Re: Conditional Select
From: Joe Celko (jcelko212_at_earthlink.net)
Date: 06/12/04
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Date: Sat, 12 Jun 2004 12:16:38 -0700
>> I would like to write a query in SQL, where the field [sic] names
included in the select statement are conditional. <<
Let's get back to the basics of an RDBMS. Rows are not records; fields
are not columns; tables are not files. A table is made up of columns
which model an entity. If you change the columns, you change the kind
of entity in that table. Furthermore, a row is always the same
structure within its table, unlike a variant record in a file system.
>> The table that I am actually using has about 40 fields [sic], and I
want to be able to exclude those fields [sic]that have null values from
the results I get. So, in my first example, I would only get the first
three fields [sic], but in the second example, I would get the
first[sic], third [sic] and fourth [sic] fields [sic]. <<
There are no fields in SQL and they are referenced by name, not by
position number! This is absolute RDBMS foundations!!
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.
As a final kicker, if you have forty or so NULL-able columns in one
table, your design is almost certainly screwed up beyond all hope of any
repair. There are payroll systems for Fortune 500 companies with fewer
NULLs in the entire system.
--CELKO--
===========================
Please post DDL, so that people do not have to guess what the keys,
constraints, Declarative Referential Integrity, datatypes, etc. in your
schema are.
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