Question

I cannot effectively understand the difference between these two queries.

select *
from person p, mytlc m
where p.personid = m.personid and p.personid = 3635

select *
from person p
    join mytlc m on p.personid = m.personid
where p.personid=3635

In this case, I don’t think either will be a greater performing query;but, what if the query was more complex handling much more data.

Thanks!

Answer 1

If you want to find more information on this topic, you can try googling ‘join vs where’. Here are a couple of other questions that address the same thing:

1. Inner join vs Where
2. MySQL: Inner join vs Where
3. http://forums.devx.com/showthread.php?t=19242

A quote from the third one is interesting (regarding SQL Server, but they are probably similar in behavior):

If you measure this, you will most likely discover that the two
versions use the exact same access plan. SQL Server tries very hard to
optimize a query, and in that process, a where clause which equates
columns from two tables will be converted to an inner join.

These seems to indicate that technically the join is correct and more efficient than a where, but it doesn’t matter in practice because optimization will likely correct the where into a join. However, for cases where it won’t optimize, it is better to be explicit, and as indicated by others, join is the right way to do it.