Joe Celko's SQL for Smarties, 3rd Edition

23.1. The Mode

The mode is the most frequently occurring value in a set. If there are two such values in a set, statisticians call it a bimodal distribution; three such values make it trimodal; and so forth.

Most SQL implementations do not have a mode function, since it is easy to calculate. A simple frequency table can be written as a single query in SQL-92.

This version is from Shepard Towindo, and it will handle multiple modes.

SELECT salary, COUNT(*) AS frequency
  FROM Payroll
 GROUP BY salary
HAVING COUNT(*)
      >= ALL (SELECT COUNT(*)
                   FROM Payroll
                  GROUP BY salary);

As an exercise, here is a version that does not use a GROUP BY clause to compute the mode. However, the execution time might be a bit longer than you would like.

SELECT DISTINCT salary ...