SQL left connection, internal connection writing method "recommended collection"

Hello everyone , I meet you again , I'm your friend, Quan Jun .

The meaning of left connection ： Use LEFT JOIN keyword , From The left table Back there to all the lines , Even in Right table There are no matching lines in .

  1） Left connection    
  select a.*, b.*  
  from table1 a 
  left join table2 b     
  on a.id = b.parent_id   


  2）  Internal connection    
  select a.*, b.*   
  from table1 a 
  inner join table2 b
  on a.id = b.parent_id   

   or 

  select a.*, b.*   
  from table1 a 
  join table2 b
  on a.id = b.parent_id  

   Inner connection written  inner join  or  join, No difference 

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