How to Sort Nulls Last in PostgreSQL with a Specific Value Exception?
Sorting: Nulls After All but Special Exception
When organizing data in a database table with an optional sorting field, it's common to place null values last. However, sometimes an exception is needed for specific values. In PostgreSQL, a requirement arose to sort null values after non-nulls, with -1 being an exception that should come after all others.
To achieve this, a straightforward solution presented itself:
SELECT * FROM tasks ORDER BY (sort IS NOT DISTINCT FROM -1), sort;
Postgres treats boolean values as a type, including NULL. Its default sorting order is:
- FALSE (0)
- TRUE (1)
- NULL
In the expression (sort IS NOT DISTINCT FROM -1), the condition evaluates to FALSE for all values except -1, which evaluates to TRUE and sorts last. Combining this with a secondary ordering by sort orders the data as desired.
An alternative query that produces the same result is:
SELECT * FROM tasks ORDER BY (sort IS DISTINCT FROM -1) DESC, sort;
By utilizing Postgres's boolean functionality and combining it with the ORDER BY clause, it becomes possible to handle complex sorting requirements with ease.
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