You need to use MySQL in the opening experiment. How can you forget the basic select statement? Remedy is coming~

?? Catalog

• • ??SQL The classification of sentences
  • ??SQL Rules and norms of language
  • • ?? The basic rule
    • ??SQL Case specification （ It is recommended that ）
    • ?? notes
    • ?? Naming rules
    • ?? Data import instruction —— understand
    • • ?? Command line import and export
      • ?? Graphical management tools Navicat Premium Import and export of
  • ?? Basic SELECT sentence
  • • ??SELECT … FROM
    • ?? Alias of column
    • ?? Remove duplicate information
    • ?? Null values participate in the operation
    • ?? mark of emphasis
  • ?? Display table structure
  • ?? Filtering data
  • ?? summary

??SQL The classification of sentences

SQL Language is divided according to function , It is mainly divided into the following 3 Categories: ：
1、DDL（Data Definition Languages、 Data definition language ）, These statements define different databases 、 surface 、 View 、 Index and other database objects , It can also be used to create 、 Delete 、 Modify the structure of database and data table . The main statement keywords include CREATE 、DROP 、ALTER etc. .

2、DML（Data Manipulation Language、 Data operation language ）, Used to add 、 Delete 、 Update and query database records , And check data integrity . The main statement keywords include INSERT 、DELETE 、UPDATE 、SELECT etc. .SELECT yes SQL The basis of language , Above all .

3、DCL（Data Control Language、 Data control language ）, Used to define the database 、 surface 、 Field 、 User's access rights and security level . The main statement keywords include GRANT 、REVOKE 、COMMIT 、ROLLBACK 、SAVEPOINT etc.

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understand ：

Because query statements are used very frequently , Therefore, some people also bring out the query statement list , be called ：DQL（ Data query language ）.
And alone will COMMIT 、ROLLBACK Take it out and call it TCL （Transaction Control Language, Transaction control language ）.

??SQL Rules and norms of language

?? The basic rule

1、SQL It can be written on one or more lines . To improve readability , Write each clause separately , Use indent if necessary
2、 Every order is made with ; or g or G end
3、 Keywords cannot be abbreviated or separated
4、 About punctuation ：

1. All parentheses must be guaranteed ()、 Single quotation marks ’’、 Double quotes "" It ends in pairs
2. Half angle input mode in English must be used
3. Single quotation marks can be used for string and date time data （’ '） Express
4. Alias of column , Try to use double quotes （" "）, And it is not recommended to omit as

??SQL Case specification （ It is recommended that ）

MySQL stay Windows The environment is case insensitive MySQL stay Linux The environment is case sensitive

It is precisely because of the two development environments , Case sensitivity is different , To prevent yourself from getting used to windows How to write in the environment , stay Linux It may be embarrassing when developing , So it is recommended to follow .

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The following lists those that need to be strictly case sensitive ：

Database name 、 Table name 、 The table alias 、 Variable names are strictly case sensitive
keyword 、 Function name 、 Name ( Or field name )、 Alias of column ( The alias of the field ) It's case insensitive .

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With the development in depth , Engineers also imperceptibly formed some rules belonging to this circle ：
Database name 、 Table name 、 Table alias 、 Field name 、 Fields, aliases, etc. are all lowercase SQL Key words of 、 Function name 、 Bound variables, etc., are all capitalized

?? notes

The comments , It's something that happens in every development language , about SQL This is also the case , Because they facilitate our understanding of the code .

Single-line comments ：# Note text (MySQL In a particular way )
Single-line comments ：-- Note text (- - Must be followed by a space .)
Multiline comment ：/* Note text */

?? Naming rules

SQL The naming rules of are similar to those of other programming languages , I believe most people are familiar with it .

