表上的（SQL）联接总是创建这些表的笛卡尔积吗？

Question

does JOINING on TABLES ALWAYS create Cartesian products of those tables ?
if a problem can be solved by sub-query and Joining which should i prefer ?
which one is fast and memory saving ?

All the questions are tightly coupled, so please dont tell to open another thread :(

MySQL 5

Answer 1

1. Yes, joining always creates a Cartesian product. That's why you usually add a condition to be met: JOIN a, b ON key_a = key_b . If the key_b is a UNIQUE KEY in table b, you will have at most one row from b for each row of a.

2. In most cases, joins are faster, but there are exceptions ( WHERE x IN ANY (subquery) )

3. See 2.

Answer 2

does JOINING on TABLES ALWAYS create Cartesian products of those tables ?

Yes, but normall that is then filtered down and the query optimizer is smart enough to take a more efficient approach. Anyhow, all joins without conditions ARE cartesian products ni the start.

if a problem can be solved by sub-query and Joining which should i prefer ?

Depends a lot on the query optimizer and the data in question. As in: Try it out. Prettym uch the only way.

which one is fast and memory saving ?

Again - try it out.

This stuff really depends no the table sizes involved AND THE FILTER CONDITIONS. Basically a materialized cartesian product in most cases is bad programming (ups, forgot the fitler conditions) or a very bad query plan. In most cases the cartesian product iwll never materialize and the query optimizer will use a different approach.

Answer 3

Yes, JOINING on tables always create Cartesian products of those tables.
you can't prefer join or sub-query because it depends on your data size
so, you have to try the two ways then you can decide which one is more appropriate to your query