Publish TIL on counting nulls in Postgres

articles/20241114-til-counting-nulls-in-postgres.md

@@ -0,0 +1,43 @@
+---
+title: "Counting nulls and non-nulls in Postgres"
+slug: "til-counting-nulls-in-postgres"
+date: "2024-11-14"
+published: true
+tags:
+  - TIL
+  - Postgres
+description: "The num_nonnulls and num_nulls functions in Postgres"
+---
+
+`num_nonnulls(...)` counts the number of provided arguments that are not null. 
+
+```sql
+SELECT num_nonnulls('a', 'b', null); 
+-- Returns 2
+```
+
+`num_nulls(...)` counts the number of provided arguments that are null.
+
+```sql
+SELECT num_nulls('a', 'b', null); 
+-- Returns 1
+```
+
+This is useful in a check constraint, if you need to ensure that exactly some number of columns are provided
+
+```sql
+CREATE TABLE purchase_order (
+    id UUID PRIMARY KEY,
+    contact_email TEXT NULLABLE,
+    contact_phone TEXT NULLABLE
+);
+
+-- If we want to ensure that at least one of email or phone is provided 
+-- when making a purchase order, we can add a check constraint using 
+-- `num_nonnulls`
+ALTER TABLE purchase_order ADD CONSTRAINT contact_info_check CHECK (
+    num_nonnulls(contact_email, contact_phone) >= 1
+);
+```
+
+`num_nonnulls` and `num_nulls` are available in Postgres 9.6+ ([docs](https://www.postgresql.org/docs/9.6/functions-comparison.html#:~:text=Table%209%2D3.%20Comparison%20Functions))