Nested case statements in inner joins - sql

Question

Having issues with that I think can be solved by a case statement, but it keeps rejecting my sql. Basically, I'm introducing a new parameter to existing SQL and need to react different ways depending on what it is... sounded like a case statement to me, but it keeps rejecting it. Any thoughts?
Original
INNER JOIN T_CASE CF ON
(
A.AWESOME_ID = CF.AWESOME_ID
// Several more awesome conditions
)
AND
(
(
cy.SOMEFLAG = 0
AND CF.SOMEDATE IS NOT NULL
)
OR
(
cy.SOMEFLAG = 1
AND CF.CASE_TYPE = 1
AND CF.SOMEDATE IS NOT NULL
)
)
Attempt #1
INNER JOIN T_CASE CF ON
(
A.AWESOME_ID = CF.AWESOME_ID
// Several more conditions
)
AND
(
// Just this part is what the new parameter applies to
CASE WHEN #NEWPARAM = 1 THEN
(
(
cy.SOMEFLAG = 0
AND CF.SOMEDATE IS NOT NULL
)
OR
(
cy.SOMEFLAG = 1
AND CF.CASE_TYPE = 1
AND CF.SOMEDATE IS NOT NULL
)
)
ELSE
(
(
cy.SOMEFLAG = 0
AND CF.SOMEDATE IS NULL
)
OR
(
cy.SOMEFLAG = 1
AND CF.CASE_TYPE = 0
OR CF.SOMEDATE IS NULL
)
)
END
)

Answer 1

In general, I think it is a better idea to use basic logical constructs in condition clauses, rather than the case. You can readily do this:
INNER JOIN T_CASE CF ON
(
A.AWESOME_ID = CF.AWESOME_ID
// Several more conditions
)
AND
(
(
#NEWPARAM = 1 AND
cy.SOMEFLAG = 0 AND
CF.SOMEDATE IS NOT NULL
)
OR
(
#NEWPARAM = 1 AND
cy.SOMEFLAG = 1 AND
CF.CASE_TYPE = 1 AND
CF.SOMEDATE IS NOT NULL
)
OR
(
#NEWPARAM <> 1 AND
cy.SOMEFLAG = 0 AND
CF.SOMEDATE IS NULL
)
OR
(
#NEWPARAM <> 1 AND
((cy.SOMEFLAG = 1 AND CF.CASE_TYPE = 0) OR
CF.SOMEDATE IS NULL
)
)
)

Answer 2

You can't use the case statement to dynamically include/exclude portions of the SQL statement, but you can still get a similar effect like this
INNER JOIN T_CASE CF ON
(
A.AWESOME_ID = CF.AWESOME_ID
// Several more conditions
)
AND
(
// Just this part is what the new parameter applies to
1 = CASE WHEN
#NEWPARAM = 1
and
(
cy.SOMEFLAG = 0
AND CF.SOMEDATE IS NOT NULL
)
OR
(
cy.SOMEFLAG = 1
AND CF.CASE_TYPE = 1
AND CF.SOMEDATE IS NOT NULL
)
THEN 1
CASE WHEN
#NEWPARAM <> 1
and
(
(
cy.SOMEFLAG = 0
AND CF.SOMEDATE IS NULL
)
OR
(
cy.SOMEFLAG = 1
AND CF.CASE_TYPE = 0
OR CF.SOMEDATE IS NULL
)
)
THEN 1
ELSE 0
END
)
Basically we're returning 1 for rows that meet the criteria, and 0 for rows that don't and comparing that against the value 1.