PostgreSQL sort by datetime asc, null first?

Question

I need to sort a PostgreSQL table ascending by a date/time field, e.g. last_updated.

But that field is allowed to be empty or null and I want records with null in last_updated come before non-null last_updated.
Is this possible?

order by last_updated asc /* and null last_updated records first ?? */

Answer 1

PostgreSQL provides the NULLS FIRST | LAST keywords for the ORDER BY clause to cater for that need exactly:

... ORDER BY last_updated NULLS FIRST

A typical use case is with descending sort order (DESC), which yields the complete inversion of the default ascending order (ASC) with null values first. Often not desirable - so, to keep null values last:

... ORDER BY last_updated DESC NULLS LAST

To support the query with an index, make it match:

CREATE INDEX foo_idx ON tbl (last_updated DESC NULLS LAST);

Postgres can read btree indexes backwards, but it matters where NULL values are appended.

Answer 2

You can create a custom ORDER BY using a CASE statement.
The CASE statement checks for your condition and assigns to rows which meet that condition a lower value than that which is assigned to rows which do not meet the condition.
It's probably easiest to understand given an example:

  SELECT last_updated 
    FROM your_table 
ORDER BY CASE WHEN last_updated IS NULL THEN 0 ELSE 1 END, 
         last_updated ASC;