Query select

1. Grammar:

select * from 表名； //查询全部
select 列名，列名，...   from  表名;

2. Make an alias

Syntax: column name AS alias;

select  列名   from  表名   where 条件

Logical operation :

AND	  与；
OR	  或；
NOT	  非

BETWEEN ...  AND  ...  在两个值之间(包含边界)

IN(set，set，...) 	等于 值列表中的一个

% represents zero or more characters (any number of characters).

_ represents a character

limit  [offset] size;
	offset代表的是起始的条目索引，默认从0开始
	size代表的是显示的条目数

Formula: limit (current page number -1) * number per page, number per page;

ASC ascending order DESC descending order

sum summation; avg average value; max maximum value; min minimum value; count count

select 列名  from  表 子查询

The result of the subquery is used as a table, and an alias must be required

One-to-one; one-to-many; many-to-many;

Primary key PRIMARY KEY, the value of this field cannot be repeated and is not unique+not null

Foreign key FOREIGN KEY, the value of this field refers to the field of another table

外键语法：FK_引用表\_连接表

classification:

Internal connection;

Outer connection: left outer connection; right outer connection

11.1. Internal connection syntax:

select 查询列表
from 表1 ,表2 
where 表1.key=表2.key

Internal connection main table: the table after the comma

11.2. Outer join syntax:

	select 查询列表
	from 表1 left join  表2 
	on 表1.key=表2.key

Overview of external connections:

外连接，主表的数据都会显示，不论从表有没有对应的数据。

左外连接，左边的表数据都会显示。

右外连接，右边的表数据都会显示。

left join 左边的就是主表，right join 右边的就是主表

1. Do not judge the database for null (judgment of null value);

2. Comparison operations will reduce the efficiency of the database, and may cause index failure;

3. The use of left blur is generally not allowed, and the use of left blur will cause the index to become invalid;

4. When the inner connection query, the data dictionary table is the main table (the table after the comma);

5. Try not to use or.