MySql：计算行，直到值更改

Question

I know how to count rows with a certain value, but how do I count the rows up until that value changes?

For instance:

1 cat
2 cat
3 cat
4 dog
5 dog
6 cat
7 cat

How would I count the first three cats without counting the last two cats?

Answer 1

You can find the id of the first thing that is not a cat and count the ids less than that:

select count(*)
from t
where id < (select min(t2.id) from t t2 where t2.value <> 'cat');

Answer 2

You can do something like this:

SELECT @val := 1;
SELECT COUNT(IF(@val = 1 AND animal = 'cat', @val := 1, @val := NULL)) cnt FROM animals;

What this does is:

• Initialize a variable @val to 1.
• For every row that matches the if condition, return 1; else return NULL.
• Count the number of (non-NULL) rows.

That will give you the count you need. If you need the actual values of the rows, you can do something like this:

SELECT @val := 1;
SELECT * FROM (
        SELECT id, animal, IF(@val = 1 AND animal = 'cat', @val := 1, @val := NULL) ok FROM animals
    ) _animals WHERE ok = 1;

What the if statement does is, as long as @val = 1 (the initial condition) and you're looking at a cat, set @val to 1 and return 1. As soon as the animal is no longer a cat, @val becomes NULL, which means all future rows will also return NULL (because @val will no longer be 1).