Chapter 6

1.	A subquery is allowed in which parts of a SQL SELECT statement?
2.	True or false: A correlated subquery references a table in the SELECT clause.
3.	Which set operator will not remove duplicate rows from the result of a compound query?
4.	What characteristics of the columns in a compound query using INTERSECT must match?
5.	How are NULL values handled using set operators in a compound UNION query?
6.	Why are ROLLUP and CUBE the preferred method for generating subtotals and grand totals for an aggregate query?
7.	Which operators can be used to compare a column to a single-row subquery?
8.	A compound query that needs to find only the rows that are the same between the two queries should use the set operator.
9.	True or false: The IN operator cannot be used with a single-row subquery.
10.	Put the set operators UNION, UNION ALL, INTERSECT, and MINUS in order of precedence.
11.	What can be used to change the precedence of a pair of queries in a compound query with more than two queries?

Answers

1.	A subquery is allowed in the SELECT clause, the FROM clause, and the WHERE clause.
2.	False, the correlated subquery references a column in the main query.
3.	UNION ALL will not remove duplicate rows from the result of a compound query.
4.	The number of columns and their datatypes must match in a compound query using INTERSECT. The lengths of the columns and the names do not need to match.
5.	NULL values in one query are considered equal to NULL values in the other query, for the purposes of eliminating duplicates in a UNION.
6.	ROLLUP and CUBE need to make only one pass over the source table(s). Other methods, such as using a UNION between two similar queries, will make more than one pass.
7.	The following operators can be used to compare a column to a single-row subquery: =, !=, >, <, >=, and <=.
8.	INTERSECT
9.	False, using IN with a single-row subquery would be equivalent to using =.
10.	All of those operators have equal precedence and are evaluated left to right in a compound query.
11.	As with any other part of a SQL query, parentheses may be used to change the evaluation order of the set operators.