Basic syntax of MySQL DDL and DML and DQL

SQL The concept of

（1）SQL(Structured Query Language)： Structured query language ; In fact, it defines the rules for operating all relational databases . Is a special purpose programming language , Is a database query and programming language , For data access and query 、 Update and manage relational database system , It is also the extension of database script file .

（2）SQL General grammar

① SQL Statements can be written in one or more lines , It ends with a semicolon .

② You can use spaces and indents to enhance the readability of statements .

③ MySQL Database SQL Statement is case insensitive , It is recommended to use uppercase .

④ 3 Species notes

* Single-line comments : -- The comment or # The comment (mysql specific )

* Multiline comment : /* notes */

DDL: Operating the database 、 surface

1. Operating the database

① C(Create): establish

Create database ：

create database Database name ;

② R(Retrieve)： Inquire about

Query all database names :

show databases;

Query the character set of a database : Query the creation statement of a database

show create database Database name ; 

③ U(Update): modify

Modify the character set of the database

alter database Database name character set Character set name ;

④ D(Delete): Delete

Delete database

drop database Database name ;

Determine that the database exists , Exist and delete

drop database if exists Database name ;

⑤ Using a database

Query the database name currently in use

select database();

Using a database

use Database name ;

1. Operation table

① C(Create): establish

        grammar ：

         create table Table name (
            -- Field name   type   attribute ,
            -- Field name   type   attribute ,
            -- ....
            -- Field name   type attribute
        -- );

Be careful :

a. The last column , There is no need to add comma （,）

b.[] Part can be omitted

c.`` The quotation marks ( Invalidate keywords ) It can be omitted , Mainly to distinguish between keywords and ordinary words

Table name naming convention :

a. Table and field names ： The table name is the unique identifier in the database , The field name is the unique identifier in the table

b. use 26 English letters ( Case sensitive ) and 0-9 The natural number of ( Often you don't need ) Underline '_' form , The naming is concise and clear , Underline multiple words '_' Separate .

c. All lowercase names , Uppercase is prohibited

d. The field name should not be too long （ Generally no more than three English words ）

② Database type ( Commonly used )：

String type ：

1. char[(M)]: Fixed long string , Retrieval is fast, but it takes space (0<= M <=255)
2. varchar[(M)]: Variable string (0 <= M <=65535)
3. text： Text string (4 individual G Size left and right )

value type ：

1. tinyint: Very small data (1 byte )
2. int: Standard integers (4 byte )
3. bigint： A large integer , amount to long(8 byte )
4. Decimal( money )： Floating point numbers in string form (m,d),m Bytes , After the decimal point d position

Date and time type

datetime: date , Including month, day, hour, minute, second yyyy-MM-dd HH:mm:ss

If you don't assign a value to this field in the future , Or assign the value to null, The current system time is used by default , To automatically assign

Null type ：

• Understood as a “ No value ” or “ Unknown value ”;

• Do not use NULL Perform arithmetic operations , The result is still the NULL;

• MySQL in ,0 or NULL All means false ,1 It's true . 

Applicable scenarios of the two tables ：

Use MyISAM： Save space and speed ;

Use InnoDB： Security , Transaction processing and multi-user operation data table .

Create table

create table student(

        id int,

        name varchar(32),

        age int ,

        score double(4,1),

        birthday date,

        insert_time timestamp

);

Copy table ：create table Table name like The name of the copied table ; 

2. R(Retrieve)： Inquire about

Query all table names in a database

show tables;

Query table structure

desc Table name ;

View the SQL sentence

show creat table  Table name ;

3. U(Update): modify

1>. Modify the name of the table

alter table used Table name rename as  The new name of the table ;

2>. Modify table structure

a. Change the properties of the field

alter table Table name modify Field name Properties to modify ;

b. Modify fields

alter tale Table name change Old field name New field name Properties to modify ;

3>. Add fields

alter table Table name add Field name type attribute ;

4>. Delete field

alter table Table name drop Field name ;

4. D(Delete): Delete

drop table Table name ;

drop table if exists Table name ;

DML： Add, delete and modify the data in the table

1. newly added ：

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

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

Be careful ：

① Column name and value should correspond one by one .

② The date is expressed as a string

③ Except for the number type , Other types require quotation marks ( Single and double can ) Lead up

eg：

Let the primary key increase ( Full field statement )

1. Write default
2. Write null

Insert multiple pieces of data at once

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

Unusual new methods ：

1. Copy table ( Tables must exist )

insert into Table name to copy select Field name ... from The table name of the copied data

1. Insert data when creating a new table ( The new table does not exist )

create table The new name of the table select Field name ... from The table name of the copied data

insert into Table name value( value 1, value 2...);[ The following value must be full field ]

2. Delete data ：

delete from Table name ;( Delete entire table )

delete from Table name where Clause ;

Clear the table ：truncate Table name ;

Be careful ：

① If there is no condition , Then delete all records in the table .

② If you want to delete all records

(1). delete from Table name ; -- It is not recommended to use . How many records will be deleted

(2). truncate table  Table name ; -- Recommended , More efficient Delete the table first , Then create the same table .

3. Modifying data ：

grammar ：

update Table name set Field name 1= value 1, Field name 2= value 2,... Field name n= value n... where Clause

modify where The following field name corresponds to where The value of the previous field name

Be careful ：

1. Without any conditions , All records in the table will be modified .
2. -- Change the data in a range (>= and <=) Equivalent to between Little data and big data

DQL： Look up the records in the table

1 Basic query

（1） Query the data of all rows and columns of the table ( What you get is a virtual table )

        select * from  Table name

        select Full field from Table name ;

（2） Query the specified field

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

Be careful ： If you query all fields , You can use * To replace the field list .

