SQL Left JOIN keyword tutorial

SQL Left JOIN keyword tutorial

The LEFT JOIN keyword returns all rows from the left-hand table (TABLE_NAME1), even if there is no contest for the right table (table_name2).

Left-joined SQL syntax

SELECT column_name (s) from
table_name1 left
JOIN table_name2 on 
table_name1.column_name=table_ Name2.column_name

Okay, let's take a look at the example of the left join.

 
   
    Address 
       City 
     

  
    
    
 
     
 
      
Id
 
      
LastName
 
      
FirstName
 
      
 
     
 
      
1
 
      
Hansen
 
      
Ola
 
      
TIMOTEIVN 10
 
      
Sandnes
 
     
 
     
 
      
2
 
      
Svendson
 
      
Tove
 
      
BORGVN 23
 
      
Sandnes
 
     
 
     
 
      
3
 
      
Pettersen
 
      
Kari
 
      
STORGT 20
 
      
Stavanger
 
     
 
    
 
   
   

Let's look at the Orders table.

 
   
   

  
    
    
 
     
 
      
o_id
 
      
OrderNo
 
      
p_id
 
     
 
     
 
      
1
 
      
77895
 
      
3
 
     
 
     
 
      
2
 
      
44678
 
      
3
 
     
 
     
 
      
3
 
      
22456
 
      
1
 
     
 
     
 
      
4
 
      
24562
 
      
1
 
     
 
     
 
      
5
 
      
34764
 
      
15
 
     
 
    
 
   
   

  
  

   
   

   
Now we want to list all the people and their orders-if any, from the above table.

We use the following SELECT statement:
   

   
   
SELECT Persons.lastname, Persons.firstname, Orders.orderno from
Persons left
JOIN Orders on
persons.p_ id=orders.p_id ORDER BY
Persons.lastname

   
   


   
   
 
     
     

  
      
      
 
       
 
        
LastName
 
        
FirstName
 
        
OrderNo
 
       
 
       
 
        
Hansen
 
        
Ola
 
        
22456
 
       
 
       
 
        
Hansen
 
        
Ola
 
        
24562
 
       
 
       
 
        
Pettersen
 
        
Kari
 
        
77895
 
       
 
       
 
        
Pettersen
 
        
Kari
 
        
44678
 
       
 
       
 
        
Svendson
 
        
Tove
 
        
 
       
 
      
 
     
     

   
   


   
   
 
     
     

      
      
  The LEFT JOIN keyword returns all rows from the left-hand table (person), even if there is no game right table (order).