OCP-1Z0-新051-61题版本-44

QUESTION NO: 44

See the Exhibits and examine the structures of PRODUCTS, SALES and CUSTOMERS table:

You issue the following query:

Which statement is true regarding the outcome of this query?

A. It produces an error because the NATURAL join can be used only with two tables

B. It produces an error because a column used in the NATURAL join cannot have a qualifier

C. It produces an error because all columns used in the NATURAL join should have a qualifier

D. It executes successfully

Answer: B

答案解析：

参考：120：http://blog.csdn.net/rlhua/article/details/11554471

Explanation:

Creating Joins with the USING Clause

Natural joins use all columns with matching names and data types to join the tables. The

USING clause can be used to specify only those columns that should be used for an equijoin.

The Natural JOIN USING Clause

The format of the syntax for the natural JOIN USING clause is as follows:

SELECT table1.column, table2.column

FROM table1

JOIN table2 USING (join_column1, join_column2…);

While the pure natural join contains the NATURAL keyword in its syntax, the JOIN…USING syntax

does not.

An error is raised if the keywords NATURAL and USING occur in the same join clause. The

JOIN…USING clause allows one or more equijoin columns to be explicitly specified in brackets

after the USING keyword. This avoids the shortcomings associated with the pure natural join.

Many situations demand that tables be joined only on certain columns, and this format caters to

this requirement.