What Looks Like A Basic SQL Query Still Not Resolved

ARRRRRRRGGGGGHHHHH!!

Please can you help, I'm going round the bend with this.

I have a simple and small table called STOCKCATS, which I need to query to
get back a dataset in a particular order, but although it looks simple I
can't get it to work. My table schema plus sample data to see the problem
is as follows:

DROP TABLE IF EXISTS `STOCKCATS`;
CREATE TABLE `STOCKCATS` (
`CATID` varchar(30) NOT NULL default '',
`LEVEL` varchar(30) default NULL,

PRIMARY KEY (`CATID`),
KEY `indxCATEGORYID` (`CATID`)
);

INSERT INTO `STOCKCATS` (`CATID`,`LEVEL`) VALUES
('A001',''),
('A002','A001'),
('A003','A001'),
('A004','A001'),
('A005','A001'),
('PCHW01',''),
('MHW01',''),
('FD01',''),
('ELEC01',''),
('MHW02','MHW01');
INSERT INTO `STOCKCATS` (`CATID`,`LEVEL`) VALUES
('MHW03','MHW01'),
('MHW04','MHW01'),
('MHW05','MHW01'),
('PCHW02','PCHW01'),
('PCHW03','PCHW01'),
('PCHW04','PCHW01'),
('PCHW05','PCHW01'),
('PCSW01',''),
('MSW01',''),
('C001',''),
('C002','C001'),
('C003','C001'),
('MV',''),
('SUZ','MV'),
('ALF','MV'),
('PLASMA','ELEC01'),
('T01','ELEC01'),
('HEATING',''),
('RAD','HEATING'),
('P01',''),
('B01','P01'),
('BB','HEATING'),
('FS','HEATING'),
('WM','HEATING'),
('AEROSOL',''),
('SOLVENTS','AEROSOL'),
('DGC','');
INSERT INTO `STOCKCATS` (`CATID`,`LEVEL`) VALUES
('DGXWINDOWS','DGC'),
('DGXEXTRA','DGC'),
('DGXCON','DGC');

As you can see from the table structure, this table consists of 2 field
values. The 1st is the category code and the 2nd is the level is at. If a
catid has a level of nothing, eg '', then it means that it is a root level
category. If a catid has a another cat's catid in it's level, eg B01 has
P01, then it is a sub-category of this category, eg B01 is a sub-cat of P01.

All I want to do is query this table and bring back the data so that
alphabetically it goes root level cat A1, then all the sub-cats for this
root level, then root level A2, then all sub-cats for this root level and so
on. An example using the above would be as follows:

^ ^ A to G of root level cats plus their sub-cats....

HEATING << root level
BB << sub-cat of heating
FS << sub-cat of heating
WM << sub-cat of heating

/ / I to Z of root level cats plus their sub-cats....

A few posters kindly gave me a solution of ORDER BY COALESCE(CATID,LEVEL),
CATID and I thought this had done it, but I was looking at the ('A001',''),
('A002','A001'), ('A003','A001'), ('A004','A001'), ('A005','A001')
entries as these naturally fell into place. If you use this order command
on the above you will see that ('P01','') and it's associated ('B01','P01')
sub-cat just don't come together.

Does any body have any ideas?

Thanks

Laphan