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Which three statements are true about dropping and unused columns in an oracle database?

2020.2.1 071 135

(关于删除和未使用oracle数据库中的列的哪三个陈述是正确的?)

A A primary key column referenced by another column as a foreign key can be dropped if using (如果使用,可以删除另一列作为外键引用的主键列)

B A DROP COLUN command can be rolled back (可以回滚DROP COLUMN命令)

C An UNUSED column’s space is reclaimed automatically when the block containing that column is next queried.(下次查询包含该列的块时,将自动回收UNUSED列的空间。)

D An UNUSED column’s space is reclaimed automatically when the row containing that column is next queried. the CASCADE option.(下一次查询包含该列的行时,将自动回收UNUSED列的空间。 CASCADE选项。)

E Partition key columns cannot be dropped (分区键列不能删除)

F A column that is set to UNUSED still counts towards the limit of 1000 columns per table(设置为UNUSED的列仍计入每个表1000列的限制)

答案:AEF

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