[NF] Conditions in JOIN clause instead of WHERE

Hey, all. I had a general SQL question. Often, I find myself needing to use the syntax:

FROM a LEFT JOIN b ON a.key = b.key AND b.field = 'Value'

because if I filter b.field in the WHERE clause, I've effectively made the LEFT JOIN an inner one.

That makes me wonder, at what point do we stop moving conditions to the JOIN clause? Obviously, some can't move for syntactical reasons, such as subqueries. But short of that, when does it make sense to limit a non-outer join in the join criteria? Never?

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