Question

I have 2 tables like this :

category_table:

data_id  cat_id
      1       1
      1       2
      3       1
      3       3
      4       5
      4       6

data_table:

data_id  example_data
      1             x
      2             y
      3             m
      4             i

what I need is to count number of data_table records that are grouped with category 1 AND 2

Answer 1

Edit: After re-reading your question, is this what you’re looking for? A count of elements from data_table where there exists an entry in the category_table for both categories 1 and 2?

select
    count(*)
from
    data_table d
    inner join category_table c1
        on  d.data_id = c1.data_id
    inner join category_table c2
        on  d.data_id = c2.data_id
where
    c1.cat_id = 1
    c2.cat_id = 2

(old answer below this…)

If you want a straight count of all records:

select
    count(*)
from
    data_table d
    inner join category_table c
        on  d.data_id = c.data_id
where
    c.cat_id in (1, 2)

If you want a count for each category:

select
    c.cat_id, count(*)
from
    data_table d
    inner join category_table c
        on  d.data_id = c.data_id
where
    c.cat_id in (1, 2)
group by
    c.cat_id