Should a column with a large percentage of NULL values be indexed?

No. A column with a large percentage of NULL values should not be indexed because the speed of accessing these rows degrades when the value of a large percentage of rows is the same.

Is the main purpose of an index to stop duplicate values in a table?

No. The main purpose of an index is to enhance data retrieval speed; although a unique index stops duplicate values in a table.

True or false: The main reason for a composite index is for aggregate function usage in an index.

False. The main reason for composite indexes is for two or more columns in the same table to be indexed.

What does cardinality refer to? What is considered a column of high-cardinality?

Cardinality refers to the uniqueness of the data within a column. The SSN column is an example of such a column.

Would the use of a unique index on a small table be of any benefit?

The index might not be of any use for performance issues, but the unique index would keep referential integrity intact.

What happens when the optimizer chooses not to use an index on a table when a query has been executed?

A full table scan occurs.

Should the most restrictive clause(s) be placed before the join condition(s) or after the join conditions in the WHERE clause?

The most restrictive clause(s) should be evaluated before the join condition(s) because join conditions normally return a large number of rows.

What command establishes a session?

The CONNECT TO statement establishes this.

Which option drops a schema that still contains database objects?

The CASCADE option allows the schema to be dropped if there are still objects under that schema.

Which command in SQL Server creates a schema?

The CREATE SCHEMA command creates a schema.