To answer this question, let's go through each option to understand why it is correct or incorrect:
Option A) A single row subquery can retrieve data from only one table. - This option is incorrect because a single row subquery can retrieve data from multiple tables. The number of tables involved in the subquery depends on the specific query and its requirements.
Option B) A SQL query statement cannot display data from table B that is referred to in its subquery, unless table B is included in the main query's FROM clause. - This option is correct. In SQL, a subquery is a query nested within another query. The main query cannot directly access tables that are referred to in the subquery, unless those tables are included in the main query's FROM clause. This is because the main query needs to have access to the table's data in order to display it.
Option C) A SQL query statement can display data from table B that is referred to in its subquery, without including table B in its own FROM clause. - This option is incorrect. As mentioned in the explanation for option B, the main query needs to include table B in its FROM clause in order to display data from it that is referred to in the subquery.
Option D) A single row subquery can retrieve data from more than one table. - This option is correct. A single row subquery can retrieve data from multiple tables, depending on the specific query and its requirements.
The correct answer is B, D. These options are correct because a subquery cannot directly access tables referred to in it unless those tables are included in the main query's FROM clause (option B), and a single row subquery can retrieve data from more than one table (option D).