ADQL20Err4 < IVOA

---+ ADQL-2.0 Erratum 4: NULL as valid value expression

Author: Grégory Mantelet

Date last changed: 2023-01-31

Date accepted: -

---++ Rationale

The in-development validator for ADQL (<a href="https://github.com/ivoa/lyonetia/pull/16#issuecomment-1406523355" title="ivoa/lyonetia#16 (comment)">ivoa/lyonetia#16 (comment)</a>) as described in the official standard document highlighted a miss since ADQL-2.0. NULL values are not allowed anywhere except in the constraint IS NULL. However, it is a valid value in any existing DBMS.

This Erratum aims to fix this miss by adding the NULL in the value expression definition.

---++ Erratum Content

This Erratum changes the BNF grammar of [[https://www.ivoa.net/documents/cover/ADQL-20081030.html][ADQL-2.0]], in the Appendix A, p.35, from:

<pre><br />&lt;value_expression&gt; ::=
      &lt;numeric_value_expression&gt;
    | &lt;string_value_expression&gt;
    | &lt;geometry_value_expression&gt;</pre>

into:

<pre><br />&lt;value_expression&gt; ::=
      NULL
    | &lt;numeric_value_expression&gt;
    | &lt;string_value_expression&gt;
    | &lt;geometry_value_expression&gt; </pre>

---++ Impact Assessment

All existing Database Management Systems (e.g. PostgreSQL, MySQL, SQLite, SQLServer, ...) allow =NULL= values in all places where a value can be provided.

DACHS and CADC already support it, and so, it should not be a problem for all existing services based on their TAP framework.

However, services based on VOLLT are impacted until a new version is released or if it has been forked and already fixed on this particular issue. Anyway, the amount of ADQL queries where a =NULL= has to be explicitly written (apart from the special constraint =IS NULL=) is extremely low. Hence, very few inconvenience is expected while waiting for a migration for a newer version resolving this issue. The next version of VOLLT will anyway embed ADQL-2.1, in which this problem is already fixed (see <a href="https://github.com/ivoa-std/ADQL/pull/72" title="ivoa-std/ADQL#72">ivoa-std/ADQL#72</a>).
This topic: IVOA > WebHome > IvoaDAL > ADQL > ADQL-2_0-Errata > ADQL20Err4
Topic revision: r1 - 2023-01-31 - GregoryMantelet