If you look up the word Coalesce in the dictionary you would see the following:

1. to unite so as to form one mass, community
2. to blend or come together

In MySQL COALESCE() is a comparison operator that returns the first non null argument fed to it. This removes the need to use if statements in determining the value that has to be returned.

Example:

People Table

Id      Name       college    high_school      elementary
---     ---        ---        ---              ---
1       Juan       Univ A     HS A             Elem A
2       Pedro                                  Elem A
3       Dada                  HS B             Elem A
4       Pepito     Univ B     HS C             Elem B
5       Maria                                  Elem C

Say we have the People table and we'd like to see which school the everybody got their highest educational attainment. We execute the following statement wich uses COALESCE:

Select id, name, COALESCE(college, high_school, elementary) as highest_ed_school from people;

Then we get a result like:

Id      Name       highest_ed_school
---     ---        ---
1       Juan       Univ A
2       Pedro      Elem A
3       Dada       HS B
4       Pepito     Univ B
5       Maria      Elem C