Question

My sql query to get “posts” along with all tags associated with it:

SELECT 
posts.*,
GROUP_CONCAT(tags.tag_name) tags
FROM
posts
LEFT JOIN relation ON relation.post_id  = posts.id
LEFT JOIN tags ON tags.tag_id = relation.tag_id
GROUP BY posts.id

(This is only a example code).

If no tags exists, “tags” field will return NULL. How to change this NULL value to empty string ?

We can use SELECT posts.*, IF(ISNULL(GROUP_CONCAT(tags.tag_name)),'',GROUP_CONCAT(tags.tag_name)) AS tags ....

But it GROUP_CONCATs two times. Correct ? What is the best method ?

Answer 1

You can shorten the if by using COASLESCE

SELECT 
  posts.*,
  COALESCE(GROUP_CONCAT(tags.tag_name),'') AS tags
FROM
  posts
  LEFT JOIN relation ON relation.post_id  = posts.id
  LEFT JOIN tags ON tags.tag_id = relation.tag_id
GROUP BY posts.id

MySQL also supports IFNULL which does the same, but IFNULL only allows 2 parameters, not an unlimited number as COALESCE does.