MySQL basic knowledge learning (I)

Catalog

One 、 Core elements of relational database

Two 、SQL Main classification of language

3、 ... and 、 notes

Four 、 Common data types

5、 ... and 、 frequently-used SQL Language （ Case insensitive ）

6、 ... and 、 Common field constraints

7、 ... and 、 Conditions of the query

8、 ... and 、 Sort

Nine 、 Aggregate functions

Ten 、 The data packet

11、 ... and 、 Data paging display

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One 、 Core elements of relational database

• database （ Set of tables , There can be multiple tables in a database ）——database
• surface （ A two-dimensional table consisting of rows and columns ）, The basic unit of data stored in a database ——table
• That's ok （ Record ）, A line in the table , It's called a record in a database ——record
• Column （ Field ）, A column in the table , It's called a field in a database ——field

Two 、SQL Main classification of language

• DQL： Data query language , Used to query data , Such as select;
• DDL： Data definition language , Do database 、 The management of the table , Such as create、drop;
• DML： Data operation language , Add to the data 、 Delete 、 Change , Such as insert、delete、update;
• TPL： The language of things , Deal with things , Include begin transaction,commit,rollback;

3、 ... and 、 notes

• Single-line comments ：-- Content of notes
• Multiline comment ：/* Content of notes */

Four 、 Common data types

• Integers ：int
• Small integers ：tinyint
• decimal ：decimal
• character string ：varchar
• Date time ：datetime

5、 ... and 、 frequently-used SQL Language （ Case insensitive ）

1、 Create table ：create table

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

2、 Add data ：insert

• Type 1 ： Set values for all fields , The order of values corresponds to the order of fields in the table

insert into  Table name  values(...);

• Type 2 ： Some fields set values , The order of the values corresponds to the order of the fields given

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

• One insert Statement to add multiple records , The data are separated by English commas

insert into  Table name  values(...),(...)...;    -- All fields 

insert into  Table name ( Field 1,...) values( value 1,...),( value 1,...)...;    -- Partial field 

 3、 Simple query ：select

•   Query all fields

select * from  Table name ;

•   Query the specified field

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

select  Field 1 as  Alias , Field 2 as  Alias ,... from  Table name  as  Alias ;    -- Alias fields and tables ,as It can be omitted 

notes ：distinct It can eliminate the duplication of query results , Add to select after

4、 Modifying data ：update

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

5、 Delete records in table ：delete

delete from  Table name  where  Conditions ;

6、 Delete records in table ：truncate

truncate table  Table name ;    -- Delete the entire table directly 

truncate And delete The difference between ：

• In speed ,truncate>delete;
• If you want to delete some data, use delete, Pay attention to take where Words and expressions ;
• If you want to keep the table and delete all the data , The self growth field is restored from 1 Start , use truncate;

7、 Delete table ：drop table

drop table  Table name ;    

drop table if exists  Table name ;    -- First judge whether the table exists , Then execute the delete command 

6、 ... and 、 Common field constraints

1、 Primary key （primary key）： Value cannot be duplicate ,auto_increment The representative value increases automatically

create table  Table name (
     Field name      data type     primary key auto_increment,
     Field name      data type      constraint 
    ...
    constraint primary key( Field name )    -- Primary keys can also be created at the end , Choose one of them 
);

notes ： When inserting, the value of the primary key self growth field can be replaced by a placeholder ,0 or null

2、 Non empty （not null）： This field is not allowed to fill in null value

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

3、 only （unique）： Duplicate values are not allowed for this field

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

4、 The default value is （default）： When this value is not filled in, the default value is used , If it is filled in, the filling shall prevail

create table  Table name (
     Field name      data type     default  value ,
    ...
);

7、 ... and 、 Conditions of the query

1、 Comparison operator

• be equal to ：=
• Greater than ：>
• Greater than or equal to ：>=
• Less than ：<
• Less than or equal to ：<=
• It's not equal to ：!= or <>

2、 Logical operators

• and（ And ）,and There are two conditions , Must satisfy at the same time
• or（ or ）,or There are two conditions , Meet one of them
• not（ Not ）,not There is only one condition , The result is opposite to the condition

3、 Fuzzy query

• like
• % Represents any number of arbitrary characters , coordination like Use
• _ Represents an arbitrary character , coordination like Use

4、 Range queries

• in In a discontinuous range ,not in Indicates that it is not in the discontinuous range ,in（ value 1, value 2,...）
• between...and... In a continuous range ,not between... and... Means not in a continuous range

5、 Empty judgment

• Judge empty ：is null
• Judgment is not empty ：is not null

Be careful ：null And '' Is different ,null Means nothing ,'' The representative length is 0 String

8、 ... and 、 Sort

• asc（ The default value is ） Sort from small to large , In ascending order
• desc Sort from large to small , In descending order

select * from  Table name  where  Conditions 
order by  Field 1 asc|desc, Field 2 asc|desc,...    -- The default is ascending ,asc Don't write , Fields have high priority before 

Nine 、 Aggregate functions

1、count Total number of records

• count（*） Indicates the total number of records calculated , Write... In brackets * And field name , The result is the same

2、max Maximum

• max（ Field ） Indicates to find the maximum value of this field

3、min minimum value

• min（ Field ） Means to find the minimum value of this field

4、sum Sum up

• sum（ Field ） Means to sum this field

5、avg Average

• avg（ Field ） Means to find the average value of this field

Be careful ：（1） Aggregate function cannot be in where Used in the following conditions

           （2） Aggregate functions cannot appear in query results at the same time as ordinary fields

           （3）count Functions do not ignore null values null, The rest ignore null values null

Ten 、 The data packet

1、 grouping （group by）

• Group by fields , Indicates that the same data in this field will be placed in a group
• The purpose of grouping is to cooperate with the aggregation function , The aggregation function will count the data of each group separately

select  Field 1, Field 2, Aggregate functions ... 
from  Table name  
where  constraint condition 
group by  Field 1, Field 2...

2、 Data filtering after grouping （having）

select  Field 1, Field 2, Aggregate functions ... 
from  Table name  
where  constraint condition 
group by  Field 1, Field 2...
having  Field 1,... polymerization ...

Be careful ： Use having You must group before group by

contrast where And having：

• where It's right from The table specified later performs data filtering , It belongs to the screening of original data
• having It's right group by Results of
• having Aggregate functions can be used in the following conditions ,where The following conditions cannot use aggregate functions

11、 ... and 、 Data paging display

1、 Get some lines

• limit Start , Get the number of lines
• from start Start , obtain count Data
• start Index from 0 Start , If omitted start The default from the 0 Start

select * from  Table name  limit start,count

2、 Pagination

• When there are too many records in a table , You need to use paging display
• It is known that ： Each page shows m Data , seek ： Query the first n Pages of data

select * from  Table name  limit (n-1)*m,m