MySQL 5.7

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CREATE TABLE tran_term_taxonomy(id int, taxonomy varchar(20) default 'post_tag', description varchar(20) default '');
insert into tran_term_taxonomy (id) values (1),(2),(3),(4)

Records: 4  Duplicates: 0  Warnings: 0


select * from tran_term_taxonomy


id
taxonomy
description


1
post_tag


2
post_tag


3
post_tag


4
post_tag



create table good_keywords (keyword varchar(20))
insert into good_keywords values
('keyword 1'),
('keyword 2'),
('keyword 3')

Records: 3  Duplicates: 0  Warnings: 0


select * from good_keywords


keyword


keyword 1

keyword 2

keyword 3


UPDATE `tran_term_taxonomy` 
SET `description` = (
  SELECT keyword 
  FROM `good_keywords` 
  WHERE NOT EXISTS (SELECT * FROM (SELECT * FROM `tran_term_taxonomy`) t WHERE description = keyword)
  ORDER BY RAND()
  LIMIT 1
) 
WHERE `tran_term_taxonomy`.`taxonomy` = 'post_tag'
  AND `tran_term_taxonomy`.`description` = ''
LIMIT 1 

Rows matched: 1  Changed: 1  Warnings: 0


select * from tran_term_taxonomy


id
taxonomy
description


1
post_tag
keyword 3

2
post_tag


3
post_tag


4
post_tag



UPDATE `tran_term_taxonomy` 
SET `description` = (
  SELECT keyword 
  FROM `good_keywords` 
  WHERE NOT EXISTS (SELECT * FROM (SELECT * FROM `tran_term_taxonomy`) t WHERE description = keyword)
  ORDER BY RAND()
  LIMIT 1
) 
WHERE `tran_term_taxonomy`.`taxonomy` = 'post_tag'
  AND `tran_term_taxonomy`.`description` = ''
LIMIT 1 

Rows matched: 1  Changed: 1  Warnings: 0


select * from tran_term_taxonomy


id
taxonomy
description


1
post_tag
keyword 3

2
post_tag
keyword 2

3
post_tag


4
post_tag



UPDATE `tran_term_taxonomy` 
SET `description` = (
  SELECT keyword 
  FROM `good_keywords` 
  WHERE NOT EXISTS (SELECT * FROM (SELECT * FROM `tran_term_taxonomy`) t WHERE description = keyword)
  ORDER BY RAND()
  LIMIT 1
) 
WHERE `tran_term_taxonomy`.`taxonomy` = 'post_tag'
  AND `tran_term_taxonomy`.`description` = ''
LIMIT 1 

Rows matched: 1  Changed: 1  Warnings: 0


select * from tran_term_taxonomy


id
taxonomy
description


1
post_tag
keyword 3

2
post_tag
keyword 2

3
post_tag
keyword 1

4
post_tag



UPDATE `tran_term_taxonomy` 
SET `description` = (
  SELECT keyword 
  FROM `good_keywords` 
  WHERE NOT EXISTS (SELECT * FROM (SELECT * FROM `tran_term_taxonomy`) t WHERE description = keyword)
  ORDER BY RAND()
  LIMIT 1
) 
WHERE `tran_term_taxonomy`.`taxonomy` = 'post_tag'
  AND `tran_term_taxonomy`.`description` = ''
LIMIT 1 

Rows matched: 1  Changed: 1  Warnings: 0


select * from tran_term_taxonomy


id
taxonomy
description


1
post_tag
keyword 3

2
post_tag
keyword 2

3
post_tag
keyword 1

4
post_tag
null
id	taxonomy	description
1	post_tag	keyword 3
2	post_tag	keyword 2
3	post_tag	keyword 1
4	post_tag