Knut Anders Hatlen created DERBY-6581:
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Summary: Document simple case syntax
Key: DERBY-6581
URL: https://issues.apache.org/jira/browse/DERBY-6581
Project: Derby
Issue Type: Improvement
Components: Documentation
Affects Versions: 10.11.0.0
Reporter: Knut Anders Hatlen
We should document the new syntax added in DERBY-1576 in the CASE expression
topic of the reference manual.
The variant that is currently documented in the CASE expression topic, is
called "searched case". It should still be documented. In addition, the topic
should show this syntax for "simple case":
{noformat}
CASE valueExpression
WHEN whenOperand [, whenOperand ]* THEN thenExpression
[ WHEN whenOperand [, whenOperand ]* THEN thenExpression ]*
[ ELSE elseExpression ]
END
{noformat}
{{whenOperand}} could be defined like this:
{noformat}
valueExpression |
{ < | = | > | <= | >= | <> } expression |
IS [ NOT ] NULL |
[ NOT ] LIKE characterExpression WithWildCard [ ESCAPE 'escapeCharacter'] |
[ NOT ] BETWEEN expression AND expression |
[ NOT ] IN tableSubquery |
[ NOT ] IN ( expression [, expression ]* ) |
comparisonOperator { ALL | ANY | SOME } tableSubquery
{noformat}
Most of this syntax is copied from the table in the Boolean expressions topic,
so we may try to find some way to refactor it to avoid repetition here.
There are some inconsistencies in this definition of {{caseOperand}}. One is
that I think all occurrences of "expression" could be replaced with
"valueExpression". Another is that line 2 lists all comparison operators
explicitly, whereas line 8 uses the shorthand "comparisonOperator". I suppose
both should do the same. If we choose to use the shorthand, we should probably
also define comparisonOperator somewhere.
Examples:
{code:sql}
-- returns 'two'
VALUES
CASE 1+1
WHEN 1 THEN 'one'
WHEN 2 THEN 'two'
ELSE 'many'
END
-- returns 'odd', 'even', 'big'
SELECT
CASE X
WHEN 1, 3, 5, 7, 9 THEN 'odd'
WHEN 2, 4, 6, 8, 10 THEN 'even'
ELSE 'big'
END
FROM
(VALUES 5, 8, 12) AS V(X)
-- returns ('long', 182), ('medium', 340), ('short', 20)
SELECT DISTANCE, COUNT(*)
FROM (SELECT
CASE MILES
WHEN < 250 THEN 'short'
WHEN BETWEEN 250 AND 2000 THEN 'medium'
WHEN > 2000 THEN 'long'
END
FROM FLIGHTS) AS F(DISTANCE)
GROUP BY DISTANCE
{code}
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