The tables that are referenced in an ON clause must be part of the join that the ON clause modifies. For example, the following is invalid:
FROM ( A KEY JOIN B ) JOIN ( C JOIN D ON A.x = C.x )
The problem is that the join condition
A.x = C.x references table A, which is not part of the join it modifies (in this case,
C JOIN D).
However, as of the ANSI/ISO standard SQL99 and Adaptive Server Anywhere 7.0, there is an exception to this rule: if you use commas between table expressions, an ON condition of a join can reference a table that precedes it syntactically in the FROM clause. Therefore, the following is valid:
FROM (A KEY JOIN B) , (C JOIN D ON A.x = C.x)
See also: Key joins.
For more information about commas, see Commas.
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