create table reddit_9ozhkk
(
    dt date,
    stock_id int,
    price decimal(10,4)
);

create unique clustered index uxc_reddit_9ozhkk_dt on reddit_9ozhkk (dt)

;with recursively as
(
    -- initial data point
    SELECT 1 as i
         , CONVERT(DATE, '2008-01-01') AS dt
         , CONVERT(DECIMAL(10,4), 65.678) AS price
    UNION ALL

   SELECT i+1
        , DATEADD(DAY, 1, dt)
        , CONVERT(DECIMAL(10, 4), price + RAND(CONVERT(VARBINARY, NEWID()))*2.000-1) -- vary price by (-1,+1) each day
     FROM recursively
    WHERE dt < GETDATE() -- until today
)
INSERT INTO reddit_9ozhkk (dt, stock_id, price)
SELECT dt, 2, price FROM recursively OPTION (MAXRECURSION 0)

-- multiplier
-- select 2.0000 / (14+1) -- = 0.133333333333333

;WITH SMA AS
(
    -- calculate SMA for each row, across the entire range
    SELECT stock_id
         , dt
         , price
         , SUM(price) OVER (PARTITION BY stock_id ORDER BY dt ROWS BETWEEN 13 PRECEDING AND CURRENT ROW)/14 AS sma
      FROM reddit_9ozhkk
     WHERE stock_id = 2
),
-- Recursively; first row, here from 2018-02-01, will use SMA as the EMA
-- then, recursively, each row from a subsequent date will calculate the EMA using the formula
-- Multiplier is 2.0000 / (14+1) -- = 0.133333333333333
-- If SMA value is removed from the SELECT statement, it will run much faster as the database will
-- only need to calculate one value for the rather expensive window SUM
EMA AS
(
    SELECT dt, price, sma, CONVERT(DECIMAL(10, 4), sma) AS ema
      FROM sma
     WHERE dt = '2018-02-01'

     UNION ALL

     SELECT curr.dt, curr.price, curr.sma, convert(decimal(10,4), calc.ema) as EMA
       FROM ema previous
      INNER
       JOIN sma curr
         -- This join will only work if there are no gaps inbetween data points
         -- Otherwise, add a ROW_NUMBER() OVER (PARTITION BY stock_id ORDER BY dt) AS rn
         -- and join on that
         ON curr.dt = dateadd(day, 1, previous.dt)
      CROSS
      APPLY (SELECT 0.133333333333333 * (curr.price - previous.ema) + previous.ema as ema) calc
)
SELECT * FROM EMA OPTION (MAXRECURSION 0)