I need to sort a PostgreSQL table ascending by a date/time field, eg 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 ??
Postgres provides the NULLS FIRST | LAST
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.
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;
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