MySQL - database query - sort query, paging query

Sort query

Through the conditional query statement, you can query the data that meets the needs of users , However, the queried data is generally displayed in the order in which the data was initially added to the table , What is added first is displayed first , Added after the display . In order to make the order of query results meet the requirements of users ,MySQL Provides ORDER BY Keyword to sort the query results .

In practical applications, it is often necessary to sort the query results , such as , When shopping online , The goods can be priced 、 Sort by quantity ; In the hospital registration system , You can sort according to the order of registration .

grammar

SELECT  Field name  FROM  Table name  ... ORDER BY  Sort field names  [ASC|DESC],[ Sort field names  [ASC|DESC]];

Syntax description

• Sort field names ： Indicates the name of the field to be sorted , If there are multiple fields separated by commas .

• Sort rule ：ASC|DESC：ASC Indicates that the fields are sorted in ascending order ;DESC Indicates that the fields are sorted in descending order . among [ ] Represents optional keywords ASC As the default value , The default is in ascending order .

characteristic

• ORDER BY Clauses are usually placed at the end of the query statement ,LIMIT Except for words .

• When there is a null value in the sorted field NULL when ,ORDER BY The null value will be treated as the minimum value .

• ORDER BY When specifying multiple fields to sort ,MySQL It will be sorted from left to right according to the order of fields . for example , At first according to job Sort , When job When there are duplicate values , It will be sorted by the next specified field

• Be careful ： When sorting multiple fields , The first field of sorting must have the same value , To sort the second field . If all values in the first field data are unique ,MySQL The second field will no longer be sorted .

Query requirement

1. Sort employees in ascending order by name

SELECT * FROM emp ORDER BY ename ASC;

2. The query entry time is greater than 1981 year 6 month 6 Daily employee information , And in ascending order of employment time

-- SQL surface  --

--  The query entry time is greater than 1981 year 6 month 6 Daily employee information 

SELECT * FROM emp WHERE hiredate>'1981-6-6';

--  The query entry time is greater than 1981 year 6 month 6 Daily employee information , And in ascending order of employment time 

SELECT * FROM emp WHERE hiredate>'1981-6-6' ORDER BY hiredate ASC;

3. According to the job position , In ascending order , With the same position , Then arrange them in descending order according to the employment time

Sort a field first , If the first sorting field is repeated , Just sort by the second field

-- SQL surface  --

--  Arrange in ascending order according to the job position 
SELECT * FROM emp ORDER BY job ASC;

--  Arrange in ascending order according to the job position , In the same position , Then arrange them in descending order according to the employment time   Separate the fields to be sorted with commas 
SELECT * FROM emp ORDER BY job ASC,hiredate DESC;

4. The salary inquiry is not available 1000 To 2000 The name and salary of the employee between , Wages are in descending order

-- SQL surface  --

--  The salary inquiry is not available 1000 To 2000 The name and salary of the employee between 
SELECT ename,sal FROM emp WHERE sal NOT BETWEEN 1000 AND 2000;

--  The salary inquiry is not available 1000 To 2000 The name and salary of the employee between , Wages are in descending order 
SELECT ename,sal FROM emp WHERE sal NOT BETWEEN 1000 AND 2000 ORDER BY sal DESC;

5. Group by job position , Check the number of people in each job position , Jobs are arranged in ascending order

-- SQL surface  --

--  Group by job position , Check the number of people in each job position 
SELECT job,COUNT(*) FROM emp GROUP BY job;

--  Group by job position , Check the number of people in each job position , Jobs are arranged in ascending order 
SELECT job,COUNT(*) FROM emp GROUP BY job ORDER BY job ASC;

Paging query

In the previous query , Generally, all fields are directly queried , For more data , If all are displayed on one page , It will be dazzling to check . If you can display in pages , Will be faster 、 Fresh browsing ！

grammar

SELECT  Field name  FROM  Table name  LIMIT  Starting index , Number of query records ;

characteristic

• Starting index from 0 Numbered starting , Starting index = ( Look up the page number - 1) * Each page shows the number of records

• Yes 5 Data ,3 Data is divided into one page , If the query is the first page of data , The starting index can be omitted , Directly abbreviated as LIMIT 3, If you are querying the second page of data , Starting index from 3 Start , The number of query records can be written directly 3, If it is missing, only two will be displayed , If there is more, only 3 strip

Query requirement

1. Query the employee data on the first page , Each page is displayed 10 Bar record

--  Query the employee data on the first page , Each page is displayed 10 Bar record 
SELECT * FROM emp LIMIT 0,10;

--  Query the employee data on the second page , Each page shows 10 Data 
SELECT * FROM emp LIMIT 10,10;
--  Starting index from 0 Start , Starting index  = ( Look up the page number  - 1) *  Each page shows the number of records  (2-1)*10

2. Query salary greater than 1500 Employee data for , In ascending order of salary , And paging , Each page is displayed 5 Data

--  Query salary greater than 1500 Employee data for , Sort and sort by salary ascending order , Each page is displayed 5 Data 
SELECT * FROM emp WHERE sal>1500 ORDER BY sal ASC LIMIT 0,5;