PLSQL Converting if then to CASE stmt - plsql

Question

Converting if then to CASE stmt. Please let me know what mistake I m making here
DECLARE
salary NUMBER;
bonus NUMBER;
hdate DATE;
empno NUMBER;
BEGIN
SELECT hiredate INTO hdate FROM emp where empno=7788 ;
CASE hdate
WHEN hdate > TO_DATE('01-JAN-82') THEN bonus := 500 DBMS_OUTPUT.PUT_LINE(bonus);
WHEN hdate > TO_DATE('23-JAN-16') THEN bonus := 1000 DBMS_OUTPUT.PUT_LINE(bonus);
ELSE bonus := 1500 DBMS_OUTPUT.PUT_LINE(bonus);
END CASE;
END;
/

Answer 1

Use the following syntax for CASE:
DECLARE
salary NUMBER;
bonus NUMBER;
hdate DATE;
empno NUMBER;
BEGIN
SELECT sysdate INTO hdate FROM dual ;
CASE
WHEN hdate > TO_DATE('01-JAN-82') THEN bonus := 500;
WHEN hdate > TO_DATE('23-JAN-16') THEN bonus := 1000 ;
ELSE bonus := 1500 ;
END CASE;
DBMS_OUTPUT.PUT_LINE(bonus);
END;
Notice that the WHEN clauses can use different conditions rather than all testing the same variable or using the same operator.

Answer 2

Perhaps another proposition; instead of CASE statement in WHERE clause, it is rather in the SELECT list to deter the usage of hdate, thus a single SQL to achieve the desired output.
DECLARE
salary NUMBER;
p_bonus NUMBER;
hdate DATE;
empno NUMBER;
BEGIN
SELECT CASE
WHEN hiredate > TO_DATE ('01-JAN-82') THEN 500
WHEN hiredate > TO_DATE ('23-JAN-16') THEN 1000
ELSE 1500
END
INTO p_bonus
FROM emp
WHERE empno = 7788;
DBMS_OUTPUT.put_line (p_bonus);
END;
/

Answer 3

In addition to the other answers, there are a couple of other things wrong with your case statement.
Dates
When you use to_date to explicitly convert a string into a date, you should also use a format model to describe the format of the string. By not doing so, you rely on the default NLS_DATE_FORMAT parameter, which could well be different on different machines.
Also, years have 4 digits - use all of them, rather than make Oracle guess. Does the 2 digit year 16 mean 2016 or 1916?
Far better to be explicit, in both cases!
Therefore, your date conditions should actually be to_date('01-JAN-1982', 'dd-MON-yyyy', 'nls_date_language=english') and to_date('23-JAN-2016', 'dd-MON-yyyy').
Note the presence of the optional third parameter - I used that because you specified the month in words, and again, your NLS_DATE_LANGUAGE parameter might not be the same on someone else's machine. Adding the third parameter means the string will be converted to a date regardless of your NLS settings.
You can avoid the use of the 3rd parameter in to_date by using numbers for the day, month and year, e.g. to_date('23/01/2016', 'dd/mm/yyyy', 'nls_date_language=english').
CASE and logic short circuiting
CASE uses logic short circuiting, meaning that when it evaluates a condition to be true, it doesn't process any further conditions.
It seems like you intend a hiredate of 23rd Feb 2017 to get a bonus of 1000, but since it meets the first condition (it's later than 1st Jan 1982), it gets a bonus of 500.
Therefore, you need to change the order of the conditions, so that the most restrictive is at the top. In your case, your procedure becomes:
DECLARE
salary NUMBER;
bonus NUMBER;
hdate DATE;
empno NUMBER;
BEGIN
SELECT hiredate
INTO hdate
FROM emp
WHERE empno=7788;
CASE hdate
WHEN hdate > TO_DATE('23-JAN-16') THEN bonus := 1000;
WHEN hdate > TO_DATE('01-JAN-82') THEN bonus := 500;
ELSE bonus := 1500 DBMS_OUTPUT.PUT_LINE(bonus);
END CASE;
DBMS_OUTPUT.PUT_LINE(bonus);
END;
/