Add Check If User Role Exists For Database as a Postgres TIL

postgres/check-if-user-role-exists-for-database.md

@@ -0,0 +1,33 @@
+# Check If User Role Exists For Database
+
+User roles define who can access a database cluster and broadly what level of
+control they have over that cluster.
+
+The most straightforward way to check if a user role exists is to connect to
+one of the databases in the cluster and run a query against the `pg_roles`
+table.
+
+```sql
+select * from pg_roles where rolename='dev';
+
+  rolname
+------------
+    dev
+(1 row)
+```
+
+This same concept can be used in a script when automating some database setup.
+To do that, we'll use `-c` (and some other flags) to dispatch a query to `psql`
+from a shell context.
+
+```bash
+psql postgres -tXAc "SELECT 1 FROM pg_roles WHERE rolname='dev'" \
+  | grep -q 1 \
+  || createuser --interactive dev
+```
+
+This queries for the value `1` if the user role named `dev` exists. The output
+of that is piped to `grep` (in quiet mode, `-q`) to check if `1` is in the
+output. If user roles doesn't exist and grep doesn't match on `1`, then the
+right side of the _or_ (`||`) gets called. That command could be whatever. I've
+chosen to call PostgreSQL's `createuser` to create the `dev` user role.