COALESCE accepts one
or more column names of the same data type. The COALESCE function
checks the value of each column in the order in which they are listed
and returns the first nonmissing value. If only one column is listed,
the COALESCE function returns the value of that column. If all the
values of all arguments are missing, the COALESCE function returns
a missing value.
Note: If your query contains a
large number of COALESCE function calls, it might be more efficient
to use a natural join instead. See
Natural Joins.