Joins with WHERE - splitting WHERE clauses - sql

Question

I solved the query at this link
Can you return a list of characters and TV shows that are not named "Willow Rosenberg" and not in the show "How I Met Your Mother"?
with the following code:
SELECT ch.name,sh.name
FROM character ch
INNER JOIN character_tv_show chat
ON ch.id = chat.character_id
INNER JOIN tv_show sh
ON chat.tv_show_id=sh.id
WHERE ch.name != "Willow Rosenberg" AND sh.name !="How I Met Your Mother"
;
However, my first try was:
SELECT ch.name,sh.name
FROM character ch
WHERE ch.name != "Willow Rosenberg" /*This here*/
INNER JOIN character_tv_show chat
ON ch.id = chat.character_id
INNER JOIN tv_show sh
ON chat.tv_show_id=sh.id
WHERE sh.name !="How I Met Your Mother"
;
because I thought that in this way only the table character would have been filtered before doing the joins and, therefore, it would have been less computationally heavy.
Does it make any sense?
Is there a way to "split" the WHERE clause when joining multiple tables?

Answer 1

Think of JOINs as a cross-product of two tables, which is filtered using the conditions specified in the ON clause. Your WHERE clause is then applied on the result set, and not on the individual tables participating in the join.
If you want to apply WHERE on only one of the joined tables, you'll have to use a sub-query. The filtered result of that sub-query will then be treated as a normal table and joined with a real table using JOIN again.
If you are doing this for performance, remember though that a join is almost always faster on standard JOINs compared to sub-queries, for properly indexed tables. You'll find that queries using JOIN will be orders of magnitude faster than the ones using sub-queries, except for rare cases.

Answer 2

You can using subqueries
SELECT ch.name,sh.name
FROM (
SELECT ch.name
FROM character ch
WHERE ch.name != "Willow Rosenberg") ch
INNER JOIN character_tv_show chat
ON ch.id = chat.character_id
INNER JOIN tv_show sh
ON chat.tv_show_id=sh.id
WHERE sh.name !="How I Met Your Mother"
but i think it don't have sense. subqueries will make temp table.
First query will be optimized by database server, and likely select only rows from character table that need

Answer 3

JOIN and WHERE clauses are not necessarily executed in the order you write them. In general, the query optimizer will rearrange things to make them as efficient as possible (or at least what it thinks is most efficient), so adding a second WHERE clause wouldn't be any different from adding another AND condition (which is why it's not allowed).
Your idea wasn't bad, but it's just not how databases actually work.

Answer 4

A SELECT can only have 1 WHERE clause.
And it comes after the JOIN's.
But you can have additional WHERE clauses in the sub-queries you join.
And sometimes a criteria that you've added to a WHERE clause can be moved to the ON of a JOIN.
For example the queries below would return the same results
SELECT *
FROM Table1 AS t1
JOIN Table2 AS t2 ON t2.ID = t1.table2ID
WHERE t1.Col1 = 'foo'
AND t2.Col1 = 'bar'
SELECT *
FROM
(
SELECT *
FROM Table1
WHERE Col1 = 'foo'
) AS t1
JOIN Table2 AS t2 ON t2.ID = t1.table2ID
WHERE t2.Col1 = 'bar'
SELECT *
FROM Table1 AS t1
JOIN Table2 AS t2 ON (t2.ID = t1.table2ID AND t2.Col1 = 'bar')
WHERE t1.Col1 = 'foo'