How to use CASE statement and a parameter in the WHERE clause? - oracle

Question

I have an SSRS report where there is a parameter that asks the user to include records where revenue is greater than zero, or records with revenue values that are just zero.
Since the query is not a stored procedure and it is not an option to put it into a procedure, I need to use some case logic for the embedded query. I need to do this in the where clause in the end.
I am trying to do something like this:
SELECT * FROM TABLE
WHERE MY_DATE BETWEEN D_START AND D_END
AND
CASE
WHEN :REVENUE = 1 THEN REV != 0
WHEN :REVENUE = 2 THEN REV = 0
END
However, when I run this query I get the following error:
ORA-00905: missing keyword
Is what I am doing not possible? Or is there an error that someone can see and help me with?
Please help. Thanks!
UPDATE: Just to clarify, the user is passing a value of 1 or 2. And the query should filter the data according to what value is passed to it. If 1 is passed in the parameter, then filter out all revenue not equal to zero. Else if two is passed, then filter so that only records where revenue is zero is returned.

Answer 1

You can write it better with a bit of boolean logic:
SELECT * FROM TABLE
WHERE MY_DATE BETWEEN D_START AND D_END
AND (
(:REVENUE = 1 AND REV != 0)
OR
(:REVENUE = 2 AND REV = 0 )
)
CASE is meant to extract different values based on conditions, so you can use it to check conditions, but you need to use it as a value to check against a condition

Answer 2

It's not necessary to use a CASE expression to get this particular result.
But it is possible to make use of one.
The problem in the original query is that Oracle is more strict than other databases (like MySQL) in that Oracle doesn't implicitly convert a boolean expression to a value, or convert a value into boolean.
I suspect that Oracle is choking in a couple of places. The error message is only showing us one of those.
The CASE expression returns a value, and Oracle is balking that he won't evaluate the value as a boolean.
To get that value evaluated as a boolean, we could do a comparison of the value to some other value.
If we fix that, I think Oracle is still going to choke on the expression following THEN. Oracle is expecting to return a value, and it's finding a comparison, which evaluates to a boolean.
Okay, so we know the CASE expression needs to return a value, and we need to use that in a boolean expression. If we move that conditional test into the WHEN part, and specify a value to be returned in the THEN, we can compare the return from the CASE expression to another value.
(As an aside... I strongly recommend that you qualify the column references in the SQL statement. That makes the intent more clear. Looking at the statement, it looks like MY_DATE, D_START and D_END are all column references. That's perfectly valid, it just seems a bit odd to me.)
As an example, we could do something like this with the CASE expression:
SELECT t.*
FROM TABLE t
WHERE t.MY_DATE BETWEEN t.D_START AND t.D_END
AND CASE
WHEN ( :REVENUE = 1 AND t.REV != 0 ) THEN 1
WHEN ( :REVENUE = 2 AND t.REV = 0 ) THEN 1
ELSE NULL
END = 1
The parens inside the CASE aren't necessary; I just included them to highlight the part that Oracle is evaluating in a boolean context.
So, does that work? If the value passed in for :REVENUE is 2, the condition in the first WHEN won't evaluate to TRUE (the result of first comparison is guaranteed to be FALSE). The condition in the second WHEN may evaluate to TRUE (first comparison will yield TRUE, the result from second comparison will depend on the value in the REV column.)
That CASE expression is either going to return a value of 1 or NULL. (We could just as easily use a 0 or a -1, or 999 in place of NULL if we wanted.)
Once the CASE expression is evaluated, the value returned will be compared to a literal value, as if we wrote e.g. val = 1. That comparison is evaluated as boolean. If it evaluates to TRUE, the row will be returned...
To get Oracle to behave similarly to other databases (like MySQL), we would need to make the conversion from boolean to value and value to boolean explicit. We would still need the return from the CASE compared to 1, like we did above. In place of REV != 0 we could use another CASE expression. I'm not recommending this, just shown here for illustration, converting a boolean to a value.
WHERE CASE
WHEN ( :REVENUE = 1 )
THEN CASE WHEN ( t.REV != 0 ) THEN 1 ELSE NULL END
WHEN ( :REVENUE = 2 )
THEN CASE WHEN ( t.REV = 0 ) THEN 1 ELSE NULL END
ELSE
NULL
END = 1
Note that the return from the outermost CASE expression is being compared to a value, so we get a boolean (where Oracle expects a boolean.)
All of the ELSE NULL in the statements above can be omitted for an equivalent result, since that's the default when ELSE is omitted.)
Again, it's not necessary to use a CASE expression. You can get equivalent results without it. For example:
SELECT t.*
FROM TABLE t
WHERE t.MY_DATE BETWEEN t.D_START AND t.D_END
AND ( ( :REVENUE = 1 AND t.REV != 0 )
OR ( :REVENUE = 2 AND t.REV = 0 )
)
In these queries that all return an equivalent result, the CASE expression doesn't buy us anything. But in some circumstances, it can have some advantages over a regular OR, because the CASE expression stops evaluation when a condition in a WHEN clause evaluates to TRUE.

Answer 3

The problem is that Oracle SQL does not have the boolean data type, so you cannot have columns of type boolean, pass boolean parameters to a query, have boolean expressions etc. So they have the somewhat unnatural concept of "condition" which is something that goes into logical conditions (like in the WHERE clause). Unfortunately, when they introduced the case EXPRESSION, which can be used wherever any other expression can be used (but this excludes boolean), they DID NOT introduce a "case CONDITION" - which could be used where other conditions can be used. This omission is odd, since the code for a case condition would probably use 95% of the code for the case expression. All the more weird since PL/SQL does have the boolean type, and the case expression there works seamlessly for Booleans.