Regarding single-row subqueries:
* A. THEY CAN BE USED in the where clause: Single-row subqueries can be used in the WHERE clause to compare a column's value against the result of the subquery.
* B. A SQL STATEMENT MAY HAVE MULTIPLE SINGLE ROW SUBQUERY BLOCKS: It is possible to use more than one single-row subquery within a SQL statement, each potentially in different parts of the statement (e.g., SELECT clause, WHERE clause).
* E. THEY CAN BE USED IN THE HAVING CLAUSE: Similar to the WHERE clause, single-row subqueries can also be used in the HAVING clause to filter groups based on a condition evaluated against a subquery result.
Incorrect options are:
* C and D: There is no such limitation on the placement of single-row subqueries relative to the comparison operator; they can be used on either side.
* F: Single-row subqueries must return exactly one row to avoid errors, but they do not necessarily have to return a row; if no row is returned, the query may still execute, potentially returning no rows in the outer query.