This is what he means, and I see it all the time: SELECT FROM table1 JOIN table2 WHERE table1.field = table2.field

An unsettling number of SQL developers never actually learned join syntax and write it that way.

It's functionally equivalent to putting the equality in the JOIN ... ON clause, and any modern database will optimize it to execute the same way, but it's a worse syntactical representation of what's actually happening.

Great, thanks for that - I've been using (typically INNER) JOIN .. ON .. for all my life, well 20 odd years of development, and not once did I ever put the ON clause into WHERE. :)

I can vaguely recall seeing it a few times now, actually, and being rather confused as to why you'd do it that way - as perhaps it was a more optimal way of putting it, and that I'd done it wrong all the time?

As you state though, it's just bad; WHERE clauses ought to be used for filtering the data, not declaring how the tables join. Glad to have cleared that up. :)

You'd do it that way if you just didn't know any better. "Two tables where A = B" falls into a natural line of thinking for someone who picked up SQL ad hoc rather than formally learning what a join is and the syntax for it.