1、 database 、 The table name must not exceed 30 Characters , Variable names are limited to 29 individual
2、 Must contain only A–Z, a–z, 0–9, _ common 63 Characters
3、 Database name 、 Table name 、 Do not include spaces in object names such as field names
4、 The same MySQL In software , The database cannot have the same name ; In the same library , A watch cannot have the same name ; In the same table , The field cannot have the same name

Here is an additional explanation of a special ：

We must ensure that the fields we use , No and reserved words 、 Database systems or common methods conflict . If you insist on using , To be in SQL Use in statement `（ mark of emphasis ） Lead up

# order yes sql A common command in order by Part of 
SELECT * from `order`;

?? Data import instruction —— understand

?? Command line import and export

Command line sql File export ：

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1、 Specify a storage location sql File folder
Such as : E:StudyMyDB
Export database file ：mysqldump -u user name -p Database name -P Port number > Exported file name
For example, the code of the command line I entered is :mysqldump -u root -p test -P 3306>test.sql ( enter , Input MySQL Password )

If you want to export the specified table in the database ： Just add the table name after the database name
mysqldump -u user name -p Database name Table name -P Port number > Exported file name

On the command line sql Import of files

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Be careful ： Import means , Is to take what I have now sql The file is imported into the database .

Here it is , You need to open up a space for this upcoming little guy in the database , Used to store the data of the database to be imported later .

therefore , The whole process is ：

1、 Connect to database ：mysql -u { user name } -p enter Enter the password again
The implementation code is ：

mysql -u root -p

2、 Specify the target database name ：use { Library name }
The code is implemented as ：

use newtest 

3、 Import files source { route }
The code is implemented as ：

 source E:Studysql	est.sql

It should be noted that we need to have this file , If there is no exception

?? Graphical management tools Navicat Premium Import and export of

If not Navicat Premium This software , But if you want to try, you can download it from the link below ~
Navicat Premium
One 、 Import operation

Step one ： New database

The second step ： Name the newly created database ( Recommendations and sql Same file name )

The third step ： Click on " function SQL file "

Step four ： Select the file to import

Two 、 Export operation

stay Navicat Premium Export in is very concise .

?? Basic SELECT sentence

??SELECT … FROM

The grammar is ：
SELECT Specific query column name / Field name
FROM From which table to query

When you need to select all columns , Can use wildcard * To achieve

The grammar is ：
SELECT *
FROM From which table to query and select ;

Tips: ：

?? Alias of column

Usage mode ：
1、 Keep up with the column name , You can also add keywords between column names and aliases AS,
2、 When an alias contains spaces or special characters and is case sensitive , Aliases need to be enclosed in double quotes .

?? Remove duplicate information

By default , The query will return all rows , Include repeating lines . Because there's duplicate information , The whole result set will appear very cumbersome and bloated .
therefore ：

stay SELECT Use keywords in statements DISTINCT Remove duplicate lines

The syntax is ：
SELECT DISTINCT Name / Field name
FROM The table where the query field is located

?? Null values participate in the operation

All operators or column values encountered null value , The result of the operation is null

Be careful ：
stay MySQL Inside , A null value is not equal to an empty string . An empty string The length is 0, And a null value The length is empty . stay MySQL The empty value in the is the space occupied .

?? mark of emphasis

We need to ensure that the fields in the table 、 Table names, etc. have no and reserved words 、 Database systems or common methods conflict . If it's really the same , It needs to be in SQL A pair of... Is used in the statement ``（ mark of emphasis ） Lead up .

?? Display table structure

Use DESCRIBE or DESC command , Represents the table structure

The syntax is ：

DESCRIBE employees;
 or 
DESC employees;

?? Filtering data

Use the syntax ：
SELECT Field 1, Field 2
FROM Table name
WHERE Filter conditions

Use WHERE Clause , Filter out rows that do not meet the criteria

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WHERE Clause in the wake of FROM Clause

Sample demonstration

?? summary

This article mainly reviews SQL Some naming rules and specifications of statements , as well as SELECT Statement and FROM And WHERE The data needed to combine to query . Take a brief look at the usage Syntax , Write one or two by yourself SQL Query cases , You can master .

that , Here are some small exercises , Interested partners can start to write it ~
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