MYSQL入门

1.字段类型

• 数值类型：
  int 整型
  long 长整型
  bigint
  float 单精度
  double 双精度
  decimal 小数 跟钱有关

• 字符串
  char 字符
  0-255 长度 自动补齐
  varchar 字符串 变长
  text 文本

• 日期：
  date 日期 YYYY-MM-DD
  time 时间 HH:mm:SS
  datetime 年月日时分秒 YYYY-MM-DD HH:mm:SS
  timestamp 年月日时分秒【时间戳】 YYYY-MM-DD HH:mm:SS

2.sql类型

ddl 数据定义语言：create drop
dml 数据操作语言：select insert update delete
dcl 数据控制语言：grant

3.基本语法【sql】

• 1.创建数据库
  CREATE {DATABASE | SCHEMA} [IF NOT EXISTS] db_name
  create database gh;

• 2.查看数据库
  show databases;

• 3.使用数据库 [切换数据库] default
  use gh;

• 4.创建表
  CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name
  (create_definition,...)
  create_definition: {
  col_name data_type
  }
  create table student(
  id int(11) not null auto_increment,
  name varchar(255),
  age int(3),
  create_user varchar(255),
  create_time timestamp not null default current_timestamp,
  update_user varchar(255),
  update_time timestamp not null default current_timestamp on update current_timestamp,
  primary key(id)
  );
  要求：规范
  1.表名字 不要写中文
  2.建表风格：
  1.id这个字段 一般是自增主键
  2.

• 5.查看表
  show tables;

• 6.插入数据
  INSERT
  [INTO] tbl_name
  [(col_name [, col_name] ...)]
  { {VALUES | VALUE} (value_list) [, (value_list)] ... }
  insert into gh.student (name,age) VALUES ("haoge",21),("bangzhang",20);
  insert into gh.student (name,age) VALUES ("zuoshao",22);
  insert into gh.student VALUES 全部指定;
  insert into gh.student (name,age) VALUES ("zuoshao",22);
  update gh.student set name="左少" where name="zuoshao";
  insert into gh.student (name,age) VALUES ("浩哥",22);

• 7.查看数据
  select 语法结构：
  select 字段名字 from 表名字 [where]
  * 表示 显示表里面所有字段
  select * from gh.student;

• 8.更新数据
  UPDATE table_reference
  SET assignment_list
  [WHERE where_condition]
  update gh.student set age=80 where name="bangzhang";

• 9.删除数据
  DELETE FROM tbl_name
  [WHERE where_condition]
  delete from gh.student where name="banzhang";

其他语法：

• 1.条件过滤 where
  =
  >
  <
  and
  or
  in
  not in
  select * from gh.student where age <20;
  select * from gh.student where age=20 and name="zuoshao" ;
  select * from gh.student where age=20 or name="zuoshao" ;
  select * from gh.student where age in (20,12,11,99) ;
  select * from gh.student where age not in (20,12,11,99) ;

• 2.排序语法
  order by
  select * from gh.student order by age asc;
  select * from gh.student order by age desc;
  select * from gh.student order by age desc,name desc;

• 3.模糊查询
  like ：
  1.%
  2._ 占位符
  regexp: 【正则表达式】【课后作业】
  select * from gh.student where name like 'z%' ;
  select * from gh.student where name like '%z%' ;
  select * from gh.student where name like '%z' ;
  先思考： name 第三个字母是n
  select * from gh.student where name like '__n%' ;

• 4.合并表
  union 去重
  union all 不去重

  create table a(id int ,name varchar(255));
  create table b(id int ,name varchar(255));
  insert into a VALUES(1,'左少');
  insert into b VALUES(1,'左少');
  insert into b VALUES(1,'左少');
  select * from a
  union
  select * from b;

  select * from a
  union all
  select * from b

• 5.null
  "" '' 'null' null
  1.过滤：
  is
  is not
  select * from student where create_user is null;

  2.数据清洗（etl）
  脏数据 =》 规范的数据
  函数 :ifnull ,coalesce
  null -> --

select 
id
,name    
,age 
,ifnull(create_user,"--") as elt_create_user
,create_time        
, ifnull(update_user,"--")  as elt_update_user
,update_time
from gh.student ;

select 
id
,name    
,age 
,coalesce(create_user,"--") as elt_create_user
,create_time        
, coalesce(update_user,"--")  as elt_update_user
,update_time
from gh.student ;

• 6.聚合语法 【聚合函数】
  1.聚合函数
  sum
  count
  max
  min
  avg

  聚合函数:多行数据 按照一定规则【聚合函数】 聚合为一行
  理论上说：
  聚合后的行数 <=聚合前的行数

  2.group by 【分组】

  select avg(age) from student; //整个班 平均年龄
  需求：
  1.求每个name 的平均年龄
  name ：zuoshaoxx
  name：其他

        select 
        'zuoshao' as name_1,
        avg(age) as avg_age
        from student 
        where 
        name like "%zuoshao%"
        group by name_1

        union all 

        select 
        name,
        avg(age) as avg_age
        from student 
        where 
        name not like "%zuoshao%"
        group by name;

if(条件，A ， B)条件成立结果是A，否则是B

        select 
        if(name like "%zuoshao%","zuoshao",name) as name_1,
        avg(age) as avg_age
        from student 
        group by name_1;

case when
条件 then A
else B end
条件成立结果是A，否则是B

        select 
        case when 
            name like "%zuoshao%" then "zuoshao" 
            else name end as name_1,
        avg(age) as avg_age
        from student 
        group by name_1;

