salah jubeh <s_ju...@yahoo.com> writes:
> On the row level, the value of a and b are know, so why we need a temporary
> table or CTE. Why I can not use it directly as shown in the example below.
> why a and b can not be used as aliases for the column names ? I want to know
> the theoretical reason behind it?
The SQL standard envisions all the expressions in a SELECT list being
computed concurrently and independently. So they can't refer to each
other. If they could, it would introduce ambiguity. Consider a table
t that provides columns x,y,z, and suppose we have
SELECT x AS z, y, y+z AS sum FROM t;
If cross-references were allowed, it would be unclear what the last "z"
is supposed to refer to.
regards, tom lane
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