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

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。