java更新mysql表内容出错

sql语句：
public static void deleteUser(String loginName){
String sql = "update user set flag=3 where loginName= "+ loginName ;
DBUtil.getInstance().delete(sql);
}
错误信息：
java.sql.SQLException: Data truncation: Truncated incorrect DOUBLE value: 'admin' Query: update user set flag=3 where loginName= 123654 Parameters: []
不太懂是怎么错了