Publish post about the FILTER clause

articles/til-filter-clause.md

@@ -0,0 +1,35 @@
+---
+title: "TIL: The FILTER clause in SQL"
+slug: "til-filter-clause"
+date: "2024-11-10"
+published: true
+tags:
+  - TIL
+  - Postgres
+  - SQL
+description: "A useful SQL keyword that I didn't know about until recently."
+---
+
+_( H/T [@winand.at](https://bsky.app/profile/winand.at/post/3lagizjuo4e2y) )_
+
+In SQL, if you wanted to perform two or more aggregations with different filters, you might be able to use the `FILTER` clause.
+
+```sql
+SELECT 
+    count(*) as user_count,
+    count(*) filter (where verified = true) as verified_user_count
+FROM users;
+```
+
+is the equivalent of:
+
+```sql
+SELECT 
+    count(*) as user_count,
+    sum(case when verified = true then 1 else 0 end) as verified_user_count
+FROM users;
+```
+
+except that a) it reads more naturally, and b) you don't need to write a complicated `case` expression.
+
+`FILTER` is supported in Postgres 9.4 and above, and in sqlite. It is unfortunately not widely supported in other databases.