Mysql database: introduction to database 𞓜 addition, deletion, modification and query

 database (database) It's for organizing , Warehouse for storing and managing data 
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database

Traditional database ( Also known as ： Relational database or SQL database ) MySQL,Oracle,SQL Server New database ( Also known as ： Non relational database or NoSQL database ) MongoDB

The data organization structure of traditional database

• database (database)
  • In development , Usually each project corresponds to a separate database
• Data sheet (table)
  • Different data , Stored in different tables
  • For example, the user data is in users surface , Book data in books In the table
• data row (row)
  • The rows in the table , Represents each specific piece of data
• Field (field)
  • What information is stored in each table , Determined by the field
  • Such as users Table design id,username,password These three fields
  • Fields are similar to Excel The column of ,
  • Each field has a corresponding data type

Create database

  Don't use Chinese , Space with '_' Instead of 
 
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Create data table

• DataType data type
  • int Integers
  • varchar(length) character string
  • tinyint(int) Boolean value
• Special identification of the field :
  • PK(Primary Key) Primary key , Unique identification
  • NN(Not Null) Value cannot be empty
  • UQ(Unique) It's worth it
  • AI(Auto Increment) The value increases automatically ( Usually to id Set up auto grow )
  • Default/Expression The default value is ( When not assigned , Automatically set to default )

hypothesis id Is an automatically increasing and unique value , If the row data is deleted , The new data will not be used again id value

SQL： Structured query language

SQL Is a database programming language 
SQL Code written in language , namely SQL sentence 
 Can only be used in relational databases (MySQL,Oracle,SQL Server)
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grammar

SQL Keywords in statements are case insensitive , therefore SELECT,FROM Equivalent to select,from
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• Select Statement is used to query data from a table , The results of the execution are stored in a result table ( Called result set ). The syntax is as follows

-- notes 
-- from from designated 【 In the table 】, Query out 【 all 】 The data of ,* Represents all columns 
SELECT * FROM  The name of the table 

-- from FROM designated 【 In the table 】, Query the specified   Column name ( Field )  The data of 
SELECT  Column name  FROM  The name of the table 

-- Between multiple columns ',' separate 
SELECT username,password FROM users
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• Insert into Statement is used to insert a new data row into a data table

    --  Be careful ： Columns and values should correspond to each other , Between multiple columns and multiple values , Use English commas to separate 
            INSERT INTO table_name ( Column 1, Column 2,...) VALUES ( value 1, value 2,...)
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• Update Statement is used to modify the data in the table

    --  use UPDATE  Specify which table data to update 
    --  use SET Specify the new value corresponding to the column 
    --  use WHERE  Specify the conditions for updating 
    -- UPDATE  The name of the table  SET  Column name  =  The new value  WHERE  Column name  =  Certain value 
        UPDATE users SET password = 'zs8888' WHERE id = 7
    --  Update several columns in a row 
        UPDATE users SET password = '123456',username = 'zs' WHERE ID = 2
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• Delete Statement to delete rows in a table

     Be careful ： Use delete If you forget to add where The condition will delete the data of the whole table 
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--  From the specified table , according to where The conditions offered , Delete the corresponding data row 
DELETE FROM TABLE_NAME WHERE COL_NAME = VALUE
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• where Clause is used to qualify the selection criteria

   stay select,update,delete In the sentence , Can be used where Clause to limit the selection criteria 
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• Operator

  • = be equal to
  • <> It's not equal to ( In some versions ,<> It can be written. '!=')
  • > Greater than
  • < Less than
  • >= Greater than or equal to
  • <= Less than or equal to
  • BETWEEN In a certain range
  • LIKE Search for a pattern

• and/or Statements in where Combining two or more conditions in a clause

    SELECT * FROM users WHERE uername<>'zs' AND id<3
    SELECT * FROM users WHERE uername<>'zs' OR id<3
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