From 4a72c63e4260b4b0e4a4a0fb08816ac970979e05 Mon Sep 17 00:00:00 2001
From: jbranchaud <jbranchaud@gmail.com>
Date: Sat, 19 Oct 2024 19:13:30 -0500
Subject: [PATCH] Add Enforce Uniqueness On Column Expression as a Postgres TIL

---
 README.md                                     |  3 +-
 ...enforce-uniqueness-on-column-expression.md | 35 +++++++++++++++++++
 2 files changed, 37 insertions(+), 1 deletion(-)
 create mode 100644 postgres/enforce-uniqueness-on-column-expression.md

diff --git a/README.md b/README.md
index 40d8576..1d359b3 100644
--- a/README.md
+++ b/README.md
@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1475 TILs and counting..._
+_1476 TILs and counting..._
 
 ---
 
@@ -744,6 +744,7 @@ _1475 TILs and counting..._
 - [Duplicate A Local Database](postgres/duplicate-a-local-database.md)
 - [Edit Existing Functions](postgres/edit-existing-functions.md)
 - [Enable Logging Of Database Activity](postgres/enable-logging-of-database-activity.md)
+- [Enforce Uniqueness On Column Expression](postgres/enforce-uniqueness-on-column-expression.md)
 - [Escaping A Quote In A String](postgres/escaping-a-quote-in-a-string.md)
 - [Escaping String Literals With Dollar Quoting](postgres/escaping-string-literals-with-dollar-quoting.md)
 - [Export Query Results To A CSV](postgres/export-query-results-to-a-csv.md)
diff --git a/postgres/enforce-uniqueness-on-column-expression.md b/postgres/enforce-uniqueness-on-column-expression.md
new file mode 100644
index 0000000..ee756da
--- /dev/null
+++ b/postgres/enforce-uniqueness-on-column-expression.md
@@ -0,0 +1,35 @@
+# Enforce Uniqueness On Column Expression
+
+When creating a table for, say `users`, where you will store `email` addresses,
+you'll likely want to enforce uniqueness on the that `email` field. And because
+people have all sorts of ways of entering their emails, in terms of casing, you
+may be tempted to try to enforce uniqueness on a lowercased version of `email`.
+
+```sql
+create table users (
+  id integer generated always as identity primary key,
+  email text not null,
+  unique ( lower(email) ) -- !! this won't work
+);
+```
+
+A unique _constraint_ must be on one or more columns. You cannot include an
+_expression_ when defining the unique constraint.
+
+You can however accomplish similar aims with [a _unique index_ on the
+expression](https://www.postgresql.org/docs/current/indexes-expressional.html).
+That is because the index is able to store the result of the expression in
+itself.
+
+```sql
+create table users (
+  id integer generated always as identity primary key,
+  email text not null
+);
+
+create unique index unq_lower_email on users ( lower(email) );
+```
+
+This is likely what you want for this example anyway because it will probably
+be a common query to look up the user by `lower(email)` and the index will
+speed up those queries.