code: func dbUserExists(ctx sessionctx.Context, name string, host string, db string) (bool, error) {
  	sql := fmt.Sprintf(`SELECT * FROM %s.%s WHERE User='%s' AND Host='%s' AND DB='%s';`, mysql.SystemDB, mysql.DBTable, name, host, db)
  	return recordExists(ctx, sql)
  }
docs: // dbUserExists checks if there is an entry with key user-host-db in mysql.DB.