SQL most commonly used basic operation syntax

One 、 brief introduction

SQL It's English Structured Query Language Abbreviation , Structured query language , A programming language for operating relational databases , It is a unified standard for defining and operating all relational databases , For the same need , There may be some differences in each way of database operation , This is called a “ dialect ”.
General grammar ：SQL Statements can be written in one or more lines , With a semicolon ; ending .MySQL Database SQL Statement is case insensitive . You can add comments , Where single line comments use -- The comment perhaps # The comment （MySQL specific ） Multiline comments use /* The comment */

Two 、 Database operation statement

1、 Inquire about

Query all databases

show databases;

2、 establish

Create database

create database  Database name ;

Create database （ Judge , Create... If it doesn't exist , This method is generally used to avoid mistakes ）

create database if not exists  Database name ;

3、 Delete

Delete database

drop database  Database name ;

Delete database （ Judge , Delete... If it exists , Generally, this method is used to delete to avoid errors ）

drop database if exists  Database name ;

4、 Use

View currently used databases

select database();

Using a database

use  Database name ;

3、 ... and 、 Table operation statement

1、 Inquire about

Query the names of all tables in the current database

show tables;

Query table structure

desc  The name of the table ;

2、 establish

Create table

create table  Table name (
	 Field name 1  data type 1,
	 Field name 1  data type 1,
	...
	 Field name n  data type n
);

Be careful ： You cannot add a comma at the end of the last set of data ,
MySQL Support multiple data types , Here is a table ：

3、 Delete

Delete table

drop table  Table name ;

Delete table （ Judge , Delete... If it exists , Generally, this method is used to delete to avoid errors ）

drop table if exists  The name of the table ;

4、 modify

Modify the name of the table

alter table  Table name  rename to  The new name of the table ;

Add a column

alter table  Table name  add  Name   data type ;

Modify column data type

alter table  Table name  modify  Name   New data types ;

Change column name and data type

alter table  Table name  change  Name   New column names   New data types ;

Delete column

alter table  Table name  drop  Name ;

Four 、 Table data operation statement

1、 Add data

Add data to the specified column

insert into  Table name ( Name 1,  Name 2, ...) values( value 1,  value 2, ...);

Add data to all columns

insert into  Table name  values( value 1,  value 2, ...);

Batch add data

insert into  Table name ( Name 1,  Name 2, ...) values( value 1,  value 2, ...), ( value 1,  value 2, ...), ...;
insert into  Table name  values( value 1,  value 2, ...), ( value 1,  value 2, ...), ...;

2、 Modify table data

Modify the specified column data （ Be careful ： If no condition is added in the modification statement , Then all data in the corresponding column will be modified ）

update  Table name  set  Name 1= value 1,  Name 2= value 2, ... where  Conditions ;

3、 Delete table data

Delete data （ Be careful ： Delete statements without conditions , Then all data will be deleted ）

delete from  Table name  where  Conditions ;

4、 Query table data

a. Basic query

Query multiple fields

select  Name 1,  Name 2, ... from  Table name ; --  Query the specified column data 
select * from  Table name ;                --  Query all column data 

Remove duplicate record queries

select distinct  Name 1,  Name 2, ... from  Table name ;

List alias queries （as You can omit it , The output column name is the alias of ）

select  Name 1 as  Alias ,  Name 2, ... from  Table name ;

b. Conditions of the query

according to where Conditions of the query

select  Field list  from  Table name  where  List of conditions ;

The table below for where Common post condition symbols and functions

c. Sort query

sort order ：
ASC： Ascending order （ The default value is ）
DESC： Descending order

select  Field list  from  Table name  order by  Sort field names 1  sort order 1,  Sort field names 2  sort order 2, ...;

Be careful ： If there are multiple sorting conditions , When the condition values of the current edge are the same , Will be sorted according to the second condition

d. Aggregate function query

Concept ： Take a column of data as a whole , Do the longitudinal calculation .
Aggregate functions are classified in the following table

The grammar is as follows ：

select  Aggregate function name ( Name ) from  Table name ;

e. Group query

select  Field list  from  Table name  where  Conditions before grouping are limited  group by  Group field name  having  Conditional filtering after grouping ;

