MySQL 8.0

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create table test ( 
col1  varchar(255)  
); 

insert into test values 
('page1:message1,page2:message2,page3:message3,page4:message4'); 



create table numbers (
  nr int
);

insert into numbers values (1),(2),(3),(4),(5);
SELECT   SUBSTRING_INDEX(SUBSTRING_INDEX(col1, ',', nr),',', -1) AS new_col1        
FROM test t
JOIN numbers n ON CHAR_LENGTH(col1) - CHAR_LENGTH(REPLACE(col1, ',', '')) >= nr - 1 


new_col1


page1:message1

page2:message2

page3:message3

page4:message4


select SUBSTRING_INDEX(new_col1,':',1) as page,
       SUBSTRING_INDEX(new_col1,':',-1) as message
from (   SELECT  SUBSTRING_INDEX(SUBSTRING_INDEX(col1, ',', nr),',', -1) AS new_col1        
         FROM test 
         JOIN numbers n ON CHAR_LENGTH(col1) - CHAR_LENGTH(REPLACE(col1, ',', '')) >= nr - 1 
  ) as t2;


page
message


page1
message1

page2
message2

page3
message3

page4
message4