Day 7 summary & homework

Internal connection ：inner join      ： Only show the public data in the table
External connection ：left join /right join

Internal connection ： Display the data common to the table , No data is displayed ; Left connection in outer connection , Mainly the table on the left , If there is no data in the right table, it shows null, The right connection , Based on the table on the right , If there is no data in the left table, it shows null

Multiple tables associated query
     Two table operation
        select surface 1. Field , surface 2. Field from surface 1 inner join surface 2 on surface 1. Field = surface 2. Field where Conditions
     Three table operation
        select surface 1. Field , surface 2. Field , surface 3. Field from surface 1 inner join surface 2 on surface 1. Field = surface 2. Field inner join surface 3 on surface 1. Field = surface 3. Field where Conditions

Database optimization ：
     The rational use of char and varchar
     Build a reasonable table     Generally, the frequently used data is put in a table , Infrequently used in sub table   If the query uses multi table associated query
     Fair use sql sentence , Reduce subqueries   Use multi table associated query
     Index in table     
     Reduce the use of queries *   

Homework 1

Homework 2

Homework 3

Homework 4