How to connect a MySQL database to a FiveM server

Create the database in GamePanel (Databases), then paste its credentials into server.cfg on a set mysql_connection_string line.

In the GamePanel

your serverManagementDatabasesOpen

Before you start

The oxmysql resource (or mysql-async for older scripts) in resources. The ESX and QBCore frameworks include it.

GamePanelManagementDatabasesCreate: choose a host, give it a name (e.g. fivem), leave % in "Connections From", then "Submit".
Creating the database in GamePanel
In the list, note the database name, the username, the password (click it to reveal it) and the "Connection" column: the address before : is the host, the number after is the port.
The created database, with its credentials
File Manager, server.cfg: add the following line with your values, placing ensure oxmysql before the resources that use the database: set mysql_connection_string "host=HOST;port=PORT;user=USER;password=PASSWORD;database=DATABASE_NAME". If the connection fails even though the credentials are correct (an old password set by hand, containing a = or a ;), use the same line in mysql:// format instead: set mysql_connection_string "mysql://USER:PASSWORD@HOST:PORT/DATABASE_NAME"
The connection line in server.cfg
With mysql-async, the line reads server=HOST;port=PORT;userid=USER;password=PASSWORD;database=DATABASE_NAME.
Restart and check the console: oxmysql reports the connection is established, otherwise it shows the error (see the common errors below).
The console: oxmysql connected to the database
To import the .sql file that came with a script or framework: the database's "PHPMyAdmin" button, Import tab.

Common errors

Access denied: username or password mistyped, or a = or ; in an old password (use the mysql:// format from step 3). If in doubt, "Reset Password" and paste it again.
localhost or 127.0.0.1 as the host: doesn't work, the database is on a different machine. Use the host from the Connection column.
Game server port instead of the database port: use the one from the Connection column.
mysql://… format: an @, :, /, ? or # in the password breaks the line: reset the password and go back to the first format.
oxmysql not found or MySQL not ready errors: ensure oxmysql is missing or placed after your scripts.
Can't create another one: the number of databases depends on your plan, the Usage counter on the tab shows it.