@Override
  public List<Node> getChildren() {
    return ImmutableList.of(first, second);
  }
NULLIF(V1,V2): CASE WHEN V1=V2 THEN NULL ELSE V1 END
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/relational/sql/ast/NullIfExpression.java