MySQL nested queries how to select more than one row

Question

SELECT id, FIO, parent_id
FROM users
WHERE parent_id =
(
    SELECT id
    FROM users
    WHERE parent_id =
    (
        SELECT id
        FROM users
        WHERE id = 16
    )
)

So here I am making an hierarchy tree, first selecting the root parent, then the children's and so on to 24th level of depth.

The question is: How to select more than one column from the inner queries?

Because I need to get the other rows fields to display info like: name, surname, age

It looks like I can only get those columns of rows in the outer query (the topmost).

PS: I don't want to use joins because they generate duplicate fields.

Is there a solution?

Answer 1

You could iterate on the SQL side using MySQL query variables. This will return all childs with all data of one parent node without repeating yourself (and thus without imposing a limit on the depth of your tree)

something like this: (500 being the parents id to start with)

SELECT
  id,
  parent_id, 
  name, 
  '0' as depth, 
  @tree_ids := id AS foo
FROM 
   tree,
  (SELECT @tree_ids := '', @depth := -1) vars
WHERE id = 500
UNION
SELECT 
  id,
  parent_id,
  name,
  @depth := IF(parent_id = 500, 1, @depth + 1) AS depth,
  @tree_ids := CONCAT(id, ',', @tree_ids) AS foo
FROM 
  tree 
WHERE FIND_IN_SET(parent_id, @tree_ids) OR parent_id = 500

See a working example at SQLfiddle

Note that this gives a really bad performance on larger datasets because MySQL will not use your indexes and instead will do a full table scan. (i don't understand why its not using indexes, thats just how it is. if someone has advice on or explain the indexing issue, please comment!)

Answer 2

= comparisons work on only a single value. You can use in to compare against multiple values:

SELECT ...
FROM yourtable
WHERE somefield IN (select somevalue from othertable);

Answer 3

Storing heirarchical data in mysql and getting it out is not as simple as that.

Look into this: https://stackoverflow.com/a/4346009/9094

You will need more data to work with.

Answer 4

It seems your DB relationship is setup to be MPTT, here is a good blog post exaplaining how to query mysql MPTT data http://mikehillyer.com/articles/managing-hierarchical-data-in-mysql/

Have a look at Full Tree example Retrieving a Full Tree in summary it can be done with joins.

Answer 5

I am not 100% sure if I understood exactly what you mean, but if you want to select all columns separately from the table in a subselect...

col1, col2, col3, col4

you would need for each column a single subselect that always matches against the same WHERE. Example:

`SELECT * FROM main_table,
(SELECT col1 FROM inner_table WHERE inner_table.some_column=main_table.some_column),
(SELECT col2 FROM inner_table WHERE inner_table.some_column=main_table.some_column), ...`