关于sQL的问题，轻而易举拿分来！
帮我看看 为什么sum(TotalMil.TotalMil)统计了两次
SELECT GravyBoatInfo.CarBrandCode,sum(Amount) AS 总耗油量（升）, sum(TotalMil.TotalMil) as 总公里数（Km）,
sum(Amount)/sum(TotalMil.TotalMil)*100 AS 百公里油耗
FROM GravyBoatInfo, TotalMil
WHERE GravyBoatInfo.CarBrandCode=TotalMil.CarBrandCode
GROUP BY GravyBoatInfo.CarBrandCode,TotalMil.CarBrandCode
实际TotalMil表里只有一个记录，而GravyBoatInfo有两条记录 由于这种原因造成的 该怎么改动
[解决办法]
应该看是left join还是right join where条件里不能直接用“=”
[解决办法]
WHERE GravyBoatInfo.CarBrandCode=TotalMil.CarBrandCode
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两个字段相等是什么意思？如果做为连接查询的话是不是应该使用on关键字
FROM GravyBoatInfo left join TotalMil
on GravyBoatInfo.CarBrandCode=TotalMil.CarBrandCode
where GravyBoatInfo.CarBrandCode=某个特定的值
[解决办法]
CarBrandCode不唯一，直接=连接不行，用子查询吧。
[解决办法]
用max就行了.
SELECT GravyBoatInfo.CarBrandCode,sum(Amount) AS 总耗油量（升）, max(TotalMil.TotalMil) as 总公里数（Km）,
sum(Amount)/sum(TotalMil.TotalMil)*100 AS 百公里油耗
FROM GravyBoatInfo, TotalMil
WHERE GravyBoatInfo.CarBrandCode=TotalMil.CarBrandCode
GROUP BY GravyBoatInfo.CarBrandCode,TotalMil.CarBrandCode
