How to find all IDs of children recursively?

Question

I would like to get all IDs from children in a tree with MySQL only.

I have a table like this:

ID parent_id name
1  0         cat1
2  1         subca         


        

Answer 1

Here is a simple single-query MySql-solution:

SELECT GROUP_CONCAT(Level SEPARATOR ',') FROM (
   SELECT @Ids := (
       SELECT GROUP_CONCAT(`ID` SEPARATOR ',')
       FROM `table_name`
       WHERE FIND_IN_SET(`parent_id`, @Ids)
   ) Level
   FROM `table_name`
   JOIN (SELECT @Ids := ) temp1
) temp2

Just substitute with the parent element's ID.

This will return a string with the IDs of all descendants of the element with ID = , separated by ,. If you would rather have multiple rows returned, with one descendant on each row, you can use something like this:

SELECT *
FROM `table_name`
WHERE FIND_IN_SET(`ID`, (
   SELECT GROUP_CONCAT(Level SEPARATOR ',') FROM (
      SELECT @Ids := (
          SELECT GROUP_CONCAT(`ID` SEPARATOR ',')
          FROM `table_name`
          WHERE FIND_IN_SET(`parent_id`, @Ids)
      ) Level
      FROM `table_name`
      JOIN (SELECT @Ids := ) temp1
   ) temp2
))

Including the root/parent element

The OP asked for the children of an element, which is answered above. In some cases it might be useful to include the root/parent element in the result. Here are my suggested solutions:

Comma-separated string of ids:

SELECT GROUP_CONCAT(Level SEPARATOR ',') FROM (
   SELECT  Level
   UNION
   SELECT @Ids := (
       SELECT GROUP_CONCAT(`ID` SEPARATOR ',')
       FROM `table_name`
       WHERE FIND_IN_SET(`parent_id`, @Ids)
   ) Level
   FROM `table_name`
   JOIN (SELECT @Ids := ) temp1
) temp2

Multiple rows:

SELECT *
FROM `table_name`
WHERE `ID` =  OR FIND_IN_SET(`ID`, (
   SELECT GROUP_CONCAT(Level SEPARATOR ',') FROM (
      SELECT @Ids := (
          SELECT GROUP_CONCAT(`ID` SEPARATOR ',')
          FROM `table_name`
          WHERE FIND_IN_SET(`parent_id`, @Ids)
      ) Level
      FROM `table_name`
      JOIN (SELECT @Ids := ) temp1
   ) temp2
))