MySQL learning 03

Sorting and paging operations

1. Sort
The default order of query in the database is the order of data addition
Use order by Sort the queried data
Before ordering ：

Ascending sort （ASC）：
order by What do you want to sort

SELECT * FROM list
ORDER BY salary;

null （DESC）：
order by What do you want to sort DESC

SELECT * FROM list
ORDER BY salary DESC;

Be careful ：
1. Column aliases can only be used in ORDER BY Use in , Can't be in WHERE Use in , Otherwise, an error will be reported .
2. When inquiring WHERE Should and FROM together , After use ORDER BY, That is, select first and then sort .
2. paging
Why do I need paging ？

The above is the page of a website .

How to perform paging operation ？
mysql Use limit To display the data in pages .
Usage method ：limit Offset start point , Offset ;
Need one ： Twenty records per page , The first page is displayed

Demand two ： Twenty records per page , The second page... Is displayed

summary ：

expand ：

Multi-table query

Find the table associated with it through the content of a table, and then find the data you want , Here's the picture
The arrow points to .

Single selected execution , Slow query time , The efficiency is not high , We should exchange information on the Internet many times . Here's the picture ：

How to query multiple tables ？

By default, the data of two tables are Cartesian multiplied , Each data will be paired with the data of another table one by one .
The reason for the error ： The connection condition of multiple tables is missing .
Correct implementation

Connection conditions are required .

Equivalent connection and self connection

Classification of multi table query ：

Non equivalent connection ： Simply put, it's a range , I don't know the specific data

Self connection is the data connection in your own table
As the name suggests, non self connection is the data connection with other tables

SQL92 And 99 Syntax realizes inner connection and outer connection

Internal connection and external connection ：

The left outer join ： In addition to the rows that meet the conditions, the two tables also return the rows that do not meet the conditions in the left table .
Right connection ： In addition to the rows that meet the conditions, the two tables also return the rows that do not meet the conditions in the right table .

MySQL I won't support it SQL92 The external connection in grammar ：

SQL99 grammar
use join…on To realize internal connection

The left outer join ：LEFT OUTER JOIN…ON

Right connection ：RIGHT OUTER JOIN…ON
Full outer join ：MYSQL Full external connection is not supported , grammar ：FULL OUTER JOIN…ON.