Re: DECODE
From: Joe Celko (joe.celko_at_northface.edu)
Date: 03/19/04
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Date: Fri, 19 Mar 2004 12:39:36 -0800
DECODE is a weaker version of the CASE expression.
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 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
<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 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
datatype that the compiler can find to determine the highest datatype
for the expression.
--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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