Question

I have a table which records a series of values, as so …

ID.   VAL
1.    18
2.    18
3.    20
4.    20
5.    18

I’m trying to work out how to select the records at which the series changes (e.g. Record 1, 3 and 5). I’m using rails, but I’m guessing raw MySQL might be the way forward.

Would appreciate any help you could offer…

Answer 1

Assuming your table looks like this:

CREATE TABLE records (
  id INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
  val INT UNSIGNED NOT NULL
);

What you want to do is self join it against itself, like this:

SELECT 
  records2.id
FROM 
  records AS records1
  JOIN records AS records2 ON (records1.id = records2.id-1)
WHERE
  records1.val != records2.val

Such that we join a record with the one preceding it. If the values for the two records differ – we have our answer.