MySQL 8.0

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CREATE tABLE tbl_1min(id int, close int,ema9 varchar(30))
INSERT INTO tbl_1min VALUES (1,1,'test')
SET @NDays := 9;
WITH RECURSIVE anchor AS
    (
        SELECT
            a1.id,
            a1.close,
            a1.ema9
        FROM
            tbl_1min a1
),
x AS
    (
        SELECT
            anchor.id,
            anchor.close,
            x2.ema9 AS PrevEma,
            anchor.ema9,
            anchor.ema9 AS NewEma
        FROM
            anchor
        INNER JOIN
            anchor x2
        ON
            anchor.id - 1 = x2.id
        WHERE anchor.id = @NDays -- FIRST SEED ROW
UNION
   SELECT
    anchor.id,
    anchor.close,
    x.PrevEma,
    anchor.ema9,
    (((2 / (1 + @NDays)) * (anchor.close - x.PrevEma)) + x.PrevEma) AS NewEma
    FROM
        x
    JOIN anchor
    ON anchor.id = x.id + 1   -- `X` WALKS ITSELF
    )
UPDATE tbl_1min t1
        INNER JOIN
    x ON t1.id = x.id 
SET t1.ema9 = x.NewEma;
;

Rows matched: 0  Changed: 0  Warnings: 0


SELECT * FROM tbl_1min


id
close
ema9


1
1
test