Defining Right and Left Tables in SQL JOINS

Question

I wanted to ask is there a way to know which table should be right or left while using joins in SQL? for example I have two tables that I am joining, for example,

SELECT * FROM filtered_numbers LEFT JOIN telecoms ON filtered_numbers.TelecomID = telecoms.ID

Now The 'filtered_numbers' table has 200 records while 'telecoms' has only 5. My question is that will there be a difference in performance / efficiency of the db if I interchange the position of these two tables in the query? If yes, why?

Answer 1

This answer is a little complicated. The first thing to note is that

FROM filtered_numbers LEFT JOIN
     telecoms
     ON filtered_numbers.TelecomID = telecoms.ID

is semantically quite different from:

FROM telecoms LEFT JOIN
     filtered_numbers
     ON filtered_numbers.TelecomID = telecoms.ID

Switching only the tables changes the meaning of the query. The execution plan and performance will be different. You will get the same query if you change the LEFT JOIN to a RIGHT JOIN in this case, but as Strawberry points out, LEFT JOIN is much preferred.

This is simplified though and not always true. If you have a complicated query, the ordering of the JOIN s can make a difference. Now, LEFT JOIN has a "driving" table (where all rows are kept) so there are many fewer options. But if you have INNER JOIN s, then ordering can matter.

In fact, it can matter so much that MySQL offers optimizer hints just to control join order .

So, the answer to your question is:

• Switching the tables in your query will affect performance because the meaning of the query changes.
• Switching the tables and changing to a RIGHT JOIN will not affect performance because there is really only one option for the optimizer.
• In more complex queries, the ordering of joins can be important.