MySQL query operation

1. Conditions

• Use the where clause to filter the data in the table, and the rows with the result true will appear in the result set
• The syntax is as follows:

select * from 表名 where 条件;

comparison operator

• equals =
• greater than>
• Greater than or equal to >=
• less than <
• Less than or equal to <=
• Not equal to != or <>
• Search for students with numbers greater than 3

select * from students where id>3;

• Query subjects whose number is not greater than 4

select * from subjects where id<=4;

• Search for students whose name is not "Huang Rong"

select * from students where sname!='黄蓉';

• Query students who have not been deleted

select * from students where isdelete=0;

Logical Operators

• and
• or
• not
• Query female students whose number is greater than 3

select * from students where id>3 and gender=0;

• Search for students whose ID is less than 4 or has not been deleted

select * from students where id<4 or isdelete=0;

fuzzy query

• like
• % means any number of arbitrary characters
• _ means an arbitrary character
• Find students with the last name Huang

select * from students where sname like '黄%';

• Find students with the last name Huang and the first name is one character

select * from students where sname like '黄_';

• Find students with the last name Huang or Jing

select * from students where sname like '黄%' or sname like '%靖%';

range query

• in means in a non-contiguous range
• Enquiry for students whose numbers are 1 or 3 or 8

select * from students where id in(1,3,8);

• between ... and ... means in a continuous range
• Query students are 3 to 8 students

select * from students where id between 3 and 8;

• Check if the student is a boy from 3 to 8

select * from students where id between 3 and 8 and gender=1;

Empty judgment

• Note: null is not the same as ''
• Empty is null
• Check for students who do not have an address

select * from students where hometown is null;

• is not null
• Find students with addresses

select * from students where hometown is not null;

• Find the girl who filled in the address

select * from students where hometown is not null and gender=0;

priority

• parentheses, not, comparison operators, logical operators
• And is calculated before or. If it appears at the same time and you want to calculate or first, you need to use it in combination with ()

2. Aggregation

• In order to get statistics quickly, 5 aggregation functions are provided
• count(*) means to calculate the total number of rows, write asterisk and column name in parentheses, the result is the same
• Query the total number of students

select count(*) from students;

• max(column) means to find the maximum value of this column
• Query the maximum number of girls

select max(id) from students where gender=0;

• min(column) means to find the minimum value of this column
• Query the minimum number of undeleted students

select min(id) from students where isdelete=0;

• sum(column) means to find the sum of this column
• After querying the boy's number

select sum(id) from students where gender=1;

• avg(column) means to find the average of this column
• Query the average number of undeleted girls

select avg(id) from students where isdelete=0 and gender=0;

3. Grouping

• Group by field, indicating that data with the same field will be put into a group
• After grouping, only the same data column can be queried, and the different data columns cannot appear in the result set
• The grouped data can be counted and aggregated
• grammar:

select 列1,列2,聚合... from 表名 group by 列1,列2,列3...

• Query the total number of boys and girls

select gender as 性别,count(*)
from students
group by gender;

• Query the number of people in each city

select hometown as 家乡,count(*)
from students
group by hometown;

Data filtering after grouping

• grammar:

select 列1,列2,聚合... from 表名
group by 列1,列2,列3...
having 列1,...聚合...

• The conditional operator after having is the same as where
• Query the total number of boys

方案一
select count(*)
from students
where gender=1;
-----------------------------------
方案二：
select gender as 性别,count(*)
from students
group by gender
having gender=1;

Contrast where and having

• where is to filter the data of the table specified after from, which belongs to the filtering of the original data
• having is to filter the results of group by

4. Sort

• Data can be sorted for easy viewing
• grammar:

select * from 表名
order by 列1 asc|desc,列2 asc|desc,...

• Sort the row data according to column 1, if some rows and columns 1 have the same value, then sort according to column 2, and so on
• By default, the column values ​​are sorted from small to large
• asc is arranged from small to large, that is, in ascending order
• desc sort from big to small, that is, descending
• Query the information of male students that have not been deleted, in descending order by student number

select * from students
where gender=1 and isdelete=0
order by id desc;

• Query the information of undeleted accounts, in ascending order by name

select * from subject
where isdelete=0
order by stitle;

5. Get some rows

• When the amount of data is too large, it is very troublesome to view the data in one page
• grammar

select * from 表名
limit start,count

• From start, get count pieces of data
• start index starts from 0

Example: Pagination

• Known: m pieces of data are displayed on each page, and the nth page is currently displayed
• Find the total number of pages: this logic will be implemented in python later
  • The total number of queries p1
  • Divide p1 by m to get p2
  • If divisible then p2 is the total number of pages
  • If not divisible, p2+1 is the total number of pages
• Find the data of the nth page

select * from students
where isdelete=0
limit (n-1)*m,m

6. Summary

• Complete select statement

select distinct *
from 表名
where ....
group by ... having ...
order by ...
limit star,count

• The order of execution is:
  • from table name
  • where ....
  • group by ...
  • select distinct *
  • having ...
  • order by ...
  • limit star,count
• In actual use, it is only a combination of some parts of the statement, not all of them