【多选题】
A A primary key column referenced by another column as a foreign key can be dropped if using the CASCADE option.
B An UNUSED column’s space is reclaimed automatically when the block containing that column is next queried.
C An UNUSED column’s space is reclaimed automatically when the row containing that column is next queried.
D Partition key columns cannot be dropped.
E A DROP COLUMN command can be rolled back
F A column that is set to UNUSED still counts towards the limit of 1000 columns per table
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