SQL语句之DML

Posted 2020-07-13

DML：Database Manipulate Language

DML include:select,delete,insert into,update

aka == also known as

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• select
  

执行顺序

start --> [from --> [where] --> [[group by] --> [having]] --> [order by]] --> select --> [limit] --> end

select select_list from tb_name where condition;

select查询类型

1)简单查询，单表查询

2)多表查询

3)子查询，嵌套查询

所有 select * from tb_name;

选择 select * from tb_name where condition;

投影 select [distinct] field1,filed2,... from tb_name;

[distinct]去除重复选项

单表查询

• from子句，表示要查询的关系，表、多表、其他select语句
  
• where子句 
  

逻辑关系操作符 

and	&&
or	||
not	!

条件判断操作符

>,<,=,<=>,between,in,like,rkike aka regexp，is null,is not null

<=> null safe euqal to;

example

mysql> select name,age,gender from students where not age>20 and not gender=‘M‘;

mysql> select name,age,gender from students where age between 20 and 25; 连续取值

mysql> select name,age from students where age in (19,25); 离散取值

mysql> select name from students where name like ‘L%‘;

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

mysql> select name,age,gender from students where name rlike ‘^[YXN].*‘;

• order by子句,排序
  

order by field_name [ASC|DESC]

mysql> select name,age from students order by age desc;

• limit [offset,]count子句，限制显示的行数

mysql> select name as student_name,age as age_number from students order by age desc limit 2,4;

• as 字段别名
  

mysql> select name as student_name,age as age_number from students order by age desc;

聚合：sum(),min(),max(),avg(),count()

having用于将group by的结果再次进行过滤

mysql> select cid1 as course,count(cid1) as total from students group by cid1;

mysql> select cid1 as course,count(cid1) as total from students group by cid1 having total>=2;

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多表查询

连接：

• 交叉连接：笛卡尔乘积，数据量大时不宜使用
  
• 自然连接
• 外连接
  

  左外连接：... LEFT JOIN ... ON ...

  右外连接: ... RIGHT JOIN ... ON ...

• 自连接，一张表变成两张表
  

自然连接

mysql> select name,cname from students,courses where courses.cid=students.cid1;

mysql> select students.name,courses.cname from students,courses where courses.cid=students.cid1;标准写法

mysql> select s.name,c.cname from students as s,courses as c where c.cid=s.cid1; 表的别名

外连接

左外连接 mysql> select s.name,c.cname from students as s left join courses as c on c.cid=s.cid1;

右外连接 mysql> select s.name,c.cname from students as s right join courses as c on c.cid=s.cid1;

自连接

mysql> select s.name as student,t.name as teacher from students as s,students as t where t.sid=s.tid;

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子查询

1）比较操作中使用子查询：子查询只能返回单个值；

mysql> select name,age from students where age>(select avg(age) from students);

2）IN(): 使用子查询；

mysql> select name from students where age in (select age from tutors);

3）在FROM中使用子查询，可改写为多条件查询

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联合查询UNION，只做联合不做判断

mysql> (select name,age from students) union (select tname,age from tutors);

mysql> (select name,age from students) union (select tname,gender from tutors);

练习

1、挑选出courses表中没有被students中的CID2学习的课程的课程名称；

mysql> select cname from courses where cid not in (select distinct cid2 from students where cid2 is not null);

附加：挑选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；

mysql> select tname from tutors where tutors.tid not in (select distinct courses.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门课程的课程名称；

mysql> select cname from courses where courses.cid in (select cid1 from students group by cid1 having count(cid1)>=2);

2、显示每一位老师及其所教授的课程；没有教授的课程的保持为NULL；

mysql> select tname,cname from tutors left join courses on tutors.tid=courses.tid;

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空；

mysql> select tname,cname from tutors right join courses on tutors.tid=courses.tid;

4、显示每位同学CID1课程的课程名及其讲授了相关课程的老师的名称；

mysql> select name,cname,tname from students,courses,tutors where students.cid1=courses.cid and courses.tid=tutors.tid;

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view 视图 虚表  

    基于基表的存储下来的select查询语句

创建视图

CREATE VIEW view_name AS select_statement

    

mysql> create view information as select name,cname,tname from students,courses,tutors where students.cid1=courses.cid and courses.tid=tutors.tid;

删除视图

DROP VIEW view_name

说明：

由于每次查看视图都要执行select语句，由于视图没有索引，性能可能没有多少提升；但是物化视图（mysql不支持），查询速度快；同时，视图可以用在只想让用户查看某些字段，增加安全性的情况。

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insert

INSERT [LOW_PRIORITY | DELAYED | HIGH_PRIORITY] [IGNORE] [INTO] tbl_name [(col_name,...)] {VALUES | VALUE} ({expr | DEFAULT},...),(...),... [ ON DUPLICATE KEY UPDATE col_name=expr [, col_name=expr] ... ]

Or:一次插入一行指定字段的数据

INSERT [LOW_PRIORITY | DELAYED | HIGH_PRIORITY] [IGNORE] [INTO] tbl_name SET col_name={expr | DEFAULT}, ... [ ON DUPLICATE KEY UPDATE col_name=expr [, col_name=expr] ... ]

example

mysql> insert into students set name=‘Agan‘,age=26,gender=‘M‘,cid1=8,cid2 =7,tid=6,createtime=‘2016-05-26 18:01:30‘;

Or:将查询出来的数据加入表中

INSERT [LOW_PRIORITY | HIGH_PRIORITY] [IGNORE] [INTO] tbl_name [(col_name,...)] SELECT ...[ ON DUPLICATE KEY UPDATE col_name=expr[, col_name=expr] ... ]

mysql> insert into tutors (tname,gender,age) select name,gender,age from students where age > 10;

只显示最后插入的数据

mysql> select * from student order by sid desc limit 1,2;

mysql> select * from student order by sid desc limit 0,2;

显示计数器

mysql> select last_insert_id();

出现的问题:

使用 create table tb_name1 like tb_name2,若有auto_increment选项，则使用同一个计数器，两个表间的使用auto_increment字段的计数会累加

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replace

用法和insert类似

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• delete

1）使用delete时，未使用where，可以禁止此操作；

2）计数器不变，在原来基础上继续计数

mysql> delete from student order by sid desc limit 10;

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• truncate table

TRUNCATE [TABLE] tbl_name

清空表并清空auto_increment计数器

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• update
  

UPDATE tb_name SET col1=..., col2=... WHERE

来自为知笔记(Wiz)