我有4个关系a、b、c和d,每个关系都包含一个字段id(与其他字段)。我想在id上内部连接这些关系,这样最终的关系应该只有属于所有a,b,c和d关系的id。我可以用多种方法来实现这一点:
1. X = JOIN A BY id, B BY id, C BY id, D BY id;
2. X = JOIN A BY id, B BY id;
Y = JOIN C BY id, D BY id;
Z = JOIN X BY A::id, Y BY C::id
3. X = JOIN A BY id, B BY id;
Y = JOIN X BY B::id, C BY id;
Z = JOIN Y BY C::id, D BY id;
我如何决定以上哪种方法是最有效的。在选择上述方法之前,我需要考虑哪些因素(例如a..d的大小、最终关系中每个a..d所需的列数等)?
暂无答案!
目前还没有任何答案,快来回答吧!