Join Vs. Sub-Query
I Am an Old-School Mysql User and Have Always Preferred Join over Sub-Query. but Nowadays Everyone Uses Sub-Query, and I Hate It; I Don't Know Why. I Lack the...
I am an old-school MySQL user and have always preferred JOIN over sub-query. But nowadays everyone uses sub-query, and I hate it; I don't know why.
I lack the theoretical knowledge to judge for myself if there is any difference. Is a sub-query as good as a JOIN and therefore is there nothing to worry about?
20 Answers
Sub-queries are the logically correct way to solve problems of the form, "Get facts from A, conditional on facts from B". In such instances, it makes more logical sense to stick B in a sub-query than to do a join. It is also safer, in a practical sense, since you don't have to be cautious about getting duplicated facts from A due to multiple matches against B.
Practically speaking, however, the answer usually comes down to performance. Some optimisers suck lemons when given a join vs a sub-query, and some suck lemons the other way, and this is optimiser-specific, DBMS-version-specific and query-specific.
Historically, explicit joins usually win, hence the established wisdom that joins are better, but optimisers are getting better all the time, and so I prefer to write queries first in a logically coherent way, and then restructure if performance constraints warrant this.
In most cases JOINs are faster than sub-queries and it is very rare for a sub-query to be faster.
In JOINs RDBMS can create an execution plan that is better for your query and can predict what data should be loaded to be processed and save time, unlike the sub-query where it will run all the queries and load all their data to do the processing.
The good thing in sub-queries is that they are more readable than JOINs: that's why most new SQL people prefer them; it is the easy way; but when it comes to performance, JOINS are better in most cases even though they are not hard to read too.
Taken from the MySQL manual (13.2.10.11 Rewriting Subqueries as Joins):
A LEFT [OUTER] JOIN can be faster than an equivalent subquery because the server might be able to optimize it better—a fact that is not specific to MySQL Server alone.
So subqueries can be slower than LEFT [OUTER] JOIN, but in my opinion their strength is slightly higher readability.
In the year 2010 I would have joined the author of this questions and would have strongly voted for JOIN, but with much more experience (especially in MySQL) I can state: Yes subqueries can be better. I've read multiple answers here; some stated subqueries are faster, but it lacked a good explanation. I hope I can provide one with this (very) late answer:
First of all, let me say the most important: There are different forms of sub-queries
And the second important statement: Size matters
If you use sub-queries, you should be aware of how the DB-Server executes the sub-query. Especially if the sub-query is evaluated once or for every row! On the other side, a modern DB-Server is able to optimize a lot. In some cases a subquery helps optimizing a query, but a newer version of the DB-Server might make the optimization obsolete.
Must Read
Sub-queries in Select-Fields
SELECT moo, (SELECT roger FROM wilco WHERE moo = me) AS bar FROM foo
Be aware that a sub-query is executed for every resulting row from foo.
Avoid this if possible; it may drastically slow down your query on huge datasets. However, if the sub-query has no reference to foo it can be optimized by the DB-server as static content and could be evaluated only once.
Alexander Ross has covered the video game industry for a decade, writing deep dives on game design, esports tournaments, VR developments, and gaming culture.
