MySQL 8.0

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select version();


version()


8.0.36


CREATE TABLE mytable(
   ID INTEGER  NOT NULL
  ,a  VARCHAR(4) NOT NULL
  ,b  VARCHAR(4) NOT NULL
  ,c  VARCHAR(3) NOT NULL
);
INSERT INTO mytable(ID,a,b,c) VALUES (1,'a1','b1','c1');
INSERT INTO mytable(ID,a,b,c) VALUES (2,'a2','b2','c2');
select * from mytable;


ID
a
b
c


1
a1
b1
c1

2
a2
b2
c2


select t.id, x.*
from mytable t
cross join lateral (
	select a, 'a' 
	union all select b, 'b'
	union all select c, 'c'
) as x(col1, col2);


id
col1
col2


1
a1
a

1
b1
b

1
c1
c

2
a2
a

2
b2
b

2
c2
c


select t.id, x.*
from mytable t
cross join lateral (values 
    row(a, 'a'), 
    row(b, 'b'),
    row(c, 'c')
) as x(col1, col2);


id
col1
col2


1
a1
a

1
b1
b

1
c1
c