LEFT JOIN vs. RIGHT JOIN: When Do These SQL Joins Produce Different Results?

Understanding LEFT JOIN and RIGHT JOIN in SQL: the subtleties of table relationships

SQL’s “JOIN” operator provides a powerful mechanism when merging data from multiple tables. "LEFT JOIN" and "RIGHT JOIN" are two variations in JOIN syntax. While it seems intuitive that these two commands should produce the same result, a closer look reveals subtle differences between them.

Let’s illustrate this difference with two example tables:

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

Now, let’s fill these tables with data:

Table1

Id	Name
1	A
2	B

Table2

Id	Name
1	A
2	B
3	C

If we execute the following SQL statement:

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

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

We will get the same results because both queries perform a "LEFT JOIN" which gives priority to the rows in the left table (Table1). Therefore, all rows from Table1 are included in the output, while rows from Table2 are concatenated only if they have matching ids in Table1.

Now, let’s explore the third variation: “Table2 LEFT JOIN Table1”. By swapping the order of tables in the JOIN statement, we perform a "RIGHT JOIN" which gives priority to the rows in the right table (Table2).

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

This query will return all rows in Table2, even rows with id 3 and no matching id in Table1. Additionally, any matching rows in Table1 will also be included.

So, "LEFT JOIN" and "RIGHT JOIN" are interchangeable when the left table contains all matching ids; however, when the right table contains unmatched ids, their results will be different . "LEFT JOIN" prioritizes the left table, and "RIGHT JOIN" prioritizes the right table, ensuring that all rows from the prioritized table are included in the output.

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