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12c OCP考试专项 [1z0-071]-Q5: SCALAR SUBQUERY 标量子查询(2020.06.16)

2020-06-16 11:37  askscuti  阅读(629)  评论(0编辑  收藏  举报

which three are true about scalar subquery expressions?

A. They cannot be used in group by clauses.
B. A scalar subquery expression that returns zero rows evaluates to zero.
C. They can be nested.
D. A scalar subquery expression that returns zero rows evaluates to null.
E. They can be used as default values for columns in a create table statement.
F. They cannot be used in the values clause of an insert statement.

Answer: ADF

Explanation:
固定的理论知识,查看官方文档
A. 它们不能在group by子句中使用,正确
B. 返回0行的标量子查询表达式计算结果为0,错误,结果为null
C. 可以嵌套,错误
D. 返回零行的标量子查询表达式计算结果为null,正确
E. 它们可以在create table语句中用作列的默认值,错误,不可作为默认值
F. 它们不能在insert语句的values子句中使用,正确

提示:答案为个人理解和解答,难免有错,也欢迎大家及时指正。