MySQL 8.0

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-- Table1

CREATE TABLE Table1 (id INT,selection1 varchar(50),selection2 varchar(50),results varchar(1000));

INSERT INTO Table1 (id,selection1,selection2,results) 
VALUES
(1,null,null,null),
(2,null,null,null),
(3,null,null,null);

select * from Table1


id
selection1
selection2
results


1
null
null
null

2
null
null
null

3
null
null
null


-- Table2

CREATE TABLE Table2 (stocks1 VARCHAR(50),stocks2 VARCHAR(50));

INSERT INTO Table2 (stocks1,stocks2) 
VALUES
-- ('BRK','BRK'),
-- ('BRK','BRK');

('BRK','GOOG'),
('INTC','NKE'),
('TSLA','APPL'),
('APPL','NKE'),
('TSLA','FB'),
('NKE','BRK');


select * from Table2


stocks1
stocks2


BRK
GOOG

INTC
NKE

TSLA
APPL

APPL
NKE

TSLA
FB

NKE
BRK


CREATE TRIGGER trigger1
BEFORE UPDATE
ON Table1
FOR EACH ROW
BEGIN
set NEW.results =
(SELECT CONCAT('<br>',GROUP_CONCAT( val
                                SEPARATOR '<br>'))
FROM (SELECT stocks1 as val FROM                    
                        Table2
                    WHERE
                        NEW.selection2 = 'x'
    UNION
       SELECT stocks2 FROM                    
                        Table2
                    WHERE
                        NEW.selection2 = 'x') t1
);
end;
update Table1
set selection1 = 'x', selection2 = 'x'
where id = 1
select * from Table1


id
selection1
selection2
results


1
x
x
<br>BRK<br>INTC<br>TSLA<br>APPL<br>NKE<br>GOOG<br>FB

2
null
null
null

3
null
null
null

id	selection1	selection2	results
1	null	null	null
2	null	null	null
3	null	null	null

id	selection1	selection2	results
1	x	x	<br>BRK<br>INTC<br>TSLA<br>APPL<br>NKE<br>GOOG<br>FB
2	null	null	null
3	null	null	null