t, Sql Server - rextester

Language: Layout:

CREATE TABLE tPosition 
(
    SecNum INT,
    Position INT
);

CREATE TABLE tTrade
(
    TradeID INT,
    TradeDate DATETIME,
    SecNum INT,
    Quantity INT
);

INSERT INTO tPosition (SecNum, Position) 
SELECT 1, 100
UNION
SELECT 2, 200
UNION
SELECT 3, -300;

INSERT INTO tTrade (TradeID, TradeDate, SecNum, Quantity)
SELECT 1, '1/1/2016', 1, -50
UNION
SELECT 2, '1/2/2016', 1, -50
UNION
SELECT 3, '1/3/2016', 1, -50
UNION
SELECT 4, '1/4/2016', 1, 50
UNION
SELECT 5, '1/4/2016', 2, 50
UNION
SELECT 6, '1/5/2016', 3, 200
UNION
SELECT 7, '1/5/2016', 3, 200;

WITH DirectReports (SecNum, Position, OpenedByTradeID, ClosedByTradeID, level_id, TradeID)
AS (
    SELECT SecNum, Position, NULL as OpenedByTradeID, NULL as ClosedByTradeID, 1, null as TradeID
    FROM tPosition
    WHERE SecNum = 1 
    UNION ALL
    SELECT D.SecNum, 
           Position + Quantity as Position, 
           CASE WHEN Position = 0 and Position + Quantity <> 0 
                THEN T.TradeID
                ELSE NULL
           END as OpenedByTradeID,
           CASE WHEN Position <> 0 and Position + Quantity = 0 
                THEN T.TradeID
                ELSE NULL
           END as ClosedByTradeID,
           level_id + 1 as level_id,
           T.TradeID
           
    FROM DirectReports D
    JOIN (SELECT  *,
                  ROW_NUMBER() OVER (partition by SecNum ORDER BY TradeDate) as rn
          FROM tTrade                              
         ) T
      ON D.SecNum = T.SecNum     
     AND D.level_id = T.rn
)   
SELECT *
FROM DirectReports

View schema
Execution time: 0,02 sec, rows selected: 5, rows affected: 10, absolute service time: 0,19 sec