Concepts and differences of DQL, DML, DDL and DCL

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        Two . 
       
       SQL The classification of languages 
       
       

       
       

       
       SQL Language can be divided into four categories ： Data query language DQL, Data manipulation language DML, Data definition language DDL, Data control language DCL.
       
       

       
       

       
       1. 
       
        Data query language DQL
       
       

       
        Data query language DQL The basic structure is made up of SELECT Clause ,FROM Clause ,WHERE
       
       

       
        The query block made up of clauses ：
       
       

       
       SELECT 
       
       <
       
        Field name table 
       
       >
       
       

       
       FROM 
       
       <
       
        Table or view name 
       
       >
       
       

       
       WHERE 
       
       <
       
        Query criteria 
       
       >
       
       

       
       

       
       2 .
       
        Data manipulation language DML
       
       

       
        Data manipulation language DML There are three main forms ：
       
       

       
       1
       
       ) 
       
        Insert ：INSERT
       
       

       
       2
       
       ) 
       
        to update ：UPDATE
       
       

       
       3
       
       ) 
       
        Delete ：DELETE
       
       

       
       

       
       3. 
       
        Data definition language DDL
       
       

       
        Data definition language DDL Used to create various objects in the database 
       
       -
       
       -
       
       -
       
       -
       
       -
       
        surface 、 View 、
       
       

       
        Indexes 、 A synonym for 、 Clustering, etc ：
       
       

       
       CREATE 
       
       TABLE
       
       /
       
       VIEW
       
       /
       
       INDEX
       
       /
       
       SYN
       
       /
       
       CLUSTER
       
       

       
       | 
       
       | 
       
       | 
       
       | 
       
       |
       
       

       
        surface  
       
        View  
       
        Indexes  
       
        A synonym for  
       
        cluster 
       
       

       
       

       
       DDL The operation is implicitly submitted ！ You can't rollback 
       
       

       
       

       
       4. 
       
        Data control language DCL
       
       

       
        Data control language DCL To grant or reclaim access to a database , And control 
       
       

       
        Time and effect of database manipulation transaction , Monitor the database . Such as ：
       
       

       
       1
       
       ) 
       
       GRANT： to grant authorization .
       
       

       
       

       
       

       
       2
       
       ) 
       
       ROLLBACK [
       
       WORK] 
       
       TO [
       
       SAVEPOINT]
       
       ： Go back to a certain point .
       
       

       
        Roll back 
       
       -
       
       -
       
       -
       
       ROLLBACK
       
       

       
        The rollback command returns the database state to the last committed state . The format for ：
       
       

       
       SQL
       
       >
       
       ROLLBACK;
       
       

       
       

       
       

       
       3
       
       ) 
       
       COMMIT [
       
       WORK]
       
       ： Submit .
      
      
      
      

       
       
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