How do I lag columns in MySQL?

Question

Consider the following table:

SELECT id, value FROM table ORDER BY id ASC;
+-----+---------+
| id  | value   |
+-----+---------+
| 12  | 158     |
| 15  | 34         


        

Answer 1

Here is a solution that returns what you want in MySQL

SET @a :=0;
SET @b :=2;
SELECT r.id, r.value, r.value/r2.value AS 'lag'
FROM
(SELECT if(@a, @a:=@a+1, @a:=1) as rownum, id, value FROM results) AS r
LEFT JOIN
(SELECT if(@b, @b:=@b+1, @b:=1) as rownum, id, value FROM results) AS r2
ON r.rownum = r2.rownum

MySQL 5.1 doesn't like a self join against a subquery so you have to count rows twice, so not as tidy or scalable as it might be, but it does make specifying the lag simple.

For readers that use Oracle instead this is much easier

SELECT id, value, value/lag(value, 2) over (order by id) as lag from results;