'How to SELECT value of the column WHEN EXIST

I try to find a way to retrieve the text of a specific column when existing otherwise return 'NONE', here my query :

SELECT 
  t_operation.*, 
  CASE WHEN EXISTS (SELECT OperationID FROM t_operationtag
    WHERE t_operationtag.ElementID = t_operation.OperationID
    and t_operationtag.Property = "Category"
    and t_operation.Name LIKE CONCAT("%", "class", "%"))
  THEN 'FOUND'
  ELSE 'NONE'
  END AS TagValue  
FROM t_operation

Instead of 'FOUND' I would like to retrieve the value, like following (but this didn't work):

SELECT 
  t_operation.*, 
  CASE WHEN EXISTS (SELECT OperationID FROM t_operationtag
    WHERE t_operationtag.ElementID = t_operation.OperationID
    and t_operationtag.Property = "Category"
    and t_operation.Name LIKE CONCAT("%", "class", "%"))
  THEN t_operationtag.VALUE
  ELSE 'NONE'
  END AS NewFiled  
FROM t_operation

I opened an dbfiddle to try the query : https://dbfiddle.uk/?rdbms=mysql_8.0&fiddle=ba10216a0979f0cc8adc16b4a867a05b

Thanks Yan

mysql


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