@Override
  public String getDisplayString(String[] children) {
    return getStandardDisplayString("COALESCE", children, ",");
  }
GenericUDF Class for SQL construct "COALESCE(a, b, c)".

NOTES: 1. a, b and c should have the same TypeInfo, or an exception will be
thrown.
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDFCoalesce.java