Remember earlier when we wanted to find loans where we wanted to find loans where 0:00 the due date was after December 18th, 2015 because we wanted to send out reminders 0:04 to those who need to return their books soon. 0:08 In other words, what are the loans that are due back after December 18th, 2015? 0:11 In the returned_on column we see there's missing values in some rows. 0:17 That's because these books haven't been returned yet. 0:22 The rows with the dates in them are returned. 0:25 We don't want to bug someone who's already returned his or her book. 0:28 We need to find those rows with missing values. 0:32 We can't just write return _ on equals or nothing. 0:35 We'll get a syntax error. 0:40 There's a special way in SQL to represent a missing value, and 0:46 that's by writing null. 0:50 A missing value is known as a null value, but 0:52 when we use null, you cannot use it with the equality operator. 0:56 We have to use null in conjunction with the is key word. 1:01 See how this only brings back loans that haven't been returned. 1:07 So, what if we wanted to send a thank you email 1:11 to those who have returned their books? 1:14 We can use is not null. 1:17 Thank you patron number four. 1:24 Let's see who that is. 1:25 Select first name, 1:27 email FROM patrons WHERE id = 4 1:31 So to recap a missing value can be expressed in SQL as null. 1:40 You can search for null or 1:46 missing values using the key word is followed by the value null. 1:47 To filter out missing values you can use IS NOT NULL. 1:53