我是新手。这可能是一个简单的解决方法。
询问：
select AvgR.preavgMR - AvgR.postavgMR
from (
((select R.mID mID, avg(R.stars) preavgMR from Rating R group by R.mID)
join Movie M using (mID)
where year < 1980
)
join
((select R.mID mID, avg(R.stars) postavgMR from Rating R group by R.mID)
join Movie M using (mID)
where year >= 1980
)
) AvgR using (mID);
结果：
Query failed to execute: near "where": syntax error
想法/建议？谢谢！
尝试这种方式：
select AvgR.preavgMR - AvgR.postavgMR
from (
(
select mID,preavgMR
from (select R.mID mID, avg(R.stars) preavgMR from Rating R group by R.mID) T1
join Movie M using (mID)
where year < 1980
)
join
(
select mID,postavgMR
from (select R.mID mID, avg(R.stars) postavgMR from Rating R group by R.mID) T2
join Movie M using (mID)
where year >= 1980
)
) AvgR;
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我来说两句