The COALESCE and NVL expressions return the first non-NULL value
    from a series of value expressions.

    SQL evaluates each value expression in a COALESCE or NVL
    expression until it can return a non-NULL value. If all the
    columns specified in the COALESCE or NVL expression contain NULL
    values, then NULL is returned.

    The data type of the resulting expression is a common data type
    to which all value expressions in the list can be converted.
    For example, COALESCE(SALARY_AMOUNT, ESTIMATED_BONUS, 1.23E+5)
    results in a DOUBLE PRECISION result because at least one
    argument is a floating point value.

    The following example replaces the stored NULL value in the
    MIDDLE_INITIAL column of the EMPLOYEES table with a hyphen:

    SQL> SELECT FIRST_NAME, LAST_NAME, MIDDLE_INITIAL,
    cont> COALESCE(MIDDLE_INITIAL, '-')
    cont> FROM EMPLOYEES
    cont> WHERE LAST_NAME LIKE 'L%';
     FIRST_NAME   LAST_NAME        MIDDLE_INITIAL
     Jo Ann       Lapointe         C                C
     Hope         Lapointe         NULL             -
     Stan         Lasch            P                P
     Norman       Lasch            NULL             -
     Peter        Lengyel          A                A
     Peter        Lonergan         V                V
    6 rows selected