@Override
  public <R, C> R accept(AstVisitor<R, C> visitor, C context) {
    return visitor.visitNullIfExpression(this, context);
  }
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