The CASE expression is an *expression* and not a control statement;
that is, it returns a value of one data type. 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 operand> THEN <result>
<searched when clause> ::= WHEN <search condition> THEN <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 <data type>) 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 expression> is 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 #1> IS NOT NULL
THEN <value exp #1>
ELSE <value exp #2> END
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 #1> IS 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 #1> END
It is important to be sure that you have a THEN or ELSE clause with a
data type that the compiler can find to determine the highest data
type 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 examples of how ISNULL() and COALESCE()
work
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)
>> Stay informed about: testing for null and not null in case statement 
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