（3） Alias fields :

        a.select Old field name as ' new field name ';

        b.select Old field name ' new field name ';

        c.select Old field name new field name ;

Table reasons for using aliases ： Used for multi table query .

1. Remove duplication (distinct), Only those identical in the query field name will be removed , As long as there is a repetition, it will stay

        select distinct  Field name ... from  Table name ;

1. Conditional inquiry where Clause ;

        eg: Query the student table sid stay 3-7 Students in the range

                select * from student where sid between 3 and 7;

2 Conditions of the query

Conditions of the query : If there are no query conditions , Then all rows are queried each time . Practical application , Generally, you need to specify the query criteria . Filter records .

        select Field name from  Table name where Clause ;

*: Birthday in the query table ( The older the birthday , The younger you are )

technological process ： Take out each data in the table , The records that meet the conditions return , Records that do not meet the conditions will not be returned

（1) where Clause followed by condition

（2) Operator

> 、< 、<= 、>= 、= 、<>/！=（ It's not equal to ）

between...and （ Baotou and Baowei ）

Fuzzy query :

a.like keyword

Fuzzy symbols %： Any number of any characters 、_: An arbitrary character

b.in keyword : Find in a specific range

c.is keyword ( Query empty data ：where Field name is null/ Query non empty data ：where Field name is not null)

Aggregate functions ：

grammar ： Before we do the query is horizontal query , They are judged line by line according to the conditions , The aggregate function query is vertical query , It calculates the values of a column , And then return a result value . Aggregate functions will Ignore the null value NULL.

-- Number of Statistics count( Field )/ Fields can be written *、 Constant 、 Any field name /count The statistical data is null The number of

-- Statistical average avg( Field )

-- Statistical maximum max( Field )

-- Statistical minimum min( Field )

-- Statistical sum sum( Field )

eg:select count(*) Total number , sum(score) Total score , avg(score) average , max(score) The highest , min(score) Lowest score  from sc;

Group query :

For group query group by Statement to group query information , The same data as a group , The purpose of grouping is to make statistics , Grouping is usually used with aggregate functions .

select  Grouping field name , Aggregate functions from  Table name group by  Grouping field name ;

eg:select classid,count(*) from student group by classid;

where It's about aggregation ( grouping ) Filter every piece of data before

having It's about aggregation ( grouping ) After each piece of data filtering

having Not alone , It has to go with group by appear

eg： The average score obtained is greater than 80 The above students

select sid,avg(score) from sc group by sid having avg(score) > 80;

Sort order by( Descending desc、 Ascending asc( Default ascending order ))

You can write multiple conditions , Separated by commas , First write the data that needs to be sorted , And so on

limit( Pagination )m,n m Indicates when to start ,n Indicating step size , Take some data

In the query table 5,6,7 Student data ( The formula : Page number -1* step , step )

select * from student limit 4,3;

3 Multiple tables associated query

1. Equivalent joint query ( Inline data ), It is applicable to less tables or more tables with less data

        select * from surface 1, surface 2 where surface 1. Field 1 = surface 2. Field 2...

         Non equivalent associated query

         The cartesian product , List all the data in the query table to form a large data table

                select * from Table name 1, Table name 2,...

        2. Link query ( Outreach query )

        select * from Table name 1 inner join Table name 2 on Conditions ,inner join on,..., It is recommended to use

        left join on( Left outreach )

         From the left table （ surface 1） Return all records in , Even on the right ( surface 2) There are no matching lines in

        right join on( Right outreach )

         From the right table ( surface 2) Return all records in , Even on the left ( surface 1) There are no matching lines in

1. Link query union

        union Is to find the union of two queries

        union What is merged is the result set , It doesn't distinguish from which table , So you can merge the data queried from multiple tables

                select * from Table name 1 inner join Table name 2 on Conditions ,inner join on,... union select * from Table name 1 inner join Table name 2 on Conditions ,inner join on,... 

union characteristic ：

1. When the field names are inconsistent , The header of the first table will prevail , And
2. Duplicate rows will be filtered out , And distinct similar
3. If the number of table columns queried is not equal , Will report a mistake
4. The order in each sentence is meaningless ,mysql It will be ignored when merging
5. You can sort the consolidated whole table
6. You can check your watch 1 Alias the field name of , Is the final header

union all, You can query all the information in the linked table

union all, You can query all the information of all tables , Find the union of two queries , But it will not filter out the repetition

4 Subquery

1. where Subquery

         Inquire about id The largest student

                select * from student where sid = (select max(sid) from student);

        2. from Subquery

         Take the query results of the inner layer as a temporary table , For the outer layer sql Query again , The query result set can be treated as a table , The temporary table should use an alias .

        3. exists Subquery

Put the outer layer sql Result , Get the inner layer sql To test , If the inner layer sql establish , Then the line takes out , The inner query is exists After the query .

1. any,some,all Subquery
   1. any Subquery
      • It means that any one of the conditions is satisfied , hypothesis any The number of results returned by the internal query statement is three result1...
      • Select...from... where a > any(...);

amount to ：

Select...from... where a > result1 or a > result2 or a > result3;

• • • any and some Use consistent , interchangeable
  • all Subquery
    • All conditions are met , hypothesis any The number of results returned by the internal query statement is three result1...
    • Select...from... where a > all(...);

amount to ：

Select...from... where a > result1 and a > result2 and a > result3;

1. case when then end sentence
   1. Simple case function

        Case ssex

                When 1 then ‘ male ’

                When 2 then ‘ Woman ’

        End,

                2. Case The search function

        Case

                When ssex = 1 then ‘ male ’

                When ssex = 2 then ‘ Woman ’

        Else‘ Unknown ’

        End,

That's all MySQLDDL and DML and DQL Basic syntax .