MySQL 5.7

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CREATE TABLE MYTABLE (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `col_a` text,
  `col_b` text,
  `col_c` text,
  PRIMARY KEY (`id`)
) ;
INSERT INTO MYTABLE VALUES (1,'apple orange plum','red green blue','bill dave sue'),(2,'orange plum banana','yellow red','frank james'),(3,'kiwi fruit apple','green pink','bill sarah-jane frank');
SELECT MYWORD FROM (
SELECT
  DISTINCT SUBSTRING_INDEX(SUBSTRING_INDEX(CONCAT_WS(' ', COL_A, COL_B, COL_C), ' ', N.DIGIT+1), ' ', -1)  MYWORD 
FROM
  MYTABLE 
  INNER JOIN
  (SELECT 0 DIGIT UNION ALL SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3  UNION ALL SELECT 4 UNION ALL SELECT 5 UNION ALL SELECT 6) N
  ON (LENGTH(REPLACE(CONCAT_WS(' ', COL_A, COL_B, COL_C), ' ' , '')) <= LENGTH(CONCAT_WS(' ', COL_A, COL_B, COL_C))-N.DIGIT) ) MYRESULT
  WHERE LENGTH(MYRESULT.MYWORD)>=5


MYWORD


apple

orange

fruit

banana

yellow

green

frank

james

sarah-jane