# Time:  O(n)
# Space: O(n)

SELECT left_operand, operator, right_operand,
       CASE WHEN operator = '>' AND v1.value > v2.value THEN 'true'
            WHEN operator = '<' AND v1.value < v2.value THEN 'true'
            WHEN operator = '=' AND v1.value = v2.value THEN 'true'
            ELSE 'false' END AS value
FROM expressions e
LEFT JOIN variables v1
ON e.left_operand = v1.name
LEFT JOIN variables v2
ON e.right_operand = v2.name;