Question

Considering the following table:

rowid   |   text    |   group
1       |   a       |   1
2       |   b       |   2
3       |   c       |   2
4       |   d       |   3
5       |   e       |   4
6       |   f       |   1
7       |   g       |   1
8       |   h       |   4
9       |   j       |   5
10      |   k       |   2
11      |   i       |   3
12      |   l       |   3

Now imagine I want the row number of rowid 6 (for use in eg. pagination). Since it’s auto incrementing, it’d be very simple to just select where rowid = 6, but in this case I want to know what row number it has in group 1.

Obviously it’s easy to do this by eye in such a small example (it’s row number 2).

Row number 1 would be rowid = 1.
Row number 2 would be rowid = 6.
Row number 3 would be rowid = 7.

My first thought was something like this:

SELECT
    tmp.rowid,
    tmp.text,
    tmp.group,
    tmp.row_number
FROM
    (
        SELECT
            `rowid`,
            `text`,
            `group`,
            (@rowNumber := @rowNumber + 1) as `row_number`
        FROM
            `my_table`,
            (SELECT @rowNumber := 0) as `r`
        WHERE
            `group` = 1
        ORDER BY
            `rowid` DESC
    ) as `tmp`
WHERE
    tmp.rowid = 6

Which will give the right result (in row_number), but on a big table (half a million records) this will take a good 2.5 seconds onn a quick server, as it has to first collect all 500,000 entries, and THEN select row id 6.

What should I do to make this quicker/better?

Answer 1

The solution is actually very simple.

SQL

SELECT
    COUNT(`rowid`) as `row_number`
FROM
    `my_table`
WHERE
    `group` = 1
AND
    `rowid` >= 6

---

NOTE

You need to have a row with an incrementing value (such as a timestamp, an incrementing ID, etc.).

Here we convert the original ORDER BY rowid DESC to WHERErowid>= '6'. If you use ORDER BY ascending, you’ll want to use <= instead.

The query takes 0.00002 seconds, and returns the correct row number (in row_number). If you want the text and group columns, you’ll have to either make this a subquery or make an additional query that selects WHERE rowid = 6.