SQL novice

Some introductions

database

Document warehouse

Database management system

Manage the warehouse （ Additions and deletions ） Such as MySQL

SQL

Database language

mySQL Some orders

SQL Case insensitive ！！！

show databases;// See what databases are 

use  Database name ;// Use a database 

creat database  Database name ;// Create a database 

show tables;// Check which tables are under a database 

source  Pathname ;// Import of this path sql file ,** No Chinese in the path **

desc(describe)  Table name ;// Check the structure of the table 

select version();// Inquire about MySQL Version number of 

select database();// Query the current database name 

The most basic unit of database ： surface (table)

That's ok (row)： data 、 Record

Column (column)： Field

Each field contains ： Field name 、 data type 、 Constraints, etc

SQL Sentence classification

• DDL：
  Data query language

  ​ select…

• DML:

  ​ Data operation language

  ​ insert delete update( Data addition, deletion and modification )

• DDL：

  ​ Data definition language

  ​ creat drop alter( Addition, deletion and modification of table structure )

• TCL：

  ​ Things control language

  ​ Transaction submission （commit) Things roll back （rollback)

• DCL：

  ​ Data control language

SQL Inquire about

Simple query

select  Field name  from  Table name ;// Query a field 
select  Field name 1, Field name 2 from  Table name ;// Query multiple fields 
select * from  Table name ;// Query all fields （ Because of the need to * Convert to field name , So it's less efficient , It is not recommended to use 

select  Field name  as  Alias ;// When displaying, the field name is replaced by an alias , But the data in the table will not be modified （"," You can use a space instead of , If there are spaces in the alias, you can include the alias with a single ）
select  Field name + operation ;// Field operation 

Conditions of the query

select  Field name 1, Field name 2....from  Table name  where  Conditions ;

• Relational operator ( The equal sign is =, The unequal sign has != and <>)
• between … and…（ Note that the previous value should be smaller than the following value , Equate to …>= and …<=） Query between two values
• Logical operators and,or,not ** Be careful ：** priority and Greater than or, If necessary, brackets should be used to contain
• in contain , Can be used instead of = for example select Field name where …in(… , …);

** Be careful ：** Whether it is null Need to use is null or is not null To judge , because null It's not a value

Fuzzy query

like

% Match any number of characters

_ Match a character

like ‘%T%’// All contain T Of

like ‘T%’// Beginning with T Of

like ‘%T’// It ends with T Of

like ‘_T%’// The second letter is T Of

If the query content has _ Or is it % You can use escape characters \_ and %

Sort query

select ... from ... order by ...;// Default ascending order 
select ... from ... order by ... desc;// Descending 
select ... from ... order by ... asc;// Ascending 
select ... from ... order by  Field one  asc, Field 2  asc;// The priority fields are arranged in ascending order , If they are equal, they are arranged in ascending order of field two 

Group query

select ... from ... where ... group by ... order by...

Execution order ：

1. from
2. where
3. group by
4. select
5. order by

Data processing functions / Single line processing functions

characteristic ： One input corresponds to one output

lower/upper Turn smaller / Big   Write 
substr( Intercepted string , Start subscript , Intercept length )** Starting subscript from 1 Start **
concat(s1,s2)// String splicing 
length(s)// String length 
trim(s)// String to space 
str_to_data// String to date 
data_format// Format date 
round( The number , Number of decimal places reserved )// rounding ( The number of decimal places can be negative , Negative constrains forward )
rand()*100// Generate 100 Random number within 
ifnull( data , Change to the value of )// If the data is null Just change to a specific value   Add ： stay SQL As long as there is null The results of the operations involved are null
case..when..then..when..then..else..end// Be similar to switch

Group function / Multiline processing functions

** characteristic ：** Multiple inputs correspond to one output

min()// minimum value 
max()// Maximum 
avg()// Average 
sum()// Sum up 
count()// Count 

Be careful ：

• Grouping function must be grouped before use , If there is no grouping, the whole table is a group by default
• Grouping function calculation will ignore null
• * Represents a row of data , Because the data in one row cannot be all null, So you can use count(*) Count lines
• Grouping functions cannot be used directly in where clause ( because where The statement has not been grouped when it runs )
• Grouping functions can be used together , utilize “,” Division
• select If there is group by,select It can only be followed by the fields participating in the grouping and the grouping statements