salah jubeh <s_jubeh@xxxxxxxxx> 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 -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