2求每个name 的平均年龄

            select 
            name,
            avg(age) as avg_age
            from student
            group by 
            name;

3. 查询 平均年龄 20
   聚合之后 过滤不能用where having

select 
case when 
name like "%zuoshao%" then "zuoshao" 
else name end as name_1,
avg(age) as avg_age
from student 
group by name_1
having avg_age>20;

select 
name_1 as name ,
round(avg_age,2) as age 
from 
(
    select 
    case when 
    name like "%zuoshao%" then "zuoshao" 
    else name end as name_1,
    avg(age) as avg_age
    from student 
    group by name_1
) as tmp
where 
avg_age >20;

4.子查询：
sql 套sql

统计表中name字段数据条数【name不能重复】

• 1.select count(distinct name) as cnt from student;

• 2.使用group by 执行效率高
  select
  count(name) as cnt
  from
  (
  select
  name
  from student
  group by name
  ) as a;

group by :
x,1
y,1
z,1
x,1
=> 分组+聚合函数 =》 “拉倒一起” “去做一些事情””
“拉倒一起”： group by
x,<1,1>
y,<1>
z,<1>
“去做一些事情””avg
x,<1,1> => x,(1+1) /2
y,<1> => y, 1/1

子查询模板：

    select 

    from 
    (
        select 

        from 
    ) as a ; ***

    select 

    from  xx 
    where 
    column in (select id from xxxx );

• 7.join（多表联查）
  种类：7大类
  create table a1(id int ,name varchar(255),address varchar(255));
  create table b1(id int ,name varchar(255),age int(3));
  insert into a1 VALUES(1,'zuoshao','长春');
  insert into a1 VALUES(2,'haoge','苏州');
  insert into a1 VALUES(4,'banzhang','山东');
  insert into b1 VALUES(1,'zuoshao',21);
  insert into b1 VALUES(2,'haoge',21);
  insert into b1 VALUES(3,'xuanxuan',29);
  sql 7 join:
  1.内连接 inner join (join)

select 
a.*,
b.*
from a1 as a join b1 as b 
on a.id = b.id and a.name=b.name;
+------+---------+---------+------+---------+------+
| id   | name    | address | id   | name    | age  |
+------+---------+---------+------+---------+------+
|    1 | zuoshao | 长春    |    1 | zuoshao |   21 |
|    2 | haoge   | 苏州    |    2 | haoge   |   21 |
+------+---------+---------+------+---------+------+

select 
a.id,
a.name,
a.address,
b.age
from a1 as a join b1 as b 
on a.id = b.id and a.name=b.name;
+------+---------+---------+------+
| id   | name    | address | age  |
+------+---------+---------+------+
|    1 | zuoshao | 长春    |   21 |
|    2 | haoge   | 苏州    |   21 |
+------+---------+---------+------+

2.左连接 left join
以左表为主，数据是全的 ，右表来匹配，匹配不上就是
null

select 
a.*,
b.*
from a1 as a left join b1 as b 
on a.id = b.id and a.name=b.name;
+------+----------+---------+------+---------+------+
| id   | name     | address | id   | name    | age  |
+------+----------+---------+------+---------+------+
|    1 | zuoshao  | 长春    |    1 | zuoshao |   21 |
|    2 | haoge    | 苏州    |    2 | haoge   |   21 |
|    4 | banzhang | 山东    | NULL | NULL    | NULL |
+------+----------+---------+------+---------+------+

3.右连接 right join
以右表为主，数据是全的 ，左表来匹配，匹配不上就是 null

select 
a.*,
b.*
from a1 as a right join b1 as b 
on a.id = b.id and a.name=b.name;
+------+---------+---------+------+----------+------+
| id   | name    | address | id   | name     | age  |
+------+---------+---------+------+----------+------+
|    1 | zuoshao | 长春    |    1 | zuoshao  |   21 |
|    2 | haoge   | 苏州    |    2 | haoge    |   21 |
| NULL | NULL    | NULL    |    3 | xuanxuan |   29 |
+------+---------+---------+------+----------+------+

4.全连接 full outer join
全连接表都为主，数据是全的
左表来匹配，匹配不上就是 null
右表来匹配，匹配不上就是 null

左连接: 99.99% inner join: 00.1%
主表 维表 =》 事实表 维度表

维度组合：
通过维度去分析指标
维度：column
指标：sum max min count

三个维度：
id | name | age

指标 ： count条数

select
id,
name,
age,
count(1) as cnt
from student
group by
id,
name,
age

注意：
select 字段、指标【聚合函数】 from xx where
group by 字段

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