Question 16 :
DISTINCT and its counterpart, ALL, can be used more than once in a SELECT statement.
Question 17 :
The result of every SQL query is a table.
Question 18 :
COUNT(field_name) tallies only those rows that contain a value; it ignores all null values.
Question 19 :
SUM, AVG, MIN, and MAX can only be used with numeric columns.
Question 20 :
Most companies keep at least two versions of any database they are using.