Putting subquery into offset clause - mysql

Question

Let's say I have table (numbers) with one column:
n
---
4
5
67
23
7
89
and I want to get median (only even list of numbers). I thought it would be easy so I wrote:
SELECT SUM(ord.n)/2
FROM (
SELECT n
FROM numbers
ORDER BY n ASC
LIMIT 2 OFFSET (SELECT COUNT(n)/2-1 FROM numbers)
) AS ord
but of course it throw me syntax error. I guess I can't insert subquery into offset but I want to know what I have to do to get expected result? I know there are different ways to write query to get median but I need to know is there any possibility to insert 'variable' into offset instead of placing some number?

Answer 1

I think you are looking for the query below this should work from MySQL version 5.1 and up
SELECT
AVG(filter.n)
FROM (
SELECT
*
, (#position := #position + 1) AS init_position
FROM
t
CROSS JOIN (
SELECT
#position := 0
, #max := (SELECT COUNT(t.n) FROM t)
, #median_mode := (CASE WHEN ((#max % 2) = 0) THEN 'even' ELSE 'odd' END)
) AS init_user_param
ORDER BY
t.n ASC
) AS filter
WHERE
CASE
WHEN #median_mode = 'even'
THEN filter.init_position BETWEEN (#max / 2) AND ((#max / 2) + 1)
WHEN #median_mode = 'odd'
THEN filter.init_position = ((#max + 1) / 2)
END
Result
| AVG(filter.n) |
| ------------- |
| 15 |
see demo
Result when 89 is out the list.
| AVG(filter.n) |
| ------------- |
| 7 |
see demo

Answer 2

You can use ROW_NUMBER() window function:
WITH cte AS (SELECT COUNT(*) counter FROM numbers)
SELECT AVG(n) median
FROM (
SELECT
row_number() over (order by n) ordinal,
n
FROM numbers
) t
WHERE (SELECT counter FROM cte) IN (2 * ordinal, 2 * (ordinal - 1))
See the demo.
Result:
| median |
| ------ |
| 15 |

Answer 3

The (probably) most similar solution to your algorithm is to use two queries. First get the offset. Then insert it into your query. The SQL-only way would be to use a prepared statement:
set #offset = (SELECT COUNT(n)/2-1 FROM numbers);
set #sql = "
SELECT SUM(ord.n)/2
FROM (
SELECT n
FROM numbers
ORDER BY n ASC
LIMIT 2 OFFSET ?
) AS ord
";
prepare stmt from #sql;
execute stmt using #offset;
db-fiddle