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Posted by --CELKO-- on 11/27/07 20:32
You use reserved words for data elements. You have multiple names
for the same data element. You used vague data element names,
including the magical, universal "id" that applies to all things in
creation.
You don't know that the ORDER BY clause is part of a cursor in
Standard SQL and that it comes at the end of the SELECT statement.
You seem to have both an invoice number (the usual term) and an
invoice identifier (I am scared that you used IDENTITY as a fake
pointer and have assumed that the data is stored in phsycial order,
like a mag tape).
I think that you are trying to use ordering because you do not know
what a table is -- no ordering! You want a sequential file. That is
not how RDBMS works at all! Totally wrogn mindset.
This is the usual template for finding the latest invoice is:
SELECT I1.invoice_nbr, C.customer_name, C.shopping_addr, C.acct_nbr
FROM Invoices AS I1,
Customers AS C
WHERE I1.acct_nbr = C.acct_nbr
AND I1.posting_date
= (SELECT MAX(I2.posting_date)
FROM Invoices AS I2
WHERE I2.acct_nbr = C.acct_nbr);
Notice how I cleaned up the names. You might want to read ISO-11179
sometime soon.
>> Neither SELECT statements are supported in SQL Server 2005... Is there a logical reason WHY? Other then ANSI Standards which I'm not buying as m$ft rarely follows any standards but there own 100% of the time anyway. <<
Both logic and Standards. And Microsoft has been moving very strongly
to ANSI Standards; look at the new stuff in SQL-2005 and SQL-2008
which is pure ANSI. The TOP syntax is proprietary syntax; the ANSI
approach would use ROW_NUMBER() OVER() instead.
What sense would it make to sort a table (a contradiction by
definition) then group it? You have no idea what a SELECT does and
want it to act like a READ() in a proceudral languages.
Here is how a SELECT works in SQL ... at least in theory. Real
products will optimize things, but the code has to produce the same
results.
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 (i.e. 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, partiton the original table
into groups and reduce each grouping to a *single* row, replacing the
original working table with the new grouped table. The rows of a
grouped table must be only group characteristics: (1) a grouping
column (2) a statistic about the group (i.e. aggregate functions) (3)
a function or constant(4) an expression made up of only those three
items. The original table no longer exists and you cannot reference
anything in it (this was an error in early Sybase products).
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 have 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/procedural 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.
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