Results of using a comparison operator in the ORDER BY clause
This is common comparison operators.
When comparison is performed then some boolean result is produced. It may be TRUE, FALSE or NULL.
These 3 values are related as
NULL &lt; FALSE &lt; TRUE

This relation is used for sorting.
For example:
select * from x order by a&lt;b;

If a and/or b is NULL then the result of ordering expression (comparison result) is NULL, and such records will be at the beginning of the output.
If a less then b then the result of ordering expression is TRUE and such records will be at the end of the output.
The records where a is greater or equal to b will be in the middle.