Be careful ： After grouping , The query fields are aggregate functions and grouping fields , Querying other fields makes no sense
where and having The difference between ：
The timing of execution is different ：where Is qualified before grouping , dissatisfaction where Conditions , They don't participate in the grouping , and having Is to filter the results after grouping .
Judging conditions are different ：where Aggregate functions cannot be judged ,having Sure .
Execution order ：where > Aggregate functions > having

f. Paging query

select  Field list  from  Table name  limit  Starting index ,  Number of query entries ;

Be careful ： Starting index from 0 Start , Calculation formula ： Starting index = ( The current page number - 1) * Number of entries per page

5、 ... and 、SQL constraint

1、 Concept and classification

Concept ： Constraints are rules that act on columns in a table , Used to limit the data added to the table ; The existence of constraints ensures the correctness of data in the database 、 Effectiveness and integrity .
The classification is shown in the figure below

2、 grammar

a. Non empty constraint

Add a non NULL constraint when creating a table

create table  Table name (
	 Name   data type  not null,
	...
);

Add a non empty constraint after creating the table

alter table  Table name  modify  Field name   data type  not null;

Delete non empty constraints

alter table  Table name  modify  Field name   data type ;

b. Unique constraint

Add unique constraints when creating tables

create table  Table name (
	 Name   data type  unique,
	...
);

Add unique constraints after creating the table

alter table  Table name  modify  Field name   data type  unique;

Delete unique constraint

alter table  Table name  drop index  Field name ;

c. Primary key constraint

Add a primary key constraint when creating a table

create table  Table name (
	 Name   data type  primary key,
	...
);
create table  Table name (
    -- auto_increment： When no value is specified, the self growth 
	 Name   data type  primary key auto_increment,
	...
);

Add the primary key constraint after creating the table

alter table  Table name  add primary key( Field name );

Delete primary key constraint

alter table  Table name  drop primary key;

d. Default constraint

Add default constraints when creating tables

create table  Table name (
	 Name   data type  default  The default value is ,
	...
);

Add the default constraint after creating the table

alter table  Table name  alter  Name  set default  The default value is ;

Delete default constraint

alter table  Table name  alter  Name  drop default;

e. Foreign key constraints

Add foreign key constraints when creating tables

create table  Table name (
	 Name   data type ,
	...
	--  Foreign key names are defined for yourself 
	constraint  Name of the foreign key  foreign key( Foreign key column name ) references  Main table ( Name of main table )
);

Add foreign key constraints after creating the table

alter table  Table name  add constraint  Name of the foreign key  foreign key( Foreign key column name ) references  Main table ( Name of main table );

Delete foreign key constraint

alter table  Table name  drop foreign key  Name of the foreign key ;

6、 ... and 、 Multi-table query

Multi-table query ： Query data from multiple tables . It is mainly divided into ： Join queries and subqueries , Connection query is divided into inner connection query and outer connection query .

1、 Internal connection query

--  Implicit inner join 
select  Field list  from  surface A, surface B,... where  Conditions ;
--  Display inner connection  inner  It can be omitted 
select  Field list  from  surface A inner join  surface B on  Conditions ;

Inner join query is equivalent to query table A And table B Intersection data of

2、 External connection query

--  The left outer join  outer  It can be omitted 
select  Field list  from  surface A left outer join  surface B on  Conditions ;
--  Right connection  outer  It can be omitted 
select  Field list  from  surface A right outer join  surface B on  Conditions ;

Left outer join query is equivalent to query table A All data and intersection data
Right outer join query is equivalent to query table B All data and intersection data
In use, it is generally enough to use the left outer connection , Want to query the table B All data and intersection data , Just put the table A And table B Just change the position .

3、 Subquery

Nested query in query , Call this nested query subquery .
According to the query results , Different functions can be divided into ： Single row single row 、 Multi row single row 、 Multiple rows and columns .

a. Single row single row

Subquery as condition value , Use =!=>< Wait for conditional judgment

select  Field list  from  Table name  where  Field name  = ( Subquery );

b. Multi row single row

Subquery as condition value , Use in And other keywords for conditional judgment

select  Field list  from  Table name  where  Field name  in ( Subquery );

c. Multiple rows and columns

Subquery as virtual table

select  Field list  from ( Subquery ) where  Conditions ;

All of these are SQL Common operation syntax , Just go ahead ！