Basic syntax of MySQL DDL and DML and DQL

SQL classification ：

DDL(Data Definition Languages) sentence ： Data definition language , These statements define different data segments 、 database 、 surface 、 Column 、 Index and other database objects . Common sentence keywords mainly include create、drop、alter etc. .

DML(Data Manipulation Languages) sentence ： Data manipulation statement , Used to add 、 Delete 、 Update and query database records , And check data integrity . Common sentence keywords mainly include insert、delete、update and select etc. .

DQL (Data Query Language) sentence ： Data query language , Reserved words SELECT yes DQL（ Also all SQL） The most used verb , other DQL The common reserved words are WHERE,ORDER BY,GROUP BY and HAVING.

DDL Is the abbreviation of data defined language , Simply speaking , It is to create objects inside the database 、 Delete 、 Language for operations such as modification . It and DML The biggest difference between languages is DML Just operate on the internal data of the table , It does not involve the definition of tables , Structural modification , It will not involve other objects .DDL The statement is more by the database administrator （DBA） Use , Developers generally rarely use .

1. Create database

create database Database name ;

use Database name ; Using a database

show databases; have access to SHOW Statement to check what database currently exists on the server

show create table Table name ; With SQL Statement to show the table structure

2. Delete database

drop database Database name ;

3: Create table

　 The basic syntax for creating a table in the database is as follows ：

　　CREATE TABLE  tablename(

　　column_name_1 column_type_1 constraints,

　　column_name_2 column_type_2 constraints,

　　....

　　column_name_n column_type_n constraints

)

4： Delete table

DROP TABLE Table name

5： Modify table

(1): Modify table type , The grammar is as follows ：

　　ALTER TABLE Table name MODIFY Field name New properties

(2) Add table field , The grammar is as follows

        ALTER TABLE Table name ADD Field name attribute ;

(3) Delete table fields , The grammar is as follows ：

　　ALTER TABLE Table name DROP Field name ;

(4) Field renaming , The grammar is as follows ：

ALTER TABLE Table name  CHANGE Old field name New field name New properties ;

(6) Change table name , The grammar is as follows ：

ALTER TABLE Old table name as New table name ;

DML sentence
DML（Data Manipulation Language） sentence ： Data manipulation statement .

purpose ： Used to add 、 modify 、 Delete and query database records , And check data integrity .

DML The object of operation is the data of the library table （ Record ）.

It mainly includes inserting （insert）、 to update （update）、 Delete （delete） And query （select）.

DML Statement is the most frequently used operation by developers .

1. insert record

Insert a record

insert into Table name ( Field name , Field name ,..., Field name ) values(' Corresponding value ',' Corresponding value ',...,' Corresponding value ');

2. Update record

Update single table data

update Table name ser Field one = New value , Field 2 = New value ;

3. Simple query records

Simple condition query . Symbol “*” Query all fields on behalf of , If you just want to see some of these columns , Then write the column name of the table .

select * from Table name ;

select Field name from Table name ;

4. Delete record

Delete a single table record

delect from Table name ;

explain ： No addition where Condition rule Delete full table data

DQL be called “ Data retrieval statements ”, To get data from a table , Determine how the data is presented in the application . among , According to the structure and relationship of the table, it can be divided into single table query and multiple Table associated query .

Single table query

Query a data table in the database , It can be done by

Three kinds of query conditions and methods to do related optimization .

Multiple tables associated query

Query two or more tables in the database at the same time ,

There are complex queries and nested queries .

Query syntax rules ：

SELECT [DISTINCT] {*| surface 1.*|[  surface 1. Field 1 [as  Field alias 1] [,  surface 1. Field 2[as  Field alias 2]][, …]]}
FROM  surface 1 [ as  Table alias  ] [ left|right|inner join  surface 2 on  Between tables 
 The relationship between  ] [ WHERE ] [ GROUP BY ] 
[ HAVING] [ ORDER BY] [ LIMIT {[  Position offset ,]  Row number  }] ;
-- 

DISTINCT

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

Set up DISTINCT You can remove duplicate records .

AS

When the table name or field name is too long , It can be used AS Keywords are aliased , Convenient operation .

GROUP BY

Display the queried data by group classification .

HAVING

GROUP BY The grouping conditions on which grouping depends .

ORDER BY

Sort the queried result set in a certain order .

LIMIT

Limit the number of query results displayed .

WHERE Conditional clause

select * from Table name where Conditions ;

You can add conditions to the query

LIKE keyword

select * from Table name where Field LIKE Conditions ;

stay WHERE clause , Use LIKE Keyword fuzzy query ;

And “%” Use it together , Represents a match 0 Or any number of characters ;

And “_” Use it together , To match a single character .

IN keyword

IN Means to query within a range .

NULL Value query

select * from Table name where Field is null | is not null;

Common aggregate functions

AVG(col)

Returns the average value of the specified column

COUNT(col)

Returns the value of the specified column NULL The number of values

MIN(col)

Returns the minimum value of the specified column

MAX(col)

Returns the maximum value of the specified column

SUM(col)

Returns the sum of all values in the specified column

GROUP BY and HAVING grouping

GROUP BY

1、 Group all data ;

2、 There can be multiple fields for grouping , and

Group in turn .

HAVING

And GROUP BY Use a combination of , Grouping

Data filtering after .

LIMIT keyword

select * from Table name LIMIT [n,m]

MySql8 New keywords OFFSET

select * from Table name limit on offset n;

among ：

• LIMIT Keywords are MySQL Unique keywords ;

• LIMIT Limit SELECT The number of rows to return the result ;

• n Represents the offset of the first record ,m Indicates the number of displayed records ;

senior DQL：

1. Multiple tables associated query ：

Multi table joint query can be realized through connection operation , That is, multiple tables are associated by primary and foreign key relationships for query .

Equivalent query and non equivalent query

select * from surface 1, surface 2;

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

• Similar to single table query , All are SELECT sentence ;

• Put multiple tables in FROM after , And separated by commas ;

• You can use AS Keywords are aliases , Easy to quote ;

• If there is no duplicate name query field, the designation of the data table can be omitted

select * from surface 1 left inner join surface 2 on Conditions

LEFT JOIN :

From the left table （ surface 1） Return all records in , Even on the right

（ surface 2） There are no matching lines in .

RIGHT JOIN:

From the right table （table_2） Return all records in , Even if

On the left （table_1） There are no matching lines in .

INNER JOIN:

When at least one of the tables matches , Then return the record .

case when then end sentence

Statement is used to query conditions that meet a variety of conditions

similar java Medium if...else, There are also queries for row to column conversion , This is on select After Clause , It acts as a field . There are two specific uses , One is a simple functional form , The other is the form of expression .

--  Simple functional form 
select （case score when 'a' then ' good ' else ' good ' end) as score from student;

--  Expression form 
select (case when score between 85 and 90 then ' good ' else ' good ' end)
-- case when then end 
--  Simple case
select tid,tname,
case tsex 
	when 1 then ' male '
	when 0 then ' Woman '
	else ' Unknown '
end tsex, 
tbirthday, taddress
 from teacher

--  Search for case
select tid,tname, 
case 
	when tsex > 1 then ' Unknown '
	when tsex = 1 then ' male '
  when tsex < 1 then ' Woman '
	else ' There can be no situation '
end 
,
tbirthday,taddress from teacher

stay case There can also be nested sub queries and other complex uses .

(case when then end) Namely select A field to be queried later , When using, it can be enclosed in parentheses, which is more readable , You can use aggregate functions for it , Alias , duplicate removal , Wait for the operation .

RIGHT JOI

LEFT JO

FROM Table name