PL/SQL--How to calculate a median using PL/SQL

Question

How would I calculate a median, not an average, using PL/SQL? Thanks.

Answer 1

1. The MEDIAN value is the value of the item in the middle of a data set.
2. For a dataset with 5 rows (sorted by the value) it is the value in row 3; for any ODD number of rows n use row number 'TRUNC(n/2) + 1'.
3. For a dataset with 6 rows the average of the two middle rows (rows 3 and 4) is usually used; for any EVEN number of rows n use row numbers 'TRUNC(n/2)' and 'TRUNC(n/2) + 1'.
4. Check for the special case where there are no rows in the result set first.
Using pseudo-code it might look like:
SELECT COUNT(1) INTO num_rows FROM mytable;
IF num_rows = 0 THEN
RETURN 0;
END IF;
first_row := TRUNC(num_rows/2);
-- set 2nd row to be same as first if ODD number of rows
second_row := first_row;
IF first_row * 2 = num_rows THEN
-- set 2nd row to be row after 1st if EVEN number of rows
second_row := second_row + 1;
END IF;
-- query the average needed. Note that for an ODD number of rows this still works.
SELECT avg(myvalue) FROM (SELECT ROWNUM rnum, rvalue FROM (SELECT myvalue rvalue FROM mytable ORDER BY myvalue))
WHERE rnum BETWEEN first_row AND second_row;
END IF;
I suggest you write and test the innermost query above first (SELECT myvalue rvalue FROM MYTABLE ORDER BY myvalue).
Then use it as a subquery for the '(SELECT ROWNUM ...' query.
Then when you have both of those working wrap it in the outermost query.
At that point you can make it a PL/SQL function or if you really want to confuse people you can perform it all with one big UNION query (to take care of the case where there are no rows).
Good luck!
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