How is resolved tables visibility in sql join - sql

Question

I have problems understanding the correct syntax of sql using explicit joins.
For example in the first snippet, in the second join "ec" is visible and the restriction works. But in the second snippet "op" cannot be resolved.
First snippet:
select im.*
from gestion_corte gc
join evento_corte ec
on ec.id_corte = gc.id_corte
join op_corte op
on op.id_corte = gc.id_corte
and op.fecha_evento = ec.fecha_evento
join img_corte im
on op.id_corte = im.id_corte
where ec.fecha_evento > '01092012'
Second snippet, "op" cannot be resolved in first join:
select im.*
from gestion_corte gc
join evento_corte ec
on ec.id_corte = gc.id_corte
and op.fecha_evento = ec.fecha_evento -- This condition has moved
join op_corte op
on op.id_corte = gc.id_corte
join img_corte im
on op.id_corte = im.id_corte
where ec.fecha_evento > '01092012'
In consequence, the visibility is resolved processing from top to bottom? are any other important thing to have in consideration?

Answer 1

In the SQL join syntax, an alias is not known until it is defined. So, you can do:
from A a join
B b
on a.val = b.val join
C c
on a.val = c.val
However, you cannot do:
from A a join
B b
on a.val = b.val and
a.id = c.id join
C c
on a.val = c.val
The SQL engine simply does not know what "c" is, because "c" has not been seen yet in the from statement.
You second query is easy to fix by moving the condition, as you discovered.

Answer 2

A join is between two virtual tables.
The join predicate can only refer to columns from these two virtual tables (though each virtual table may consist of columns from multiple base tables).
It is not necessarily the case that the order of valid usage is top down as the logical join order is determined by the order of the ON clauses. So for example in the following query (SQL Fiddle) the virtual table (a join b) is joined onto the virtual table (c join d join e)
SELECT *
FROM a
JOIN b
ON a.id = b.id /*Only a and b in scope here*/
JOIN c
JOIN d
ON c.id = d.id /*Only c and d in scope here*/
JOIN e
ON e.id = d.id /*Only c, d, e in scope here*/
ON d.id = a.id /*a,b,c,d,e all in scope*/
This query (SQL Fiddle) would give an error as a is not in scope there.
SELECT *
FROM a
JOIN b
ON a.id = b.id
JOIN c
JOIN d
ON c.id = d.id
JOIN e
ON e.id = a.id -- Can't reference a here
ON d.id = a.id

Answer 3

I don't get your second question, so I have only answer to first one.
Your joins can be imagined this way:
"result" = select "some columns" from "any source"
"any source" can be a table or a subquery (i.e. another "result") or "any source" join "any source":
"any source" = table | "result" | "any source" join "any source"
So if you want a "result" you should determine "any source" first then "some columns" (let's don't focus on it). Let's go to the "any source" definition to determine it. Table is determined, "result" recursively leads to prevprevious sentence and join construction has a rule - to determine it firstly get left "any source" then right.
join is a left associative operation and isn't a commutative operation generally (inner join and cross join are, but not the left join or right join).
So, look at your 2nd snippet. This part gestion_corte join evento_corte hasn't been calculated because of op_corte hasn't been determined yet