Let say in your first data set you have data like
item , sales
1 , 100
and right table as
item , item name
1 , i1
now if you do left join on item column with first table as left table , your output will be
so your initial sales will change from 250 to 350 because of cross join.
To handle this you should go with Data Blending as mentioned by Amit Narkar .
This is happening because there are multiple rows exists in the second table with reference to key used to join the tables. You may need to take the avg(sales) or divide the sum(sales) by distinct key combination.
For example, if there are 6 rows in second table for each Country and Category and you're joining two tables on country then sum(sales) will be multiplied by 6. Then you need to Divide the Sum(sales) by distinct combination of Country and Category i.e 6 to get the initial Sum.
Hope this helps.