There is no mistake - your query is showing everything from Sales AND Purchases.
These articles might help you understand
Visual Representation of SQL Joins[
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Joining Tables in SQL[
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What you probably want is a UNION or something like this
select salesDB.id, 'SALE' as tranType, sales
from salesDB
UNION
select purchDB.id, 'PURC' as tranType, purchase
from purchDB
which should give
1 SALE 5000
1 PURC 10000
1 PURC 15000