Question

SELECT count( t1.id ) , t2.special_value
FROM table_1 AS t1, table_2 AS t2
WHERE t1.`group` = 'val'
AND t2.code = 'val'

returns

count - normal value
special_value - NULL

but

SELECT t2.special_value
FROM table_2 AS t2
WHERE t2.code = 'val'

returns

special_value - another normal value

why first query works wrong?..

currently, i need

(count( t1.id ) + t2.special_value)

Answer 1

This is why you should never use SQL’89 implicit join syntax.

You have no join condition resulting in a cross join.

Rewrite the query using explicit join syntax:

SELECT count( t1.id ) , t2.special_value
FROM table_1 AS t1
INNER JOIN table_2 AS t2 ON (t1.`group` = t2.code)    <<-- join condition here
WHERE ....                                            <<-- filter condition here
GROUP BY ....                                         <<-- group by field here

I don’t know how table t1 and t2 are linked, so you’ll have to tweak this a bit, but that’s how it supposed to work.
And please never ever use implicit where joins again.

Remarks
I’m wondering what p.`group` and pp.code are, but I’m guessing you meant to write t1.`group` and t2.code

You only need to escape fields and tablenames in ` backticks if they are reserved words.
Personally all those backticks make me dizzy, but that’s just me.