mysql类型为varchar double类型字符串求和多出多个小数

-- 错误
SELECT
    SUM(price)
FROM
    m_user

-- 正确
SELECT TRUNCATE
    ( SUM( u.price ), 2 )
FROM
    m_user u;

-- 正确
SELECT
    SUM(TRUNCATE(  u.price , 2 ) )
FROM
    m_user u;

-- 正确     但是和以上误差0.01
SELECT
    sum(cast(price as decimal(18,2)))
FROM
    m_user