Using case statement or IF inside a WHERE clause - mysql

Question

I have several stored procedures that are almost identical but have some different AND parts inside a WHERE clause.
Based on a variable deptname, I want to add additional AND/OR conditions to my already existing WHERE clause. So kind of like IF/CASE WHEN on the part that is different.
Think about it as string concatenation
query_string = 'WHERE a= XYZ AND B= 123"
if deptname = a: query_string + "AND additional conditions for dept a"
else if deptname = b:query_string + "AND additional conditions for dept b"
What is the appropriate way to use a variable?
here is some pseudo code of what I am trying to do
SELECT
personID AS pid,
personcode,
persondeptcode,
more_fields AS fields
FROM
TABLE_XYZ
WHERE
--shared parts
personcode = 'C'
AND
persondeptcode = 'MAJ'
--- NOW the different part
IF #deptname = "deptA"
AND
(
PROGRAM_LDESCR IN
(
'prog1',
'prog2',
'prog3'
)
OR
aprogram IN ('aprogram1')
OR
(aprogram IN ('aprogram2') AND PLAN_LDESCR IN ('plan123'))
);
--- THIS IS A DIFFERENT DEPT SO WE HAVE DIFFERENT AND PART
ELSE IF #deptname = "deptB"
(
PROGRAM_LDESCR IN
(
'1234'
)
OR
aprogram IN ('a1234')
);

Answer 1

You can use a CASE expression in this case, the important thing is to make sure you have an ELSE clause to ensure the expression remains true if #deptname is not one of the two values with extra conditions:
WHERE personcode = 'C'
AND persondeptcode = 'MAJ'
AND (CASE #deptname
WHEN "deptA" THEN PROGRAM_LDESCR IN ('prog1', 'prog2', 'prog3')
OR aprogram IN ('aprogram1')
OR aprogram IN ('aprogram2') AND PLAN_LDESCR IN ('plan123')
WHEN "deptB" THEN PROGRAM_LDESCR IN ('1234')
OR aprogram IN ('a1234')
ELSE 1
END)
Here is a simple demo of a CASE expression used in this fashion.

Answer 2

You seem to want something like:
AND
(#deptname = 'dept123' AND (PROGRAM_LDESCR IN ('1234') OR aprogram IN ('a1234')) OR
#deptname <> 'dept123'
)

Answer 3

To combine the last part of the WHERE clause (if I'm understanding your commented-code correctly), you could do something like the following:
SELECT
personID AS pid,
personcode,
persondeptcode,
more_fields AS fields
FROM
TABLE_XYZ
WHERE
personcode = 'C'
AND persondeptcode = 'MAJ'
AND (
(#deptname="deptA" AND (PROGRAM_LDESCR IN ('prog1', 'prog2', 'prog3') OR aprogram IN ('aprogram1') OR (aprogram IN ('aprogram2') AND PLAN_LDESCR IN ('plan123'))))
OR
(#deptname="deptB" AND (PROGRAM_LDESCR IN ('1234') OR aprogram IN ('a1234'))
)
Normally you would use the WHERE clause to filter out unnecessary rows of data and a CASE statement if you wanted to actually change the value in the SELECT statement (I rarely see CASE statements outside a SELECT clause, unless it is doing something like a complex sort).