MySQL 5.6

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CREATE TABLE `additional` ( `trip_num` VARCHAR(8) NOT NULL ,  
                            `add_id` VARCHAR(3) NOT NULL DEFAULT '??0',  
                            `add_type` VARCHAR(10) NOT NULL ,  
                            `value` VARCHAR(5) NOT NULL );
CREATE TRIGGER tr_bi
BEFORE INSERT 
ON additional
FOR EACH ROW
BEGIN
DECLARE counter INT;
SELECT 1 + COUNT(*) 
    INTO counter 
    FROM additional 
    WHERE trip_num = NEW.trip_num
      AND add_type = NEW.add_type;
SET NEW.add_id = CONCAT( CASE NEW.add_type WHEN 'type 1' THEN 'UM'
                                           WHEN 'type 2' THEN 'UB'
                                           WHEN 'type 3' THEN 'UL'
                                                         ELSE '??' END,
                         counter );
END;
INSERT INTO `additional` (`trip_num`, `add_type`, `value`) VALUES 
('TRP0001', 'type 1', '50'), 
('TRP0001', 'type 1', '27'), 
('TRP0001', 'type 2', '250'), 
('TRP0002', 'type 1', '32'), 
('TRP0002', 'type 2', '300'), 
('TRP0002', 'type 2', '120'), 
('TRP0002', 'type 3', '120');
SELECT * FROM additional;


trip_num
add_id
add_type
value


TRP0001
UM1
type 1
50

TRP0001
UM2
type 1
27

TRP0001
UB1
type 2
250

TRP0002
UM1
type 1
32

TRP0002
UB1
type 2
300

TRP0002
UB2
type 2
120

TRP0002
UL1
type 3
120

trip_num	add_id	add_type	value
TRP0001	UM1	type 1	50
TRP0001	UM2	type 1	27
TRP0001	UB1	type 2	250
TRP0002	UM1	type 1	32
TRP0002	UB1	type 2	300
TRP0002	UB2	type 2	120
TRP0002	UL1	type 3	120