Use `is distinct from` to match `null` records

Let's say you want to find all purchases that don't match a specific coupon. You can use != to filter them:

select * from purchases where coupon != 'JULY4';

The problem with that is that it doesn't match records that have null values. One way to solve that is by doing a or:

select * from purchases where coupon != 'JULY4' or coupon is null;

Better than that is to use is distinct from:

select * from purchases where coupon is distinct from 'JULY4';