OCP-1Z0-051-V9.02-170题

170. Which two statements are true regarding the DELETE and TRUNCATE commands? (Choose two.)
A. DELETE can be used to remove only rows from only one table at a time.
B. DELETE can be used to remove only rows from multiple tables at a time.
C. DELETE can be used only on a table that is a parent of a referential integrity constraint. 父级的参照完整性约束错？
D. DELETE can be used to remove data from specific columns as well as complete rows. 指定行删除
E. DELETE and TRUNCATE can be used on a table that is a parent of a referential integrity constraint
having ON DELETE rule .
Answer: AE
答案解析：
A，DELETE一次只能用于移除一个表的行，正确
B，DELETE一次只能用于移除多个表的行，错误，一次只能一个表
C，DELETE只能用于删除有引用完整性约束的父表，错误，有完整性约束的父表不能被删除，除非禁用约束等
D，DELETE可以用于删除指定列的数据以及完整的行，错误，只能删除行，不能删除列
E，DELETE和TRUNCATE可以用于有引用完整性约束ON DELETE规则的父表。正确。
建表时可以使用ON DELETE CASCADE（当删除父表数据时，子表数据也一起删除）或ON DELETE CASCAD SET NULL（当删除父表数据时，子表相关的列设置为NULL）子句，可以在SQL Language Reference里的Constraints部分查找到ON DELETE子句