mysql add some string to several rows

If a column has several rows, i.g.:

'red'  
'NULL'  
'red, yellow'  
'green'  
'NULL'  
'yellow, green'

And you want to add a new string (‘blue’) to every row, so that the result be:

'red, blue'  
'blue'  
'red, yellow, blue'  
'green, blue'  
'blue'  
'yellow, green, blue'

you can use this code

UPDATE my_table 
SET mycolumn = CONCAT('same new string ',mycolumn); 

But the above code works only for rows with some content, not for the empty (NULL) ones. Therefore you can complete your task with the following:

UPDATE my_table 
SET mycolumn = 'same new string' 
WHERE mycolumn IS NULL;