[MySQL] limit implements paging

Pagination

1. background ：

background 1： The query returned too many records , It's inconvenient to check , How to implement paging query ？

background 2： There are 4 Data , If you only want to display 2、3 What about this data ？

2. Implementation rules

• Pagination principle
  Pagination display , The result set in the database , Show the required conditions paragraph by paragraph .

• MySQL Use in LIMIT Implement paging

• Format ：

  LIMIT [ Position offset ,]  Row number 
  

• first “ Position offset ” Parameters indicate MySQL Which line to start with , Is an optional parameter , If you don't specify “ Position offset ”, It will start with the first record in the table （ The position offset of the first record is 0, The position offset of the second record is 1, And so on ）; The second parameter “ Row number ” Indicates the number of records returned .

• give an example

-- front 10 Bar record ：
SELECT * FROM  Table name  LIMIT 0,10;
 perhaps 
SELECT * FROM  Table name  LIMIT 10;

-- The first 11 to 20 Bar record ：
SELECT * FROM  Table name  LIMIT 10,10;

-- The first 21 to 30 Bar record ： 
SELECT * FROM  Table name  LIMIT 20,10;

• Use limit Realize pagination display of data

• demand 1： Each page shows 5 Bar record , The... Is displayed 1 page

SELECT employee_id,last_name
FROM employees
LIMIT 0,5;

• demand 2： Each page shows 6 Bar record , The... Is displayed 2 page

SELECT employee_id,last_name
FROM employees
LIMIT 5,6;

• demand 3： Each page shows 7 Bar record , The... Is displayed 3 page

SELECT employee_id,last_name
FROM employees
LIMIT 6,7;

• demand 4： Each page shows pageSize Bar record , The... Is displayed pageNo page ：
• The formula ：

LIMIT (pageNo-1) * pageSize, pageSize;

• Paging explicit formula ：（ The current number of pages -1）* Number of entries per page , Number of entries per page

SELECT * FROM table 
LIMIT(PageNo - 1)*PageSize, PageSize;

• Be careful ：LIMIT Clause must be placed throughout SELECT At the end of the sentence ！

• Use LIMIT The benefits of ：
  The number of results returned by the constraint can Reduce the network transmission of data tables , It's fine too Improve query efficiency . If we know that the return result is only 1 strip , You can use LIMIT 1, tell SELECT Statement only needs to return a record . The good thing is SELECT There is no need to scan the complete table , You only need to retrieve a qualified record to return .

• WHERE ... ORDER BY ...LIMIT The order of declaration is as follows ：

• LIMIT The format of ： Strictly speaking ：LIMIT Position offset , Number of entries

• structure "LIMIT 0, Number of entries " Equivalent to “LIMIT Number of entries ”

SELECT employee_id,last_name,salary
FROM employees
WHERE salary > 6000
ORDER BY salary DESC
#limit 0,10;
LIMIT 10;

• practice ： There are 107 Data , If you only want to display 32、33 What about this data ？

SELECT employee_id,last_name
FROM employees
LIMIT 31,2;

MySQL 8.0 Can be used in “LIMIT 3 OFFSET 4”, It means to get from 5 A record starts after 3 Bar record , and “LIMIT 4,3;” The results returned are the same .

• MySQL8.0 New characteristics ：LIMIT ... OFFSET ...

• practice ： There are 107 Data , If you only want to display 32、33 What about this data ？

SELECT employee_id,last_name
FROM employees
LIMIT 2 OFFSET 31;

• practice ： Query the information of the highest paid employee in the employee table

SELECT employee_id,last_name,salary
FROM employees
ORDER BY salary DESC
#limit 0,1
LIMIT 1;

3. expand

• LIMIT It can be used in MySQL、PGSQL、MariaDB、SQLite And so on , Represents paging . Cannot be used in SQL Server、DB2、Oracle in .

• In different DBMS The keywords used in may be different . stay MySQL、PostgreSQL、MariaDB and SQLite Use in LIMIT keyword , And it needs to be put in SELECT At the end of the statement .

• If it is SQL Server and Access, Need to use TOP keyword , such as ：

SELECT TOP 5 last_name, employee_id 
FROM employees
ORDER BY employee_id DESC

• If it is DB2, Use FETCH FIRST 5 ROWS ONLY Such keywords ：

SELECT last_name, employee_id 
FROM employees 
ORDER BY employee_id DESC 
FETCH FIRST 5 ROWS ONLY

• If it is Oracle, Need to be based on ROWNUM To count the number of lines ：
  Such as ： Before query 10 Bar record

SELECT rownum, employee_id, last_name
FROM employees
where rownum<= 10;

SELECT rownum,last_name,salary 
FROM employees 
WHERE rownum <= 5 ORDER BY salary DESC;

4. practice

1. Check the employee's name, department number and annual salary , In descending order of annual salary , Display by name in ascending order

SELECT last_name,department_id,salary * 12 annual_salary
FROM employees
ORDER BY annual_salary DESC,last_name ASC;

2. The choice of salary is not in 8000 To 17000 The name and salary of the employee , In descending order of wages , According to the first 21 To 40 Location data

SELECT last_name,salary
FROM employees
WHERE salary NOT BETWEEN 8000 AND 17000
ORDER BY salary DESC
LIMIT 20,20;

3. Query mailbox contains e Employee information , And first by the number of bytes in the mailbox , And then in ascending order by department number

SELECT employee_id,last_name,email,department_id
FROM employees
#where email like '%e%'
WHERE email REGEXP '[e]'
ORDER BY LENGTH(email) DESC,department_id;