Final answer:
A secondary sort specified in the ORDER BY clause is used when identical values are encountered during the primary sort; it uses an alternate column to determine the sort sequence in a SQL query. For example, a students table can be sorted primarily by last name, then by first name as the secondary sort.
Step-by-step explanation:
A secondary sort can be specified in the ORDER BY clause to indicate that if an exact match occurs during the primary sort, the alternate column listed should be used to determine the presentation sequence. In SQL, this is done by listing multiple columns in the ORDER BY clause. When the primary column has identical values for multiple rows, the secondary column's values are used to order those rows. This ensures that the result set is sorted by the first specified column, and where there are duplicates in that column, the sort order of the secondary column is applied.
For example, consider a database table that holds student records with columns for last name, first name, and grade level. To sort primarily by last name and secondarily by first name, you would use a query like:
SELECT last_name, first_name, grade_level FROM students ORDER BY last_name, first_name;
In this statement, last_name is the primary sort key, and first_name is the secondary sort key. The result set would first be ordered alphabetically by the last name, and where last names are the same, it would be ordered by the first name.