设关系R和S具有公共属性集Y,当执行RS时,会丢弃那些在Y属性上没有匹配值的元组。如果不想丢弃那些元组,应采用下列哪个操作
D
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Hewas______towithdrawfromrunningforthepresidency.
Itisdifficultto________theimplicationbetweenthelines.
When_______,waterwillbeturnedintosteam.
Thehostsapologizedfor______thatsomeoftheirguestsmightbevegetarians.
ThelongestriverinCanadais________.