Advance SQL

Dear Expert,

How can I reuse the column, instead of select the whole things again.
Example as below :

select
column1 as A,
column2 as B,
column3 * A as C
from dummy ;

in MS SQL I have to do like this,

select
column1 as A,
column2 as B,
column3 * column1 as C
from dummy ;

Thanks

DesmondOn 1 Jun 2004 03:56:08 -0700, Desmond wrote:

>Dear Expert,
>How can I reuse the column, instead of select the whole things again.
>Example as below :
>select
> column1 as A,
> column2 as B,
> column3 * A as C
>from dummy ;
>in MS SQL I have to do like this,
>select
> column1 as A,
> column2 as B,
> column3 * column1 as C
>from dummy ;
>
>Thanks
>Desmond

Hi Desmond,

You can use a derived table:

select A, B, column3 * A as C
from (select column1 as A,
column2 as B,
column3
from dummy) AS t

Best, Hugo
--

(Remove _NO_ and _SPAM_ to get my e-mail address)|||>> How can I reuse the column, instead of selecting the whole thing
again. <<

You can put it into a VIEW or derived table:

SELECT a, b, a*column3 AS c
FROM (SELECT column1, column2, column3
FROM Dummy) AS X(a, b, column3);

But that is not your problem. The real problem is that you do not
understand how SQL -- real SQL -- works.

Here is how a SELECT works in SQL ... at least in theory. Real
products will optimize things when they can.

a) Start in the FROM clause and build a working table from all of the
joins, unions, intersections, and whatever other table constructors
are there. The table expression> AS <correlation name> option allows
you give a name to this working table which you then have to use for
the rest of the containing query.

b) Go to the WHERE clause and remove rows that do not pass criteria;
that is, that do not test to TRUE (reject UNKNOWN and FALSE). The
WHERE clause is applied to the working set in the FROM clause.

c) Go to the optional GROUP BY clause, make groups and reduce each
group to a single row, replacing the original working table with the
new grouped table. The rows of a grouped table must be group
characteristics: (1) a grouping column (2) a statistic about the group
(i.e. aggregate functions) (3) a function or (4) an expression made up
those three items.

d) Go to the optional HAVING clause and apply it against the grouped
working table; if there was no GROUP BY clause, treat the entire table
as one group.

e) Go to the SELECT clause and construct the expressions in the list.
This means that the scalar subqueries, function calls and expressions
in the SELECT are done after all the other clauses are done. The "AS"
operator can also give names to expressions in the SELECT list. These
new names come into existence all at once, but after the WHERE clause,
GROUP BY clause and HAVING clause has been executed; you cannot use
them in the SELECT list or the WHERE clause for that reason.

If there is a SELECT DISTINCT, then redundant duplicate rows are
removed. For purposes of defining a duplicate row, NULLs are treated
as matching (just like in the GROUP BY).

f) Nested query expressions follow the usual scoping rules you would
expect from a block structured language like C, Pascal, Algol, etc.
Namely, the innermost queries can reference columns and tables in the
queries in which they are contained.

g) The ORDER BY clause is part of a cursor, not a query. The result
set is passed to the cursor, which can only see the names in the
SELECT clause list, and the sorting is done there. The ORDER BY
clause cannot have expression in it, or references to other columns
because the result set has been converted into a sequential file
structure and that is what is being sorted.

As you can see, things happen "all at once" in SQL, not from left to
right as they would in a sequential file/proceudral language model. In
those languages, these two statements produce different results:
**READ (a, b, c) FROM File_X;
**READ (c, a, b) FROM File_X;

while these two statements return the same data:

SELECT a, b, c FROM Table_X;
SELECT c, a, b FROM Table_X;

Think about what a confused mess this statement is in the SQL model.

SELECT f(c2) AS c1, f(c1) AS c2 FROM Foobar;

That is why such nonsense is illegal syntax.