Re: can not use the column after rename

salah jubeh <s_jubeh(at)yahoo(dot)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