql> select NAME  from students where  CID2 is NULL;

查询CID2不空的

mysql> select NAME  from students where  CID2 is NOT NULL; 

排序：order by FIELD_NAME {ASC|DESC} 默认是升序

根据姓名进行排序：升序

mysql> select NAME  from students where  CID2 is NOT NULL order by NAME;

降序

mysql> select NAME  from students where  CID2 is NOT NULL order by NAME desc;

给字段取别名：AS

mysql> select NAME AS Student_Name  from students;

直接进行算数运算

mysql> select 2+1 AS SUM; 

+-----+

| SUM |

+-----+

|   3 |

+-----+

1 row in set (0.00 sec)

limit N: 只显示查询到的前N个

limit N,X: 查询到的前N个不显示。从N+1个开始显示，共显示X个

mysql> select NAME AS Student_Name  from students limit 2;

mysql> select NAME AS Student_Name  from students limit 2，3;

聚合计算：

avg()平均数

max()

min()

sum()

count() 一共有多少个

求所有同学的平均年龄

mysql>select avg(AGE) from students;

查找年龄最大的同学

mysql>select max(AGE) from students;

查找年龄最小的同学

mysql>select min(AGE) from students;

查找年龄之和

mysql>select sum(AGE) from students;

查找一共有多少人

mysql>select count(AGE) from students;

group by 分组:

    having  QUALIFICATION  过滤条件

查找男同学和女同学的平均年龄

mysql>select AGE,Gender from  students group by Gender;

mysql>select avg(AGE) from students group by Gender;

统计选择CID1课程的同学的个数

mysql>select count(CID1) AS Persons,CID1 from students group by CID1;

显示哪些课程人数大于2个人的  这里必须用having而不能使用where

mysql>select count(CID1) AS persons,CID1 from students group by CID1 having Persons>=2;

多表查询：

 连接：

    交叉连接： 笛卡尔乘积

    自然连接：仅保留具有等值关系的

    外连接：用于显示查询包含NULL

       左外连接: ...left join ...on... 

       又外连接: ... right join ...on ...

    自连接：

自然连接：

mysql>select students.NAME,courses.CNAME from students,courses where students.CID1 = courses.CID;

mysql>select s.NAME,c.CNAME from students AS s,courses AS c where s.CID1 = c.CID;

外连接：

显示同学对应的课程，有人选就显示有。没有人选就显示人物对应的课程为NULL

select s.NAME, c.CNAME  from students as s left join   as c on s.CID1=c.CID;

显示同学对应的课程，有人选就显示。没有人选就显示课程对应的人名为NULL

select s.NAME, c.CNAME  from students as s right join as c on s.CID1=c.CID;

自连接：

select c.NAME as teacher,  s.NAME as stu from  stufents as s,students as c where c.TID=s.CID;

子查询： 

    比较操作中使用子查询：子查询只能返回一个单值 

    in()：使用子查询 

    在from中使用子查询 

联合查询： 

    union   

DISTINCT：选择唯一的结果集

                                  

找students表中年龄大雨平均年龄的同学

select NAME from students  where AGE > (select avg(AGE) from students);

找到同名和老师年龄一样的

select NAME from students  where AGE in (select AGE from tutors )；

在from中使用子查询 

select NAME ,AGE from (select NAME,AGE from students) as t where t.AGE  >=20;

union把2个查询语句的结果联合起来

（select NAME，AGE from students ）union (select TNAME ,AGE from tutors);

找出CID2中没有同学选用的课程号对应的课程名

select Cname,CID2 from ( select c.Cname,s.CID2 from students as s , courses as c where s.TID=c.TID ) as d  where d.CID2 is NULL;

挑出没有教授任何课程的老师，每个老师及其所教授课程的对应关系在courses表中

select Tname from tutors where TID not in ( select distinct TID from courses );

找出students表中CID1有两个或两个以上同学学习了的同一个课程的课程名称

select Cname from courses where CID in (select CID1 from students  group by CID1  having count (CID1) >=2);

视图：存储下来的select语句

基于基表的查询视图

view

mysql> help create view

CREATE

    [OR REPLACE]

    [ALGORITHM = {UNDEFINED | MERGE | TEMPTABLE}]

    [DEFINER = { user | CURRENT_USER }]

    [SQL SECURITY { DEFINER | INVOKER }]

    VIEW view_name [(column_list)]

    AS select_statement

    [WITH [CASCADED | LOCAL] CHECK OPTION]

create view sct as    select NAME from students  where AGE in (select AGE from tutors )；

show tables会发现select的变成了一张表

select * from sct;  

mysql> help drop view

DROP VIEW [IF EXISTS]

    view_name [, view_name] ...

    [RESTRICT | CASCADE]

show create view sct\G 可以显示创建sct视图所使用的完整语句

show create table crouses; 查看创建crouses表的完整语句

# mysql -e ‘create database edb;‘可以直接往mysql里创建数据库

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