Are "from Table1 left join Table2" and "from Table2 right join Table1" interchangeable?

Question

For example, there are two tables:

create table Table1 (id int, Name varchar (10))

create table Table2 (id int, Name varchar (10))

Table1 data as follows:

    Id     Name     
    -------------
    1      A        
    2      B    

Table2 data as follows:

    Id     Name     
    -------------
    1      A        
    2      B 
    3      C

If I execute both below mentioned SQL statements, both outputs will be the same:

select *
from Table1
  left join Table2 on Table1.id = Table2.id

select *
from Table2
  right join Table1 on Table1.id = Table2.id

Please explain the difference between left and right join in the above SQL statements.

Answer 1

Select * from Table1 left join Table2 ...

and

Select * from Table2 right join Table1 ...

are indeed completely interchangeable. Try however Table2 left join Table1 (or its identical pair, Table1 right join Table2) to see a difference. This query should give you more rows, since Table2 contains a row with an id which is not present in Table1.

Answer 2

Table from which you are taking data is 'LEFT'.
Table you are joining is 'RIGHT'.
LEFT JOIN: Take all items from left table AND (only) matching items from right table.
RIGHT JOIN: Take all items from right table AND (only) matching items from left table.
So:

Select * from Table1 left join Table2 on Table1.id = Table2.id  

gives:

Id     Name       
-------------  
1      A          
2      B      

but:

Select * from Table1 right join Table2 on Table1.id = Table2.id

gives:

Id     Name       
-------------  
1      A          
2      B   
3      C  

you were right joining table with less rows on table with more rows
AND
again, left joining table with less rows on table with more rows
Try:

 If Table1.Rows.Count > Table2.Rows.Count Then  
    ' Left Join  
 Else  
    ' Right Join  
 End If  

Answer 3

select fields 
from tableA --left
left join tableB --right
on tableA.key = tableB.key

The table in the from in this example tableA, is on the left side of relation.

tableA <- tableB
[left]------[right]

So if you want to take all rows from the left table (tableA), even if there are no matches in the right table (tableB), you'll use the "left join".

And if you want to take all rows from the right table (tableB), even if there are no matches in the left table (tableA), you will use the right join.

Thus, the following query is equivalent to that used above.

select fields
from tableB 
right join tableA on tableB.key = tableA.key

Answer 4

You seem to be asking, "If I can rewrite a RIGHT OUTER JOIN using LEFT OUTER JOIN syntax then why have a RIGHT OUTER JOIN syntax at all?" I think the answer to this question is, because the designers of the language didn't want to place such a restriction on users (and I think they would have been criticized if they did), which would force users to change the order of tables in the FROM clause in some circumstances when merely changing the join type.

Answer 5

Your two statements are equivalent.

Most people only use LEFT JOIN since it seems more intuitive, and it's universal syntax - I don't think all RDBMS support RIGHT JOIN.

Answer 6

I feel we may require AND condition in where clause of last figure of Outer Excluding JOIN so that we get the desired result of A Union B Minus A Interaction B. I feel query needs to be updated to

SELECT <select_list>
FROM Table_A A
FULL OUTER JOIN Table_B B
ON A.Key = B.Key
WHERE A.Key IS NULL AND B.Key IS NULL

If we use OR , then we will get all the results of A Union B

Answer 7

select * from Table1 left join Table2 on Table1.id = Table2.id

In the first query Left join compares left-sided table table1 to right-sided table table2.

In Which all the properties of table1 will be shown, whereas in table2 only those properties will be shown in which condition get true.

select * from Table2 right join Table1 on Table1.id = Table2.id

In the first query Right join compares right-sided table table1 to left-sided table table2.

In Which all the properties of table1 will be shown, whereas in table2 only those properties will be shown in which condition get true.

Both queries will give the same result because the order of table declaration in query are different like you are declaring table1 and table2 in left and right respectively in first left join query, and also declaring table1 and table2 in right and left respectively in second right join query.

This is the reason why you are getting the same result in both queries. So if you want different result then execute this two queries respectively,

select * from Table1 left join Table2 on Table1.id = Table2.id

select * from Table1 right join Table2 on Table1.id = Table2.id

Answer 8

Select * from Table1 t1 Left Join Table2 t2 on t1.id=t2.id By definition: Left Join selects all columns mentioned with the "select" keyword from Table 1 and the columns from Table 2 which matches the criteria after the "on" keyword.

Similarly,By definition: Right Join selects all columns mentioned with the "select" keyword from Table 2 and the columns from Table 1 which matches the criteria after the "on" keyword.

Referring to your question, id's in both the tables are compared with all the columns needed to be thrown in the output. So, ids 1 and 2 are common in the both the tables and as a result in the result you will have four columns with id and name columns from first and second tables in order.

*select * from Table1 left join Table2 on Table1.id = Table2.id

The above expression,it takes all the records (rows) from table 1 and columns, with matching id's from table 1 and table 2, from table 2.

select * from Table2 right join Table1 on Table1.id = Table2.id**

Similarly from the above expression,it takes all the records (rows) from table 1 and columns, with matching id's from table 1 and table 2, from table 2. (remember, this is a right join so all the columns from table2 and not from table1 will be considered).