Amortization, Sql Server

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CREATE TABLE contract
(  ID int
  ,outstanding float
)

CREATE TABLE amort
(  contractID int
  ,amortization float
  ,amo_date date
)

INSERT INTO contract VALUES
(1, 100000),
(2, 350000)

INSERT INTO amort VALUES
(1, 10000, '2017-08-01'),
(1, 20000, '2017-09-01'),
(1, 50000, '2017-10-01'),
(1, 10000, '2017-11-01'),
(1, 10000, '2017-12-01'),
(2, 30000, '2017-08-01'),
(2, 20000, '2017-09-01'),
(2, 50000, '2017-10-01'),
(2, 60000, '2017-11-01'),
(2, 60000, '2017-12-01'),
(2, 70000, '2018-01-01'),
(2, 60000, '2018-02-01')

;WITH CTE AS
(
        SELECT con.ID
              ,con.outstanding
              ,amo.amortization
              ,amo.amo_date
              ,ROW_NUMBER() OVER ( PARTITION BY con.ID
                                       ORDER BY amo.amo_date asc
							      ) as rn    
          FROM contract con
    INNER JOIN amort amo
            ON amo.contractID = con.ID
)
SELECT ID
      ,outstanding
      ,outstanding - amortization as new_outstanding
      ,amo_date
  FROM CTE

View schema
Execution time: 0,03 sec, rows selected: 12, rows affected: 14, absolute service time: 0,2 sec