# xSQL Sample Compound_nullable

> This sample demonstrates the use of a compound index and a null (nullable) field (column). Note also the use of aliases `a1` and `a2` to form a self-join and use of the `explain` operator to display the execution plan.

> This sample can be run from directory `samples\xsql\scripts` with the following command:

>         ..\..\..\target\bin\xsql.exe -b -c xsql.cfg -f compound_nullable.sql