OCP-1Z0-051-题目解析-第7题

7. Which two  statements are true regarding the USING and ON clauses in table joins? (Choose two.)

A. Both USING and ON clauses can be used for equijoins and nonequijoins.

B. A maximum of one pair of columns can be joined between two tables using the ON clause.

C. The ON clause can be used to join tables on columns that have different names but compatible data types.

D. The WHERE clause can be used to apply additional conditions in SELECT statements containing the ON or the  USING clause.

Answer: CD

题目解析：

A:  Using和On子句都可以用于等式和不等式连接（错误，Using只能用于等式连接）

B：在两个表之间使用On子句最大只能连接两个字段（错误，可以连接多个字段，如，on e.empno=d.empno and e.deptno=d.deptno）

C：On子句可以连接两个表中名称不同的字段，但数据类型要兼容（正确，On子句可以连接不同的字段名，Using只能用于相同的字段名）

D:  在包含On和Using子句的Select语句中，可以使用Where语句作为附加的条件（正确，例如

SELECT e.department_id, d.department_id
FROM employees e JOIN departments d
ON (e.department_id = d.department_id)
Where e.department_id>20;

）