MySQL in a Nutshell, 2nd Edition

Name

COALESCE()

Synopsis

COALESCE(column[, ...])

This function returns the leftmost non-NULL string or column in a comma-separated list. If all elements are NULL, the function returns NULL. Here is an example:

SELECT CONCAT(name_first, ' ', name_last)
  AS Student,
COALESCE(phone_dorm, phone_home, 'No Telephone Number')
  AS Telephone
FROM students;

In this example, the results will show the student’s dormitory telephone number if there is one (i.e., if the student lives in the dormitory). If not, it will show the student’s home telephone number (i.e., maybe his parent’s house). Otherwise, it will return the string given, indicating that there is no telephone number for the student.