Retrieve second table as subarray in codeigniter query - sql

Question

I have two tables A & B, and B has a many:1 relationship with A.
When querying rows from A I'd also like to have corresponding B records returned as an array and added to the result array from A, so I end up with something like this:
A-ROW
field
field
B-ITEMS
item1
item2
item3
Is there a clean way to do this with one query (perhaps a join?), or should I just perform a second query of B on the id from A and add that to the result array?

Answer 1

It would be more efficient to join table B on table A. It will not give you the data in the shape you are looking for. But you can iterate over this result and build the data into the desired shape.
Here is some code to illustrate the idea :
// Join table B on table A through a foreign key
$sql = 'select a.id, a.x, b.y
from a
left join b on b.a_id=a.id
order by a.id';
// Execute query
$result = $this->db->query($sql)->result_array();
// Initialise desired result
$shaped_result = array();
// Loop through the SQL result creating the data in your desired shape
foreach ($result as $row)
{
// The primary key of A
$id = $row['id'];
// Add a new result row for A if we have not come across this key before
if (!array_key_exists($id, $shaped_result))
{
$shaped_result[$id] = array('id' => $id, 'x' => $row['x'], 'b_items' => array());
}
if ($row['y'] != null)
{
// Push B item onto sub array
$shaped_result[$id]['b_items'][] = $row['y'];
}
}

Answer 2

"... just perform a second query of B on the id from A and add that to the result array ..." -- that is the correct solution. SQL won't comprehend nested array structure.

Answer 3

To build on what Smandoli said--
Running the secondary query separately is more efficient because even if row data on the primary table (A) has changed, unchanged data on the secondary table (B) will result in a (MySQL) query cache hit assuming the IDs never change.
This is not necessarily true of the join query approach.
There will also be less data coming over the wire since the join approach will fetch duplicate data for the primary table (A) if the secondary table (B) has multiple rows associated with a single row in the primary table.
Hopefully anyone looking to do this (relatively) common type of data retrieval may find this useful.