SQL Server 2022

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5 rows affected

CREATE TABLE #tree (
  node_id bigint PRIMARY KEY,
  value bigint,
  op varchar(2),
  side char(1),
  parent_node_id bigint,
  INDEX IX (parent_node_id) INCLUDE (side, value),
  CHECK (parent_node_id <> node_id)
);

INSERT #tree
SELECT
  et.node_id,
  CASE WHEN et.value NOT LIKE '%[^0-9]%' THEN et.value END,
  CASE WHEN et.value LIKE '%[^0-9]%' THEN et.value END,
  et.side,
  et.parent_node_id
FROM expression_tree et
WHERE et.expression_id = 1;

DECLARE @final bigint, @parent bigint;

WHILE 1=1
BEGIN
    WITH child AS (
        SELECT
          child.parent_node_id,
          MIN(CASE WHEN child.side = 'L' THEN child.value END) AS L,
          MIN(CASE WHEN child.side = 'R' THEN child.value END) AS R
        FROM #tree child
        WHERE child.parent_node_id IS NOT NULL
        GROUP BY
          child.parent_node_id
        HAVING COUNT(CASE WHEN child.value IS NULL THEN 1 END) = 0
    )
    UPDATE t
    SET @final = v.final,
        value = v.final,
        @parent = t.parent_node_id
    FROM #tree t
    JOIN child c ON c.parent_node_id = t.node_id
    CROSS APPLY (
        SELECT CASE t.op
              WHEN '+' THEN L + R
              WHEN '-' THEN ISNULL(L, 0) - R
              WHEN '*' THEN L * R
              WHEN '/' THEN L / R
              WHEN '%' THEN L % R
          END
    ) v(final)
    WHERE t.value IS NULL;
    
    IF @@ROWCOUNT = 0 OR @parent IS NULL
        BREAK;
END;

SELECT @final;

(No column name)
1

Warning: Null value is eliminated by an aggregate or other SET operation.
Warning: Null value is eliminated by an aggregate or other SET operation.