After reading it, this is not a duplicate of Explicit vs Implicit SQL Joins. The answer may be related (or even the same) but the question is different.
What is the difference and what should go in each?
If I understand the theory correctly, the query optimizer should be able to use both interchangeably.
On an inner join, they mean the same thing. However you will get different results in an outer join depending on if you put the join condition in the WHERE vs the ON clause. Take a look at this related question and this answer (by me).
I think it makes the most sense to be in the habit of always putting the join condition in the ON clause (unless it is an outer join and you actually do want it in the where clause) as it makes it clearer to anyone reading your query what conditions the tables are being joined on, and also it helps prevent the WHERE clause from being dozens of lines long.
They are not the same thing.
Consider these queries:
SELECT *
FROM Orders
LEFT JOIN OrderLines ON OrderLines.OrderID=Orders.ID
WHERE Orders.ID = 12345
and
SELECT *
FROM Orders
LEFT JOIN OrderLines ON OrderLinse.OrderID=Orders.ID AND Orders.ID=12345
The first will return an order and it's lines (if any) for order number 12345. The second will return all orders, but only order 12345 will have any lines associated with it.
On INNER JOINs they are interchangeable, and the optimizer will rearrange them at will.
On OUTER JOINs, they are not necessarily interchangeable, depending on which side of the join they depend on.
I put them in either place depending on the readability.
In terms of the optimizer, it shouldn't make a difference whether you define your join clauses with ON or WHERE.
However, IMHO, I think it's much clearer to use the ON clause when performing joins. That way you have a specific section of you query that dictates how the join is handled versus intermixed with the rest of the WHERE clauses.
The way I do it is:
Always put the join conditions in the on clause If you are doing an inner join, so not add any where conditions to the on clause, put them in the where clause
If you are doing a left join, add any where conditions to the on clause for the table in the right side of the join. This is a must because adding a where clause that references the right side of the join will convert the join to an inner join (With one exception described below).
The exception is that when you are looking for the records that are not in a particular table, you would add the refernce to a unique identifier(that is not ever null) in the right join table to the where clause this way "Where t2.idfield is null". So the only time you should reference a table on the right side of the join is to find those records which are not in the table.
this is my solution.
SELECT song_ID,songs.fullname, singers.fullname
FROM music JOIN songs ON songs.ID = music.song_ID
JOIN singers ON singers.ID = music.singer_ID
GROUP BY songs.fullname
You must have the GROUP BY to get it to work.
Hope this help.