Learning database MySQL (twenty-three) - DDL (a) Overview

Foreword '· ᴗ · `

• This section talked about the creation of the database table and so modify and delete
• In this part will help you learn ...
  • 0 DDL understand why the concept is called the data definition language
  • 1 Create Modify Delete Library
  • 2 Create Modify Delete table
  • 3
    the CREATE create
    ALTER modify
    DROP delete
  • 4 How to use DQL statement replication table

DDL

DDL data definition language
the name makes sense from a fact table represents a relationship between our real lives (relation), such as who the goddess boyfriend, such as what time of entry relationship with his staff bonus rate Yes. . . These relations turn into concrete realities abstract table will need to define the definition

Closer to home the core content is to create modify delete tables and tables of collections - Libraries

Library Operation

We played at the code to know in order to perform each line:

CREATE DATABASE IF NOT EXISTS db1;

RENAME DATABASE db1 TO db2;

DROP DATABASE db2;
DROP DATABASE IF EXISTS [旧表名];
CREATE DATABASE [新表名];

IF NOT EXISTS good sentence (remember we mentioned in Section 21 EXISTS)
refers to if we do not create a db1
any, we have to consider (Do not get no more data)

The second line is the change of name

The third sentence is deleted
changes to the database in fact, nothing to talk about
general we want to create a new database of the same name and also do not care about past data error not occurred we would write:

DROP DATABASE IF EXISTS [库名];
CREATE DATABASE [库名];
# 安全创造新的库

USE [库名]; # 库内再操作表的问题
DROP TABLE IF EXISTS [表名];
CREATE TABLE [表名];

It is noteworthy that create modify delete the table with exactly the same keywords

Create a table

Of course, do not forget to watch is to have fields (columns) attributes you so Create a table with no arguments really embarrassed
us look at a table is created Zeyang:

CREATE TABLE `departments` (
  `department_id`   INT(4)     NOT NULL AUTO_INCREMENT,
  `department_name` VARCHAR(3) DEFAULT NULL,
  `manager_id`      INT(6)     DEFAULT NULL,
  `location_id`     INT(4)     DEFAULT NULL,
)

• The first column of the field attribute name
• The second column indicates the data type will be behind the panellists data type of
• The third column indicates the constraint is also behind the panellists will be
  constrained (constrain) is not well understood improper actions such as gender attribute data of the objective world only "male", "female",
  added here is to prevent constraints do not fly to happen
  serious it XSS attack is sql injection attacks

XSS attacks: the js code to the server database via POST then the database and then return when the js code to the user of the client can be in the browser is running because it is returned by the server data General safety review is not strict like this js code may have high authority on your PC hands and feet :)

So the network security grabbed from the database (funny)

Here we are talking about several common data types give you a feel:

Code	Explanation
int	Plastic
double float	Float
char	Fixed length character string
varchar	Variable-length string
text	Longer string
date	date
time	time
timestamp	Timestamp

Wherein the double (5,2) means 5 digit 2 decimal places such as 999.99
and is very similar to other common programming languages such as C js, which do not go into detail
varchar char text where applicable each have their own are relatively common

Then there are basically these constraints:

Code	Explanation	English	Chinese
DEFAULT NULL	The default value is empty	default	Defaults
NOT	non-	not	denial
UNIQUE	The only value	unique	Unparalleled
AUTO_INCREMENT	The default increment in steps of 1	auto increase	Automatic growth
PRIMARY KEY	Primary key	primary key	The main keys
FOREIGN KEY	Foreign key	foreign key	External 键位

note

• These modifiers can be superimposed such as the default value is non-empty:
  NOT NULL(not effect a keyword DEFAULT)
• auto increment step size can be changed
  SET auto_increment_increment=3# 3 step
  but change the "Environment Variables" step all the tables are the same as the little pit is not recommended
• auto increment works: I want to achieve from the beginning has been id 10 111 213 ...
  can first insert data INSERT a id = 10 ( on a section taught )
  and then later insert other data when id there to fill in NULL without writing 1112 13 ... tables are automatically generated for you

Structure of the table

Our structure is called before the specified property name data type plus constraints

so some people say that modify the "table" is actually a modified structure of the table
so we create table to check how its structure is it desirable?
normal method:

DESC [表名]

In addition to this key constraints as well as our custom key can check out

The question is key to key what is?
Is essentially a row but we selected one or more columns then assign attributes
such as primary key PRIMARY KEY is the value of the only non-empty is
also a foreign key of course is the only value but also linked with other tables
there UNIQUE key is a unique value of

The constraints we will go into that section describes how to define the key , data definition language is defined dry live!

Also called key index (index) is actually a time just looking for something to say when the data put things said by the index key
so you can see there is a command to all of the index (key primary key foreign key and our own definition of key)

SHOW INDEX FROM [表名]

The above two commands can be combined to determine the overall look at the results

Modify table

Modification (the ALTER) is related to the structure on the column properties:

• Add column attributes
  to add a table stu classname columns:
  the ALTER TABLE stu the ADD (classname VARCHAR (100));
• Modify column attribute name
  modified stu table name to the gender column Sex:
  the ALTER TABLE stu Sex Change gender CHAR (2);
• (Therefore, the same data type is used to modify the constraint column attributes modifications) or modify the data type constraint
  modification stu gender column of the table column type is CHAR (2):
  the ALTER TABLE stu the MODIFY gender CHAR (2);
• Delete Column property
  to delete stu table classname columns:
  the ALTER TABLE DROP stu classname;

Anyway,
alter table [表名] add|drop|modify|change column [列名] [列类型] [约束]
so be sure to write

Copy the table

• Copy table structure
  that the white column data type attribute + + constraint

CREATE TABLE [复制出来的表] LIKE [被复制的表]

• Copy all (+ Data Structure)

CREATE TABLE [复制出来的表] SELECT * FROM [被复制的表];

• Copy part of the data in conjunction with DQL you will do anything
  that we check out the tables is a + a copy of the data if it is added to create a memory CREATE TABLE 【复制出来的表】that case we can write:

CREATE TABLE [复制出来的表] 
SELECT [column]
FROM [table1] AS [table1_name]
[join type] JOIN [table2] AS [table2_name]
ON [join condition]
WHERE [condition]
GROUP BY [grouping fields]
HAVING [grouping condition]
ORDER BY [order fields]
LIMIT [offset] [size]

Cattle breaking, right? :)
DQL DQL out why important because where results can be put to use!

Summary'◡`

In this section we know a bit emotional language DDL next section we look at the special data type specific content:
Next stop: the database learning MySQL (twenty-four) - DDL (b) data types

In addition,

• Well I want to learn database? You may wish to start from MySQL
  MySQL column

• python so fire you want to play with python-depth study of a simple application thing? I can see the column continues to update oh:
  Python application

• Child before doing all adults choose to be! Backend interested? Accept it now :)
  hands-on learning with your back-end (server)

• Thank you, big brother support! Meng new courtesy :)