Relations and Sets
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31-12-2020 - |
해결책
The relation One
has 20 tuples. Note that the key of One is that
(it is underlined), so we are sure that each tuple has a different values for that
, so the cardinality of One[that]
is 20, but it could have the same (or different) values for this
, so we know with certainty only that it has at least a value for this
(see the example of @ypercubeᵀᴹ).
Since we know that One[this]
⊆ Two[tweedledee]
, we know that Two
must have at least one tuple, and possibly more. Note that the fact the tweedledee
is a not a key of relation Two
means that there could be many tuples with the same value of this attribute.
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