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Posted by --CELKO-- on 10/24/07 20:46
>> But why/how is colX "available" in both tables? <<
colx is available to **the subquery** and not in both table at all.
Queries first try to reference the nearest column, and move outward in
the scope of query nesting.
>> Maybe there are some basics that I don't understand... <<
Scoping rules in SQL are similar to the scoping rules in block
structure programming languages like Algol, C, PL/I, ADA, etc.
A: BEGIN
x INTEGER;
y INTEGER;
..
B: BEGIN
x INTEGER;
x := x + y;
..
END;
END;
When you get to block B, the local x is referenced in that assignment
statement, but the y in the containing block A is referenced.
But SQL is a bit more complicated. If two query expression are on the
same level, then you have to use a LATERAL operator in Standard SQL.
When a query expression is given a name and is it called a derived
table. Only that derived table is exposed to higher nesting level.
I have a chapter in THINKING IN SETS on this; the book is due out in
February 2008.
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