Comment by hyperpape

The recommended approach is to generate SQL that looks like:

    SELECT \* FROM users
    WHERE id = $1
        AND ($2 IS NULL OR username = $2)
        AND ($3 IS NULL OR age > $3)
        AND ($4 IS NULL OR age < $4)

It's worth noting that this approach has significant dangers for execution performance--it creates a significant chance that you'll get a query plan that doesn't match your actual query. See: https://use-the-index-luke.com/sql/where-clause/obfuscation/... for some related material.