SQL Basics

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【MySQL】

1. SQL The classification of

• SQL The function of language is mainly divided into the following 4 Categories: ：
• (1) DDL（Data Definition Languages – Data definition language ）：
  • DDL It can be used to 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 .
  • DDL The main statement keywords include ( First understand the meaning of keywords )：
    1. CREATE： establish
    2. ALTER： modify
    3. DROP： Delete
    4. RENAME： rename
    5. TRUNCATE： Delete the data and create a new one as before ( Empty )
• (2) DML（Data Manipulation Language – Data operation language ）：
  • Used to add 、 Delete 、 Update database records , And check data integrity .
  • DML The main statement keywords include ( First understand the meaning of keywords )：
    1. INSERT： insert data
    2. DELETE： Delete data
    3. UPDATE： modify / Update data
• (3) DQL（Data Query Language – Data query language ）：
  • Used to query database records .
  • DQL The main statement keywords include ( First understand the meaning of keywords )：
    1. SELECT( A top priority )： Query data
• (4) DCL（Data Control Language – Data control language ）：
  • Used to define the database 、 surface 、 Field 、 User's access rights and security level .
  • DCL The main statement keywords include ( First understand the meaning of keywords )：
    1. COMMIT： Commit transaction
    2. ROLLBACK： Roll back the transaction
    3. SAVEPOINT： Rollback to a specific savepoint
    4. GRANT： To give permission
    5. REVOKE： Revoke authority
• (5) Add ：
  • Some people will COMMIT 、 ROLLBACK Take it out and call it TCL （Transaction Control Language, Transaction control language ）.

First, add some explanations SQL sentence ：
(1) Specify which database to use ：USE Database name ;
(2) View currently used databases ：SELECT DATABASE();
(3) View all databases ：SHOW DATABASES;
(4) View all tables in the current database ：SHOW TABLES;

2.SQL The rules and norms of

2.1 SQL The rules of ( Must abide by )

1. SQL It can be written on one or more lines . To improve readability , Write each clause separately , Use indent if necessary ( Sometimes a sentence SQL The statement may be very long ).

   for example ：

   DELETE FROM edge WHERE (edge_first = 2 AND edge_second = 3) OR (edge_first = 3 AND edge_second = 2);
   #  It's written in 
   DELETE FROM edge
   WHERE (edge_first = 2 AND edge_second = 3) OR
         (edge_first = 3 AND edge_second = 2);
   

2. Every order is made with ; or \g or \G end
   

   It can be seen from the screenshot that ; End and end with \g The execution results at the end are consistent , With \G Execution result at the end , Only the data is displayed in different ways , From the original use of tables for display to the use of dictionaries ( Column names in the table : result ) Display in the format of .

3. Keywords cannot be abbreviated or separated

   SHOW DATA
   BASES;
   

   [42000][1064] You have an error in your SQL syntax; 
   check the manual that corresponds to your MySQL server 
   version for the right syntax to use near 'DATA BASES' 
   at line 1
   

4. About punctuation
   ① All must be guaranteed ()、 Single quotation marks 、 Double quotes end 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

2.2 SQL The specification of ( It is recommended that )

1. MySQL stay Windows The environment is case insensitive
   stay windows Create two new folders , The names of the two folders are the same , Only the case of letters is different ,window You will think that the names of two folders are the same , So in windows Case insensitive under the system , As a result windows Under the SQL Case insensitive .
   
   

2. MySQL stay Linux The environment is case sensitive
   ① Database name 、 Table name 、 The table alias 、 Variable names are strictly case sensitive
   ② keyword 、 Function name 、 Name ( Or field name )、 Alias of column ( The alias of the field ) It's case insensitive .

3. A unified writing standard is recommended ( Prevent being affected under different systems )：
   ① Database name 、 Table name 、 Table alias 、 Field name 、 Fields, aliases, etc. are all lowercase
   ② SQL keyword 、 Function name 、 Bound variables, etc., are all capitalized

2.3 Naming rules ( understand )

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
5. You must ensure that your field has no and reserved words 、 Database systems or common methods conflict . If you insist on using , Please be there. SQL Use in statement `（ mark of emphasis ,1 To the right of , Top left of keyboard ） Lead up
6. Keep field names and types consistent , When naming fields and specifying data types for them, be sure to ensure consistency . If the data type is an integer in a table , Then don't turn into character in another table

3.MySQL Notes

1. Single-line comments ：

   grammar ：

   #  What needs to be annotated 
   
   #  The annotation method is MySQL Unique 
   

   perhaps

   --  What needs to be annotated 
   
   --  Use this method to annotate ,--  There must be a space after , Otherwise, the line is not considered to be annotated 
   

   

2. Multiline comment ：
   grammar ：

   /*
    What needs to be annotated 
    What needs to be annotated 
    What needs to be annotated 
   */
   

   Cannot nest in multiline comments