Question

I have two tables:
t1 with the following columns: name | key | length
t2 with the following columns: name | country.

I need to select all distinct keys with length>2000 group by country. So, I made

SELECT count(distinct key), country
from db.t1
inner join db.t2
on t1.name=t2.name
where length>2000
group by country;

But, when I make the query:

SELECT count(distinct key)
from db.t1
where Length>2000;

I am supposed to get equal results but I’m getting different results. For example, in the first query, I get 125494 and in the second I get: 121653.

What is the reason for this different results?? Knowing that there are some fields in the country are ''. It seems to me they don’t appear as a group and i counted them and found that they are 134 records. but I can’t find out the reason.

Answer 1

Unless key is UNIQUE (in which case, why bother with the DISTINCT keywords?), there is no reason that your two queries should return the same results.

Suppose t1 contains:

+------+-----+--------+
| name | key | length |
+------+-----+--------+
|    a |   x |   5000 |
|    b |   x |   5000 |
|    b |   y |   5000 |
|    c |   z |   5000 |
+------+-----+--------+

And t2 contains:

+------+---------+
| name | country |
+------+---------+
|    a |      uk |
|    b |      fr |
|    c |      de |
+------+---------+

Then your queries will return:

1. First query:

   SELECT count(distinct key), country
   from db.t1
   inner join db.t2
   on t1.name=t2.name
   where length>2000
   group by country;
   

   Will yield:

   +---------------------+---------+
   | count(distinct key) | country |
   +---------------------+---------+
   |                   1 |      uk |
   |                   2 |      fr |
   |                   1 |      de |
   +---------------------+---------+
   

2. Second query:

   SELECT count(distinct key)
   from db.t1
   where Length>2000;
   

   Will yield:

   +---------------------+
   | count(distinct key) |
   +---------------------+
   |                   3 |
   +---------------------+
   

See it on sqlfiddle.