Subject Re: [firebird-support] Implicit vs. Explicit Joins
Author Ann W. Harrison

Flattery will get you a long way - though Helen is the real expert
in this area, in my opinion. The SQL-89 join syntax is OK if all your
joins are inner joins and you never mix the syntaxes.

In other words, this is old-fashioned but OK:

select ... from t1 x, t2 y, t3 z
where x.a = y.b and y.a = z. b

this is not good:

select ... from t1 x
inner join t2 y, t3 z on x.a = y.b
where y.a = z.b

this is very bad:

select ... from t1 x
left outer join t2 y, t3 z on x.a = y.b
where y.a = z.b

and this is terrible:

select ... from t1 x, t2 y
left outer join t3 z on y.a = z.b
where x.a = y.b

The problem is that there is a required precedence between the SQL-89
type join and the inner and outer SQL-92 joins that affects the
optimizer's ability to distribute equalities and perform filtering on
the input streams at the initial phases of the join. Those abilities
are critical to good performance. The precedence can also trip up the
logic of the query, leading to unexpected, but not inexplicable results.

So, to answer your question, using the SQL-89 joins in program (or
procedure) code is a lazy practice that shows a lack of willingness to
learn and very little consideration for the people who will inherit the
code. It also leads to poor performance and unexpected results.

Personally, I no longer write SQL-89 joins except in interactive tools
and change code that uses that join type as I find it.

Is that clear?