Delete in one query instead of using loops and ifs - php

Question

I want to delete some posts in a database, but for some rows, only if they are the only children with the same id in it's table. Is it possible to do all of the below in one single query, instead of selecting the data and making loops and ifs?
Basically I want to delete from sessions only if there is 1 hit in session_projects and delete from rec_projects only if there is 1 hit in sessions and all of the hits in session_projects
Is this possible? (please don't mind the unprepared statements in this case)
$pid = 1; // varies
$sql = "SELECT ss_id, rp_id
FROM session_projects
INNER JOIN sessions
ON ss_id = sp_ss_id
INNER JOIN rec_projects
ON rp_id = ss_rp_id
WHERE sp_p_id = $pid";
$session_projects = mysqli_query($db_link, $sql);
while( $row = mysqli_fetch_array($session_projects) )
{
$sql = "SELECT COUNT(*) as session_projects_count FROM session_projects WHERE sp_ss_id = " . $row['ss_id'];
$result = mysqli_query($db_link, $sql);
$session_projects_count = mysqli_fetch_assoc($result);
if( $session_projects_count['session_projects_count'] == 1 )
{
$sql = "SELECT COUNT(*) as sessions_count FROM sessions WHERE ss_rp_id = " . $row['rp_id'];
$result = mysqli_query($db_link, $sql);
$sessions_count = mysqli_fetch_assoc($result);
$sql = "DELETE FROM sessions
WHERE ss_id = " . $row['ss_id'];
$delete_sessions = mysqli_query($db_link, $sql);
if( $sessions_count['sessions_count'] == 1 )
{
$sql = "DELETE FROM rec_projects
WHERE rp_id = " . $row['rp_id'];
$delete_rec_projects = mysqli_query($db_link, $sql);
}
}
}
$sql = "DELETE FROM session_projects
WHERE sp_p_id = $pid";
$delete_session_projects = mysqli_query($db_link, $sql);

Answer 1

You could always aggregate into arrays and delete with IN (), that way you'd have one delete query per table:
// ...
$delSsId = array();
$delRpId = array();
while( $row = mysqli_fetch_array($session_projects) )
{
// ...
if( $session_projects_count['session_projects_count'] == 1 )
{
// ...
$delSsId[] = $row['ss_id'];
if( $sessions_count['sessions_count'] == 1 )
$delRpId[] = $row['rp_id'];
}
}
$sql = "DELETE FROM sessions
WHERE ss_id IN (" . implode(",", $delSsId) . ")";
$delete_sessions = mysqli_query($db_link, $sql);
$sql = "DELETE FROM rec_projects
WHERE rp_id IN (" . implode(",", $delRpId) . ")";
$delete_rec_projects = mysqli_query($db_link, $sql);
The same principle can be applied to your select queries inside the loop; select all rows that could apply into arrays and then use the data from there.
However this could be a bad idea if you have enormously large tables as you might end up loading thousands of rows into arrays, wasting time and memory.
From my personal experience there isn't much performance benefit in optimizing the amount of reading (SELECT) queries anyways, it's usually writing (INSERT, UPDATE, DELETE, ...) queries that have the biggest impact.