PHP MySQL imbricate SELECT statements return null

Houari Zegai :

I tried to execute the query, but the result was null, I executed the same query in PhpMyAdmin and returns the result.

The problem is when the query contains SELECT inside another SELECT statement (imbricate), so when the query contains only one SELECT statement, it works fine and returns the result. Here is the code:

db_config.php

<?php    
$servername = "localhost";
$username = "root";
$password = "";
$dbname = "university_db";

// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);

get_planning.php

<?php
require 'db_config.php';

// Check connection
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}

$sql = "SELECT cours.jour, 
TIME_FORMAT(cours.heure_debut, '%H:%i') AS debut, 
TIME_FORMAT(cours.heure_fin, '%H:%i') AS fin, 
enseignant.nom_ens as prof, modules.nom_mod as module, salles.nom_salle as salle
FROM cours, promotion, enseignant, modules, salles 
WHERE cours.id_promo = (SELECT id_promo FROM promotion WHERE promotion.niveau = '2' 
AND promotion.id_speci = (SELECT id_speci FROM spécialité WHERE nom_speci = 'MGL')) 
AND cours.id_promo = promotion.id_promo AND cours.id_ens = enseignant.id_ens AND cours.id_salle = salles.id_salle AND cours.id_mod = modules.id_mod 
ORDER BY cours.id_cours;";

$result = $conn->query($sql);

if($result == null)
    echo "Result is empty!";

Output:

Result is empty!

Information:

PHP: Version 7.3.5
Database: MySQL

O. Jones :

Three suggestions for troubleshooting this problem:

Change your cours.id_promo = (SELECT ... to cours.id_promo IN (SELECT... and do the same to the line after it. Why? if you use = and the inner select statement returns more than one result, boom. Query fails. SQL is, at its heart, a set-processing language, and this IN that checks that this is a member of the set that.
echo your $sql value to make sure the statement in it is correct. Try running the exact statement via phpmyadmin to make sure it gives you what you expect.
You have this

  if($result == null) 
      echo "Result is empty!";

Change it to this

  if(!$result) 
      echo 'sql failure:', $conn->error, ': ', $sql;

The query() method only returns a falsey result if the query failed. If it succeeds but finds no matching rows, query() returns an empty result set. So anytime your $result is falsey, you made a programming error in your query. The echo I mentioned will diagnose it for you.

Pro tip Always check SQL operations for errors.

Pro tip 2. Bring your SQL skills into the 21st century. Use explicit join operations rather than old-timey comma-join operations. Change ...

SELECT cours.jour, whatever, whatever 
  FROM cours 
  JOIN promotion   ON cours.id_promo = promotion.id_promo
  JOIN enseignant  ON cours.id_ens = enseignant.id_en
  JOIN modules     ON cours.id_mod = modules.id_mod
  JOIN salles      ON cours.id_salle = salles.id_salle
 WHERE cours.id_promo IN (SELECT id_promo FROM promotion WHERE promotion.niveau = '2')
   AND promotion.id_speci IN (SELECT id_speci FROM spécialité WHERE nom_speci = 'MGL') 
 ORDER BY cours.id_cours

The relationships between your tables are much easier to read and understand this way. And, you can change JOIN to LEFT JOIN if your application requires it.

PHP MySQL imbricate SELECT statements return null

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