Working with NULLS

[rcamarda]

I have a field that may be null which is valid, and I am finding
something I didnt expect when working with nulls.
SELECT NULL/4.0 will return NULL (which I expect), however, when I test
it with a case it does not:

SELECT NULL/4.0 AS 'TEST1', TEST2 = CASE NULL/4.0 WHEN NULL THEN
'HURAY' ELSE 'OH DARN' END

I can work around by testing for NULL first else CASE ..., but I'd like
to understand why the CASE does not test for null.
TIA
rob
SQL 2005 Enterprise x64, SP1

[Mike C#]

Use a searched CASE expression with IS NULL. The way you're doing it is
effectively doint the comparison NULL/4.0 = NULL, which evaluates to
UNKNOWN, which means your CASE will drop through to the next WHEN (or the
ELSE in this case). NULL comparisons need to use IS NULL or IS NOT NULL.

SELECT NULL/4.0 AS 'TEST1', TEST2 =
CASE
WHEN NULL/4.0 IS NULL THEN 'HURAY'
ELSE 'OH DARN'
END

"rcamarda" <ro*****@hotmail.comwrote in message
news:11*********************@m73g2000cwd.googlegro ups.com...

>I have a field that may be null which is valid, and I am finding
something I didnt expect when working with nulls.
SELECT NULL/4.0 will return NULL (which I expect), however, when I test
it with a case it does not:

SELECT NULL/4.0 AS 'TEST1', TEST2 = CASE NULL/4.0 WHEN NULL THEN
'HURAY' ELSE 'OH DARN' END

I can work around by testing for NULL first else CASE ..., but I'd like
to understand why the CASE does not test for null.
TIA
rob
SQL 2005 Enterprise x64, SP1

[rcamarda]

DOH, forgot about IS
Thanks Mike
Mike C# wrote:

Use a searched CASE expression with IS NULL. The way you're doing it is
effectively doint the comparison NULL/4.0 = NULL, which evaluates to
UNKNOWN, which means your CASE will drop through to the next WHEN (or the
ELSE in this case). NULL comparisons need to use IS NULL or IS NOT NULL.

SELECT NULL/4.0 AS 'TEST1', TEST2 =
CASE
WHEN NULL/4.0 IS NULL THEN 'HURAY'
ELSE 'OH DARN'
END

"rcamarda" <ro*****@hotmail.comwrote in message
news:11*********************@m73g2000cwd.googlegro ups.com...

I have a field that may be null which is valid, and I am finding
something I didnt expect when working with nulls.
SELECT NULL/4.0 will return NULL (which I expect), however, when I test
it with a case it does not:

SELECT NULL/4.0 AS 'TEST1', TEST2 = CASE NULL/4.0 WHEN NULL THEN
'HURAY' ELSE 'OH DARN' END

I can work around by testing for NULL first else CASE ..., but I'd like
to understand why the CASE does not test for null.
TIA
rob
SQL 2005 Enterprise x64, SP1

[--CELKO--]

The CASE expression is an *expression* and not a control statement;
that is, it returns a value of one datatype. SQL-92 stole the idea and
the syntax from the ADA programming language. Here is the BNF for a
<case specification>:

<case specification::= <simple case| <searched case>

<simple case::=
CASE <case operand>
<simple when clause>...
[<else clause>]
END

<searched case::=
CASE
<searched when clause>...
[<else clause>]
END

<simple when clause::= WHEN <when operandTHEN <result>

<searched when clause::= WHEN <search conditionTHEN <result>

<else clause::= ELSE <result>

<case operand::= <value expression>

<when operand::= <value expression>

<result::= <result expression| NULL

<result expression::= <value expression>

The searched CASE expression is probably the most used version of the
expression. The WHEN ... THEN ... clauses are executed in left to
right order. The first WHEN clause that tests TRUE returns the value
given in its THEN clause. And, yes, you can nest CASE expressions
inside each other. If no explicit ELSE clause is given for the CASE
expression, then the database will insert a default ELSE NULL clause.
If you want to return a NULL in a THEN clause, then you must use a CAST
(NULL AS <datatype>) expression. I recommend always giving the ELSE
clause, so that you can change it later when you find something
explicit to return.

The <simple case expressionis defined as a searched CASE expression
in which all the WHEN clauses are made into equality comparisons
against the <case operand>. For example

CASE iso_sex_code
WHEN 0 THEN 'Unknown'
WHEN 1 THEN 'Male'
WHEN 2 THEN 'Female'
WHEN 9 THEN 'N/A'
ELSE NULL END

could also be written as:

CASE
WHEN iso_sex_code = 0 THEN 'Unknown'
WHEN iso_sex_code = 1 THEN 'Male'
WHEN iso_sex_code = 2 THEN 'Female'
WHEN iso_sex_code = 9 THEN 'N/A'
ELSE NULL END

There is a gimmick in this definition, however. The expression

CASE foo
WHEN 1 THEN 'bar'
WHEN NULL THEN 'no bar'
END

becomes

CASE WHEN foo = 1 THEN 'bar'
WHEN foo = NULL THEN 'no_bar' -- error!
ELSE NULL END

The second WHEN clause is always UNKNOWN.

The SQL-92 Standard defines other functions in terms of the CASE
expression, which makes the language a bit more compact and easier to
implement. For example, the COALESCE () function can be defined for
one or two expressions by

1) COALESCE (<value exp #1>) is equivalent to (<value exp #1>)

2) COALESCE (<value exp #1>, <value exp #2>) is equivalent to

CASE WHEN <value exp #1IS NOT NULL
THEN <value exp #1>
ELSE <value exp #2END

then we can recursively define it for (n) expressions, where (n >= 3),
in the list by

COALESCE (<value exp #1>, <value exp #2>, . . ., n), as equivalent to:

CASE WHEN <value exp #1IS NOT NULL
THEN <value exp #1>
ELSE COALESCE (<value exp #2>, . . ., n)
END

Likewise, NULLIF (<value exp #1>, <value exp #2>) is equivalent to:

CASE WHEN <value exp #1= <value exp #2>
THEN NULL
ELSE <value exp #1END

It is important to be sure that you have a THEN or ELSE clause with a
datatype that the compiler can find to determine the highest datatype
for the expression.

A trick in the WHERE clause is use it for a complex predicate with
material implications.

WHERE CASE
WHEN <search condition #1>
THEN 1
WHEN <search condition #2>
THEN 1
...
ELSE 0 END = 1

Gert-Jan Strik posted some exampels of how ISNULL() and COALESCE() on
2004 Aug 19

CREATE TABLE #t(a CHAR(1));
INSERT INTO #t VALUES (NULL);
SELECT ISNULL(a,'abc') FROM #t;
SELECT COALESCE(a, 'abc') FROM #t;
DROP TABLE #t;

He always use COALESCE, with the exception of the following type of
situation, because of its performance consequences:

SELECT ...,
ISNULL((SELECT COUNT(*) -- or other aggregate
FROM B
WHERE B.key = A.key), 0)
FROM A;

Likewise, Alejandro Mesa cam up with this example:

SELECT 13 / COALESCE(CAST(NULL AS INTEGER), 2.00); -- promote to
highest type (decimal)
SELECT 13 / ISNULL(CAST(NULL AS INTEGER), 2.00); -- promote to first
type (integer)