Merging Section and Sub-Section Data: A SQL Solution Using GROUP

Understanding the Problem and Query

The problem at hand involves merging data from two tables, sections and sub_sections, based on a common column (section_id). The goal is to fetch all section titles along with their corresponding sub-section titles in a structured format.

Table Structure

Table: sections
+------------+---------------+-----------------+
| section_id | section_titel | section_text    |
+------------+---------------+-----------------+
| 1          | Section One   | Test text blaaa |
| 2          | Section Two   | Test            |
| 3          | Section Three | Test            |
+------------+---------------+-----------------+

Table: sub_sections
+----------------+-------------------+------------------+-----+
| sub_section_id | sub_section_titel | sub_section_text | sId |
+----------------+-------------------+------------------+-----+
| 1              | SubOne            | x1               | 1   |
| 2              | SubTwo            | x2               | 1   |
| 3              | SubThree          | x3               | 3   |
+----------------+-------------------+------------------+-----+

SQL Query Issue

The provided SQL query attempts to solve the problem but results in multiple section titles being fetched:

SELECT section_titel as t1, sub_section_titel as t2 
FROM sections LEFT JOIN sub_sections ON section_id = sId;

This query yields an output where each row has a different section_titel value:

Array(
    [0] =>
        Array(
            [t1] => Section One
            [t2] => SubOne
        )

    [1] =>
        Array(
            [t1] => Section One
            [t2] => SubTwo 
        )
)

This is because the LEFT JOIN does not group the results by section_id, resulting in multiple rows for each section.

Solution Overview

To achieve the desired output, we will modify the SQL query to use a combination of GROUP_CONCAT and HAVING. We will also add code in PHP to handle the fetched data.

Modified SQL Query

SELECT section_titel as t1, GROUP_CONCAT(sub_section_titel) as t2 
FROM sections LEFT JOIN sub_sections ON section_id = sId
GROUP BY t1
HAVING t2 IS NOT NULL;

This query groups the results by section_titel and only includes rows where the concatenated sub_section_titel values are not empty.

PHP Code

<?php
// Assume you have a database connection $conn and a SQL query string $sql;

$result = mysqli_query($conn, $sql) or die(mysqli_error($conn));
$out = array();
while ($row = mysqli_fetch_array($result)) {
    $out[] = array('t1' => $row['t1'], 't2' => explode(',', $row['t2']));
}
print_r($out);

Explanation

The modified SQL query uses GROUP_CONCAT to concatenate the sub-section titles for each section. The HAVING t2 IS NOT NULL condition filters out rows where the concatenated string is empty.

In PHP, we use mysqli_query to execute the modified SQL query and fetch the results as an array of arrays. We then iterate through this array and split the comma-separated t2 values into separate sub-section titles using explode. The resulting data structure is an associative array where each key is a section title and its corresponding value is another array containing the sub-section titles.

Example Output

Array(
    [0] =>
        Array(
            [t1] => Section One
            [t2] =>
                Array(
                    [0] => SubOne
                    [1] => SubTwo
                )
        )

    [1] =>
        Array(
            [t1] => Section Three
            [t2] =>
                Array(
                    [0] => SubThree
                )
        )
)

This output represents the desired structure, where each section title is paired with its corresponding sub-section titles.

Last modified on 2025-01-12