mysql, index

 Index: usually when construction of the table are built. For details, video position  https://www.bilibili.com/video/av29072634/?p=5

    Category: 
        primary key index: can not be repeated. id can not be null 
        unique index: can not be repeated. null id may be 
        single-valued index: single, age; a plurality of tables can be single-valued index, name. 
        Composite Index: Index (corresponding secondary directory: z: zhao) composed of a plurality of columns (name, age) (a, b, c, d, ..., n) 
    to create an index: 
        Mode 1: 
        Create Index index type name on the table (field) 
        single-value: 
        Create index dept_index on TB (Dept); 
        unique: 
        Create uNIQUE index name_index on TB (name); 
        composite index 
        create index dept_name_index on tb (dept, name); 

        Second way: alter table table name index index type name (field) 
        
        single-value: 
        ALTER index dept_index the Add Table TB (Dept); 
        unique:
        alter table tb add unique index name_index ( name); 
        composite index 
        alter table tb add index dept_name_index (dept , name); 

        Note: If a field is a primary key, then the change field default is the primary key index     
    

        delete indexes: 
        drop index index name on table name; 
        drop index name_index ON TB; 

        query index: 
        Show index from table name; 
        Show index from the table name \ G

SQL optimization (No)