DB2 'CASE' puzzle slqerr -811

[lenygold via DBMonster.com]

Hi everybody?
here is 2 CASE EXAMPLES:

WITH CASE_SUB(A,B) AS
(VALUES('z','1'),
('z','2'),
('x','1'))
SELECT
CASE
WHEN (SELECT COUNT(A) from case_sub WHERE A='z') = 1
THEN 'A'
ELSE 'B'
END AS CASE
FROM CASE_SUB;

CASE
----
B
B
B

3 record(s) selected.

2ND EXAMPLE:

WITH CASE_SUB(A,B) AS
(VALUES('z','1'),
('z','2'),
('x','1'))
SELECT
CASE
WHEN (SELECT COUNT(A) from case_sub WHERE A='z') = 1
THEN
(select B from CASE_SUB WHERE A='z')
ELSE 'B'
END AS CASE
FROM CASE_SUB;

sqlcode: -811 The result of a scalar fullselect, SELECT INTO statement, or
VALUES INTO statement is more than one row.
WHY ELSE 'B' is not executed?

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[Tonkuma]

>sqlcode: -811 The result of a scalar fullselect, SELECT INTO statement, or VALUES INTO statement is more than one row. <<
The reson of -811 would be that the following subselect in THEN clause
retuned multiple rows for all result rows.
(select B from CASE_SUB WHERE A='z')

[lenygold via DBMonster.com]

Thank you very much Tonkuma for your help.
I have learn alot from you

Tonkuma wrote:

>>sqlcode: -811 The result of a scalar fullselect, SELECT INTO statement, or VALUES INTO statement is more than one row. <<

The reson of -811 would be that the following subselect in THEN clause
retuned multiple rows for all result rows.
(select B from CASE_SUB WHERE A='z')

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