请问 为什么MYSQL会呢? 会报 DUPLICATE COLUMN NAME
执行里面 子查询就不会,
ORACLE 好像也不会的,

SELECT 
    COUNT(*)
FROM
    (
    SELECT      c.channel_code, p.channel_code 
    FROM        channel_info c
    LEFT JOIN product_info p ON c.id = p.channel_id
    ) aa