When Are LEFT JOIN and RIGHT JOIN Truly Interchangeable?

Understanding the Subtle Differences Between LEFT JOIN and RIGHT JOIN

While LEFT JOIN and RIGHT JOIN might seem interchangeable when using the same join condition, their behavior differs significantly depending on the order of tables in the query. Let's explore this with an example.

Consider two tables:

CREATE TABLE Table1 (id INT, Name VARCHAR(10));
CREATE TABLE Table2 (id INT, Name VARCHAR(10));

Populated with the following data:

Table1		Table2
Id	Name	Id
1	A	1
2	B	2
		3

When joining using Table1.id = Table2.id, the results of LEFT JOIN and RIGHT JOIN are indeed identical:

SELECT *
FROM Table1
LEFT JOIN Table2 ON Table1.id = Table2.id;

SELECT *
FROM Table2
RIGHT JOIN Table1 ON Table1.id = Table2.id;

This is because the join condition is symmetrical. However, the illusion of interchangeability breaks down when the table order is reversed.

The Impact of Table Order

Observe the following query:

SELECT *
FROM Table2
LEFT JOIN Table1 ON Table1.id = Table2.id;

This returns all rows from Table2, regardless of a match in Table1. The result set would be:

Table2		Table1
Id	Name	Id
1	A	1
2	B	2
3	C

Conversely, a RIGHT JOIN with reversed table order:

SELECT *
FROM Table2
RIGHT JOIN Table1 ON Table1.id = Table2.id;

This only includes rows from Table2 that have a matching row in Table1. The row with id = 3 in Table2 would be excluded.

Therefore, while the join condition might appear symmetrical, the choice between LEFT JOIN and RIGHT JOIN, and the order of tables, fundamentally determines which table's rows are guaranteed to be included in the final result set, even if there is no match in the other table. Always carefully consider the table order to ensure you get the expected results.

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