Informix is an SQL-based object-relational, OLTP/OLAP, NoSQL DBMS produced by IBM. This tag is appropriate for questions about any of the products with Informix in the product name.

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Why does NOT IN with a set containing NULL always return FALSE/NULL?

I had a query (for Postgres and Informix) with a NOT IN clause containing a subquery that in some cases returned NULL values, causing that clause (and the entire query) to fail to return anything. ...