Question

Basically I store data in MySql 5.5. I use qt to connect to mysql. I want to compare two columns, if col1 is greater than col2, the count continues, but when col1 is less than col2, count finishes and exits. So this is to count how many rows under some condition at the beginning of column. Is it possible in mysql?

An example:

Col1 Col2

2 1

2 3

2 1

The count I need should return 1, because the first row meets the condition of Col1 > Col2, but the second row doesn’t. Whenever the condition is not meet, counting exits no matter if following rows meet the condition or not.

Answer 1

This is a bit ugly, but will do the job. It’ll need adjusting depending on any ORDER etc you would like to apply to someTable but the principle is sound.

SELECT COUNT(*) 
FROM (
    SELECT
        @multiplier:=@multiplier*IF(t.`col1`<t.`col2`,0,1) AS counter
    FROM `someTable` t, (SELECT @multiplier := 1) v
    HAVING counter = 1
) scanQuery

The @multiplier variable will keep multiplying itself by 1. When it encounters a row where col1 < col2 it multiplies by 0. It will then continue multiplying 0 x 1. The outer query then just sums them up.

It’s not ideal, but would suffice. This could be expanded to allow you to get those rows before the break by doing the following

SELECT 
    `someTable`.*
FROM `someTable`
INNER JOIN (
    SELECT
        `someTable`.`PrimaryKeyField`
        @multiplier:=@multiplier*IF(`col1`<`col2`,0,1) AS counter
    FROM `someTable` t, (SELECT @multiplier := 1) v
    HAVING counter = 1
) t
ON scanQuery.`PrimaryKeyField` = `someTable`.`PrimaryKeyField`

Or possibly simply

    SELECT
        `someTable`.*
        @multiplier:=@multiplier*IF(`col1`<`col2`,0,1) AS counter
    FROM `someTable` t, (SELECT @multiplier := 1) v
    HAVING counter = 1