How is the COALESCE function used in SQL?

In SQL, the COALESCE function is used to return the first non-NULL expression in a list of parameters. It accepts any number of parameters and returns the value of the first non-NULL parameter. If all parameters are NULL, it returns NULL.

For example, here is an example of how to use the COALESCE function:

SELECT COALESCE(column1, column2, 'default value') AS result
FROM table_name;

In this example, if the value of column1 is NULL, then return the value of column2; if the value of column2 is also NULL, then return ‘default value’.