It is usually not a problem as most times you are using the query to fill out the contents of a card variable.
However, if you had a two databases columns and you would like to take the contents of both to fill out a single data card variable you might use concatentaion in the query.
For example: let's say you has two columns FirstName and LastName and you wante to fill put a card variable ContactName
SELECT FirstName + ' ' + LastName FROM tablename
If however, you had a data card that had two card variables (FirstName and LastName) and you wanted a query to get the FirstName and the related last name, you would need to write an add-in (see sample below).