Note: Try not to use the wildcard * in the query statement. Reason: When the amount of data is large, the query efficiency will be reduced. Use the column name instead of *, and develop a good habit at ordinary times.

MySQL database command

1. Query the database

show databases; (plural for multiple databases)

2. Create a database

create database database name charset utf8;

(The database name is composed of letters, numbers, and underscores, and the numbers cannot be at the top)

Note: here is database, there is no s behind it, and adding charset utf8 to the database can make the database insert Chinese, otherwise it will report an error when inserting Chinese

Error case:

3. Delete the database

drop database database name;

4. Select the database

use database name;

create table

Points to note (select a specific database before creating a table)

view all tables

show tables;

create table

create table table name (column name type, column name type, column name type);

Note: the types are int, varchar(), decimal(m, n) m means there are m significant digits, n means there are n digits after the decimal point

View the structure of the specified table

desc table name;

delete table

drop table table name;

Commenting on database commands

In SQL, you can use "--space + description" to indicate the comment description

CRUD detailed description

Increase

Insert into table name (id, name, age) values (1, 'Xiaohong', 111) Note: When inserting strings, remember to separate them with semicolons' '

Note: insert multiple at a time, remember to separate them with commas

The SQL library provides functions about time: now()

Inquire

Select * from table name;

Select column name, column name from table name;

Query table for column-to-column operations

select column name, column name + column name + column name from table name;

The query specifies a column as an alias, using the keyword as

To deduplicate query data, use the keyword distinct

Query result sorting

When the query results need to be sorted, you can use order by asc (asc can not be written by default), that is, order by

Perform ascending operations on the results (from small to large): order by

Perform descending operations on the results (from large to small): order by desc

Points to note: ( orde by can be followed by an alias , for example, when calculating the sum, there is as total in front, and order by total can be followed), Where cannot be followed by an alias

Specify condition query where

Conditional query, you can directly compare two columns

Example: Query the list of students whose Chinese scores are greater than their math scores

select name from table where chinese > math;

Range search difference: between A and B (closed before and closed after)

Query a specific value: use in to represent

Fuzzy search for a character: use like

Paging query, use limit to query

Example: Query the first 3 pieces of data in the table (the default is to query from the first page)

If you want to customize the query from a certain page, use offset

The query result is added to the table as data

String interception of SQL query results: substring

Usage: substring(string, start position of interception, number of characters of interception)

String concatenation of SQL query results: concat

Usage: concat(string1, string2, string3, ...)

Character capitalization in SQL query results: upper

Usage: upper(string)

aggregation query

Data summation: sum

Sum summation is only valid for numbers, and strings cannot be summed. When summing columns, rows with null results will be automatically skipped

Calculate the average: avg

Avg can calculate the average value of a column, the method of use is the same as sum

avg can also be used with expressions

Find the maximum and minimum values of a column: max \ min

Group query: group by

A group query will group the same columns in the same group.

For example, if you need to calculate the salary of a certain position, you need to divide each position into the same group, and then calculate the average salary of this position

Conditional filter: having \ where

where: filter the specified line

having: filter groups, used in conjunction with group by

Note: After the group by clause is grouped, when you need to conditionally filter the grouped results, you cannot use the where statement, but you need to use having

Example: Display roles whose average salary is less than 1500 and their average salary

select role ,max(salary),min(salary),avg(salary) from emp group by role having avg(salary)< 1500 ;

Count the number of rows in the table: count

select count(*) from table name;

Note: When it comes to multiple data calculations, use count and remember to use group by

Multi-table query: join on

When querying data in different columns from multiple tables, you need to use the multi-table query statement join on

Statement usage: select *from table 1 join table 2 on condition 1 join table 3 on condition 2

Example:

select * from student join score on student.id = score.id join course on course.id = score.course_id;

First consider the outer join between the left table and the middle table to get a "temporary" table, and then take this "temporary" table and the right table for outer join

Left outer join: left join table on

left join ---- Left join, take the columns of the left table as the main, take the intersection of the two columns, and take null for the columns that do not exist in the right column

Left outer join: table 1 left join table 2 on connection condition, the right table (table 2) returns the data that exactly matches the connection condition, and the left table (table 1) not only returns the data that matches the connection condition, but also the unmatched data will return;

Right outer join: right join table on

right join ---- Right join, take the columns of the right table as the main, take the intersection of the two columns, and take null for the columns that do not exist in the left column

Right outer join: table 1 right join table 2 on connection condition, the left table (table 1) returns the data that exactly matches the connection condition, and the right table (table 2) returns the data that matches the connection condition, and the data that does not match will return

Combine multi-table query results: union \ union all

union can combine the query results of multiple tables, the prerequisite is that multiple result columns must have one-to-one correspondence.

union : Perform a union operation on two query results, automatically remove duplicates, excluding duplicate rows , that is, remove duplicate results before displaying

union all : Perform a union operation on two query results without deduplication, including duplicate rows , that is, all query results will be displayed

database index

view index

show index from table name;

create index

create index index name on table name (column name);

delete index

drop index index name on table name;

MySQL database addition, deletion, modification and aggregation query SQL statement learning summary

Database addition, deletion, modification and query SQL statements

MySQL database command

1. Query the database

2. Create a database

3. Delete the database

4. Select the database

create table

view all tables

create table

View the structure of the specified table

delete table

Commenting on database commands

CRUD detailed description

Increase

The SQL library provides functions about time: now()

Inquire

Query table for column-to-column operations

select column name, column name + column name + column name from table name;

The query specifies a column as an alias, using the keyword as

To deduplicate query data, use the keyword distinct

Query result sorting

Specify condition query where

Range search difference: between A and B (closed before and closed after)

Query a specific value: use in to represent

Fuzzy search for a character: use like

Paging query, use limit to query

The query result is added to the table as data

String interception of SQL query results: substring

String concatenation of SQL query results: concat

Character capitalization in SQL query results: upper

aggregation query

Data summation: sum

Calculate the average: avg

Find the maximum and minimum values ​​of a column: max \ min

Group query: group by

Conditional filter: having \ where

Count the number of rows in the table: count

Multi-table query: join on

Left outer join: left join table on

Right outer join: right join table on

Combine multi-table query results: union \ union all

database index

view index

create index

delete index

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Find the maximum and minimum values of a column: max \ min