Mysql – Add constraint on number of entries of certain type in referenced table

Here are the layers of abstraction that you need to go through to track who has (pays for) a GSM phone:

Note that this is good for a point in time. If you need to track changes over time then everything becomes many-to-many with start/end dates in the intersection entities.

If you look at incomes and expenses tables, you see they are the same, they differ only in a sign of the change of user balance. You can easily create only one table of "transactions" to keep both and either add a column with a type [Income, Expense] or decide it only by a sing on a value column (I suppose you want that one as you talk about summing all expenses and incomes, but it is not defined in your question).

Then you can do simple

select sum(value) as balance from transactions where user_id = X and datetime <= Y;

As you can see, I use datetime as one column - I suggest you do do that as otherwise all conditions for checking ranges get quite cumbersome:

where date < xxx or (date = xxx and time <= yyy)
-- and now add once more for the upper limit

This is good enough if you need that only sometimes and have a good index with user_id and datetime and preferably a value too. But if know that you will check that often or that there will be really lots of transactions for each user, it makes sense adding one more column where you will store the value of user balance after applying that transaction. That way getting the user balance for a given datetime is like

select balance_value 
from transactions 
where user_id = X and datetime <= Y
order by datetime desc
limit 1;

with again using index on (user_id, datetime) for a really fast retrieval.

This can have a secondary usage too - it lets you simply verify the users_balance records and check that there is no transaction missing. That can come heplful in a case of a bug or problem with data storage.