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Write your query using a subquery instead of a join when it
is easier for you to understand what the purpose or intent of the query is.
Remember, you not only have to write it, you may have
to come back six months later and figure out what it's doing!
I'll demonstrate with two examples:
Example 1
Do you prefer:
select a.foo, a.bar
from table1 a
inner
join table2 b
on a.id = b.id
and b.qux = '937'
group
by a.foo, a.bar
or:
select foo, bar
from table1
where id in
( select id
from table2
where qux = '937' )
Example 2
Do you prefer:
select a.foo, a.bar
from table1 a
left outer
join table2 b
on a.id = b.id
and b.qux = '937'
where b.id is null
or:
select foo, bar
from table1
where not exists
( select 1
from table2
where id = table1.id
and qux = '937' )
Ordinarily, when writing SQL, you should not be concerned with
performance. Instead, you should focus all your intellect on ensuring that you get
the correct results, and let the database optimizer figure out how to
satisfy the query. So the first consideration is whether the join and subquery techniques
actually do both produce the same correct results in either example.
The other consideration is maintainability. The first example returns rows from
table1 with matching qualified rows in table2, while the second example returns
only rows without qualified matches. Now consider how you would have to modify each
query in both examples to add another condition, to return only those
rows from table1 that also have a qualifying matching row in table3.
You may discover that the join technique is severely limiting.
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