SQL Server Left Join producing duplicates - sql

Question

I have 3 tables, Budgets, Income, and Expenses.
Budget table:
Income table:
Expenses table:
This is my SQL statement:
SELECT
Budgets.BudgetID, Budgets.BudgetName, Budgets.Username_FK,
Budgets.BudgetAmount, Budgets.SavePercentage,
Expenses.ExpensesID, Expenses.ExpensesAmount, Expenses.ExpensesCategory,
Income.IncomeID, Income.IncomeAmount, Income.IncomeCategory
FROM
Budgets
LEFT JOIN
Income ON Budgets.BudgetID = Income.BudgetID_FK
LEFT JOIN
Expenses ON Budgets.BudgetID = Expenses.BudgetID_FK
WHERE
BudgetName = '2019
And the results are as follows:
Based on my Income table, there is only 1 record tied to BudgetID = 3, but in the left join, it duplicates.
Ideally, I would want it to return "null" on the duplicates. How do I do this?

Answer 1

You have several rows in expenses per budgetID, so your join produces that many rows. I tend to suspect that the same situation could happen with income too.
If you want one row per budgetID, then one option is pre-aggregation and a left join (or outer apply). Say you want the total expense and income per budget, you would do:
select b.*, e.expenseAmount, i.amountAmount
from budgets b
left join (
select budgetID_FK, sum(expenseAmount) expenseAmount
from expenses
group by budgetID_FK
) e on e.budgetID_FK = b.budgetID
left join (
select budgetID_FK, sum(incomeAmount) incomeAmount
from income
group by budgetID_FK
) i on i.budgetID_FK = b.budgetID
Now you are grouping rows in the dependent tables by budgetID, so you can't see the other columns of these tables, such as incomeCategory or expenseCategory (which have multiple values per budgetID).

Answer 2

For budgetID 3 , you have 4 expenses( distinct expense IDs) of 50.00 in your expense table for the same user. I think what you would want is the total expenses of same category and budget ID i.e budgetID 3 in this case
SELECT
Budgets.BudgetID, Budgets.BudgetName, Budgets.Username_FK,
Budgets.BudgetAmount, Budgets.SavePercentage,
Income.IncomeID, Income.IncomeAmount, Income.IncomeCategory,
ex.ExpensesCategory,
ex.Total_ExpensesAmount,
FROM Budgets
LEFT JOIN Income ON Budgets.BudgetID = Income.BudgetID_FK
LEFT JOIN (Select BudgetID_FK, ExpensesCategory, SUM(ExpensesAmount) as Total_ExpensesAmount
FROM Expenses
GROUP BY BudgetID_FK, ExpensesCategory) ex ON Budgets.BudgetID = ex.BudgetID_FK
WHERE BudgetName = '2019'