Mysql database DDL and DML

SQL Introduce

What is? SQL？

SQL General grammar

SQL classification

DDL Data definition language

DDL database

Query the database

Create database

modify , Delete , Using a database

DDL Data sheet

Query of data table

Data table creation

Modification of data sheet （ Structural changes ）

Deletion of data table

DML Data operation language

New table data

modify 、 Delete table data

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SQL Introduce

What is? SQL？

Structured Query Language： Structured query language . In fact, it defines the operation of all relational databases The rules .

There may be some differences in each way of database operation , We call it “ dialect ”.

SQL General grammar

SQL Statement can One or more lines Writing , With End of semicolon .

You can use Spaces and indents To enhance the Readability .

MySQL Database SQL sentence Case insensitive , It is recommended to use uppercase .

Comments for the database ：

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

         Multiline comment ：/* The comment */

SQL classification

DDL(Data Definition Language) Data definition language

Used to define database objects ： database , surface , Column, etc. . keyword ：create, drop,alter etc.

DML(Data Manipulation Language) Data operation language

It is used to update the data of tables in the database Additions and deletions . keyword ：insert, delete, update etc.

DQL(Data Query Language) Data query language （ above all ！！！）

be used for Inquire about Records of tables in the database ( data ). keyword ：select, where etc.  

DCL(Data Control Language) Data control language ( understand )

It is used to define the access rights and security level of the database , And create users . keyword ：GRANT, REVOKE etc.

DDL Data definition language

DDL database

Query the database

（1） Query all databases  

SHOW DATABASES;

（2） Query the creation statement of a database

SHOW CREATE DATABASE demo;

Create database

（1） Create a demo The database of

CREATE DATABASE demo;

（2） Create database （ Judge , If it doesn't exist, create it ） （exists）

CREATE DATABASE IF NOT EXISTS demo;

If you have already created this database ——demo, The program will not report an error , But there will be warnings

（3） Create database （ Specify character set ） （character set）

CREATE DATABASE demo01 CHARACTER SET utf8;

Don't worry about the warning ！ In this way, a new one named demo01 And the character set is utf8 The database of .

Small cases

establish demo02 database 、 Create... If it doesn't exist , The specified character set is gbk

CREATE DATABASE IF NOT EXISTS demo02 CHARACTER SET gbk;

have access to show create database demo02;  View character set

modify , Delete , Using a database

（1） Modify the character set of the database   （character set）

ALTER DATABASE demo CHARACTER SET gbk;

Above SQL Code handle demo The character set of is modified to gbk

In order to deepen the memory , We compare it with the above （3） Create database （ Specify character set ） Contrast

--  Modify the character set of the database

ALTER DATABASE Database name CHARACTER SET Character set ;

--  Specify character set to create database

CREATE DATABASE Database name CHARACTER SET Character set ;

（2） Delete database   （drop）

DROP DATABASE demo01;

（3） Delete database （ Judge , Delete... If it exists ） 

DROP DATABASE IF EXISTS demo02;

（4） Using a database

USE demo;

（4.1） Query the database in use

SELECT DATABASE();

DDL Data sheet

Query of data table

（1） Query all data tables

SHOW TABLES;

Because of the db1 There is only one... In the database surface  

（2） The structure of the query table   （desc）

DESC student;

Detailed structure information of the table  

（3） Query the status information of the table   （status,like）

SHOW TABLE STATUS FROM database LIKE ' Data sheet ';

SHOW TABLE STATUS FROM db1 LIKE 'student';

Data table creation

（1） Create data table

among constraint It will be explained in detail in the following chapters ！

--  Create a product list （ Product id , Name of commodity , commodity price , inventory , Shelf time ）
CREATE TABLE product(
	id INT,
	NAME VARCHAR(20),
	price DOUBLE,
	num INT,
	insert_time DATE
);

--  see product Detailed structure of the table 
DESC product;

（ repair ） data type （ part ）

1. int： Integer types
2. double: Decimal type
3. date: date , Include only mm / DD / yyyy     yyyy-MM-dd
4. datetime: date , Including month, day, hour, minute, second      yyyy-MM-dd HH:mm:ss
5. timestamp: Timestamp type      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, be The current system time is used by default , To automatically assign
6. varchar： character string

Modification of data sheet （ Structural changes ）

（1） Modify the name of the table （rename to）

take product Change the name of the table to product2

ALTER TABLE product RENAME TO product2;

（2） Modify the character set of the table （character set）

take demo In the database product2 The character set of the table is changed to utf8.

ALTER TABLE product2 CHARACTER SET utf8;

notes ： You can compare it with the above

--  Modify the character set of the database

ALTER DATABASE Database name CHARACTER SET Character set ;

--  Specify character set to create database

CREATE DATABASE Database name CHARACTER SET Character set ;

--  modify surface Character set for

ALTER TABLE Table name CHARACTER SET Character set ;

（3） Add a separate column （add）

take product2 Add a column to the table color

ALTER TABLE product2 ADD color VARCHAR(10);

（4） Modify the data type of a column in the table （modify）

ALTER TABLE product2 MODIFY color INT;

Have already put varchar(10) Change it to int data type . 

（5） Change column name and data type （change）

ALTER TABLE Table name CHANGE Old column names New column names   The data type of the new column ;

ALTER TABLE product2 CHANGE color address VARCHAR(200);

（6） Delete a column （drop）

Delete address Column

ALTER TABLE product2 DROP address;

Deletion of data table

（1） Delete data table

DROP TABLE product2;

（2） Delete data table （ Judge , Delete if it exists ）

DROP TABLE IF EXISTS product2;

DML Data operation language

New table data

（1） Add data to the specified column

The number of column names and values as well as the data type should be Corresponding , Except for numeric types , All other data types should be quoted ,（ single 、 Both are OK , Single quotation marks are recommended ）

-- Complete writing

INSERT INTO Table name ( Name 1, Name 2,...) VALUES ( value 1, value 2,...); 

INSERT INTO product (id,NAME,price,num) VALUES (100,' Zhang San ',5,5);

SELECT * FROM product;

Be careful ：

In this way, add a row of data , A data item can have no data , When the position of the corresponding column name should be removed , as follows ：

id by 100,name For Li Si ,price It's empty ,num It's empty .

INSERT INTO product (id,NAME) VALUES (100,' Li Si ');

（2） Add data to all columns by default

-- Omit column names

INSERT INTO Table name VALUES ( value 1, value 2, value 3,...);

INSERT INTO product VALUES (101,' The computer ',9999.99,23);

（3） Batch add data

-- Use between multiple values "," separate

INSERT INTO Table name VALUES ( value 1, value 2, value 3,...),( value 1, value 2, value 3,...),( value 1, value 2, value 3,...);

INSERT INTO product VALUES 
		(101,' The computer ',9999.99,23),
		(102,' The notebook ',2.99,2),
		(103,' Rice cooker ',99.99,53);			

SELECT * FROM product;

modify 、 Delete table data

（1） Modify the data in the table   （update）

UPDATE Table name SET Name 1= value 1, Name 2= value 2,... [WHERE Conditions ];

Modify the statement There must be conditions , If there is no condition , All data will be All modified .

UPDATE product SET price=3500 WHERE NAME = ' The computer ';

take name The value is computer price Value changed to 3500.

（2） Delete data from table （delete）

DELETE FROM Table name [WHERE Conditions ];

Delete... In the statement There must be conditions , If there is no condition , All data will be deleted .

DELETE FROM product WHERE NAME = ' The computer ';

take name The value is the computer record deleted .