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.