SQL query involving partial group with condition?

Question

Below is a SQL query problem for which I am not able to understand correct approach:

DB tables:

Employee: emp_id, emp_name
Credit: credit_id, emp_id, credit_date, credit_amount
debit: debit_id, emp_id, debit_date, debit_amount

Here, each person can have multiple incomes and expenses.

Query requirement: At the end of each day, each employee will have some asset('credit till now' - 'debit till now'). We need to find top five employees in terms of maximum asset and the date on which they had this maximum asset.

I have tried the below query but seems like I am missing something:

select Credit.emp_id, Credit.date, (Credit.income_amount - Debit.credit_amount) from 
(select emp_id, sum(amount) as credit_amount 
from credit) Credit
LEFT JOIN LATERAL (
     select emp_id, sum(amount) as debit_amount
     from debits
     where debits.emp_id = Credit.emp_id and Credit.date >= debits.date
     group by debits.emp_id
    ) Debit
ON true

Answer 1

Here I'm breaking the query to make it more readable.

First of all, we need to get the total amount on a day-level for both credit and debit both, so that we can join the credit and debit table on the day level with the same emp_id.

with 
credit as(
    select emp_id,credit_date date,sum(credit_amount) as amount
    from credit  
    group by 1,2),

debit as(
    select emp_id,debit_date,sum(debit_amount) as amount
    from expenses 
    group by 1,2),

Now we need to full outer join the "credit" and "debit" subqueries

payments as (
    select distinct
    case when c.emp_id is null then d.person_id else c.emp_id  end as emp_id ,
    case when c.emp_id is null then d.date else c.date end as date,
    case when c.emp_id is null then 0 else i.amount end as credit ,
    case when d.emp_id is null then 0 else d.amount end as debit 
    from credit c
    full outer join debit d on d.emp_id=c.emp_id and d.date=c.date
    ),

Now we will take day-wise cumulative sum for credit, debit and total balance as shown below.

total_balance as(
    SELECT emp_id, date, 
    sum(credit) OVER (PARTITION BY emp_id ORDER BY date asc) AS total_credit,
    sum(debit) OVER (PARTITION BY emp_id ORDER BY date asc) AS total_debit,
    (sum(income) OVER (PARTITION BY person_id ORDER BY date asc) - 
    sum(expense) OVER (PARTITION BY person_id ORDER BY date asc)) as total_balance
    FROM group_payment
    ORDER BY person_id, date),

Now we need to use the rank() function to assign rank based on total balance (desc) for an emp_id (ie. rank=1 will be assigned to the largest total balance on a day for a particular emp_id). The query is shown below.

ranks as (select emp_id,date,total_balance,
rank() over (partition by emp_id order by total_balance desc) as rank
from total_balance ),

Now pick the rows having rank=1 (ie. MAX of total_balance on a day for an emp_id and the date on which it was MAX). Order it by total_balance descending and pick the top 5 rows

emp_order as (select emp_id,date,total_balance 
from ranks
where rank=1
order by 3 desc
limit 5)

Now pick the name from the employee table.

select emp_id,name, date, total_balance as balance
from emp_order eo
join Employee e on e.emp_id = eo.emp_id
order by 4 desc

Answer 2

Group by and sum allows you to get the total credit for each person into 1 record. You can do a similar thing in a subquery to subtract the debit.

Select top 5 emp_id, credit_date, (sum(credit_amount) - 
(select sum(debit_amount) from debit d 
where c.emp_id = d.emp_id and c.credit_date = d.debit_date)
) as total
from Credit c group by emp_id, credit_date order by total