What is worse about a 'where' or 'join'? Which is more expensive?

In the background a join will work by searching the way where is done. How this where will be mounted depends on the SQL parser of the database in question. The analyzer can be better or worse according to the vendor and this can give better or worse optimizations depending on the situation. There is no magic.

It depends on how it is written. A simple% of% is certainly less costly. A where that makes relationships between two or more tables may have the same cost as where , depending on how it is written. If the programmer is very clever and the situation allows, he can make a join that does the same thing that a certain where would do more efficiently. If the programmer is not so good, there is a good chance he will write join to get the same result that is worse than where .

If you have a concrete case, write with join and write another with join , take the test and come to a conclusion. It's the only way to guarantee this. Even if you have a lot of experience and believe you have good intuition (you are experienced really do not believe it), you still should not trust it, you should test.

Send a command where , excute plan or something like this.

This example will likely produce the same results for most databases:

SELECT * FROM a, b WHERE a.ID = b.ID;

SELECT * FROM a JOIN b ON a.ID = b.ID;

In a better context the answer could be better contextualized, but overall this is it.