Add Enforce Uniqueness On Column Expression as a Postgres TIL

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
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@@ -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)

postgres/enforce-uniqueness-on-column-expression.md

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+# 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.