SQL - select both "child" and "parent" rows in a single query - mysql

Question

I have a database filled with a lot of "empty" records and I want to get rid of those orphaned records.
The database is structured like this;
A child row that contains the real entry (meta_value)
A parent row that is "linked" to its child (by it's meta_key)
| meta_id post_id meta_key meta_value |
| |
| 011 301 ACF__P_01_01 Foo |
| 012 309 _ACF__P_01_01 field_5874d5 |
| 013 321 ACF__P_01_02 |
| 014 316 _ACF__P_01_02 field_54290a |
| 015 119 ACF__P_01_03 Bar |
| 016 101 _ACF__P_01_03 field_a04a88 |
| 017 119 ACF__P_01_03 |
| 018 101 _ACF__P_01_03 field_a04a88 |
| 019 149 ACF__P_01_03 |
| 020 111 _ACF__P_01_03 field_a04a88 |
| 021 169 ACF__P_01_03 Foo Bar |
| 022 171 _ACF__P_01_03 field_a04a88 |
It's easy to select all the (empty) "child" rows, with a query like this;
SELECT
*
FROM
wp_postmeta
WHERE
wp_postmeta.meta_key LIKE '%ACF__%'
AND
wp_postmeta.meta_value LIKE ''
But this query only fetches the (empty) "child" rows, not their "parent" row.
There are two logical conditions that bind the parent to the child;
The first row is always the child, immediately followed by its parent.
The parent row has the same "meta_key" value as it's child, but pre-fixed by an underscore.
Is there any way to create a SQL query that selects the empty child records (like I did above) and it's parent as well?
I prefer a single query to fetch them both - but when that's not possible, I could run two queries as well (first one to fetch the parent, second one to get the empty children, I guess).
I use Navicat for my database management, so it must be plain SQL - no PHP.
The query should spit out the results like this;
| meta_id post_id meta_key meta_value |
| |
| 013 321 ACF__P_01_02 |
| 014 316 _ACF__P_01_02 field_54290a |
| 017 119 ACF__P_01_03 |
| 018 101 _ACF__P_01_03 field_a04a88 |
| 019 149 ACF__P_01_03 |
| 020 111 _ACF__P_01_03 field_a04a88 |
So both the empty-childs (no value in the "meta_value") and it's parent (both have the same "meta_key", where the parent has an underscore.

Answer 1

With a self-join
Select *
from wp_postmeta c
join wp_postmeta p
on p.meta_key = '_' + c.meta_key
The above will only fetch "pairs" of parent-child records. If it's possible to have parents without children (I assume it's not possible to have children without parents), and you want all the parents, including the childless ones, use an outer join
Select *
from wp_postmeta p
left join wp_postmeta c
on '_' + c.meta_key = p.meta_key
In relational databases, there is no such concept as "after" or "before" unless you define it yourself and add data into the tables that implements that concept (like a datetimestamp, or sequentially increasing numeric key, or whatever. Without such artifacts, records in a relational database table do not have any implicit order.

Answer 2

Using PHP I was able to solve it. I iterate over the first result from a query (that fetches all the empty children) and fetch every row AFTER that child, inside the loop.
But that's PHP - I wanted pure SQL :)
<?php
$connection = new mysqli(DB_HOST, DB_USER, DB_PASSWORD, DB_NAME);
if ($connection -> connect_errno > 0) {
die ("Unable to connect to database [" . $connection->connect_error . "]");
}
$query_1 = "SELECT * FROM `wp_postmeta` WHERE `meta_key` LIKE '%ACF%__' AND `meta_value` LIKE ''";
if (!$result_1 = $connection -> query($query_1)) {
die ("There was an error running query[" . $connection -> error . "]");
}
if ($result_1) {
$i = 0;
echo "<h2>ACF - empty fields</h2>";
echo "<table border=1 cellpadding=4 cellspacing=0 width=800>";
echo "<tr style='background:#eee;font-weight:bold;'><td>row</td><td>meta_id</td><td>post_id</td><td>meta_key</td><td>meta_value</td></tr>";
while ($row_1 = $result_1 -> fetch_assoc()) {
$i++;
echo "<tr>";
echo "<td width=100>" . sprintf('%03d', $i) . "</td>";
echo "<td width=100 style='background:#aaa;color:#fff;font-weight:bold;'>" . $row_1["meta_id"] . "</td>";
echo "<td width=100>" . $row_1["post_id"] . "</td>";
echo "<td width=100>" . $row_1["meta_key"] . "</td>";
echo "<td width=500>" . $row_1["meta_value"] . "</td>";
echo "</tr>";
$query_2 = "SELECT * FROM `wp_postmeta` WHERE `meta_id` = " . ($row_1["meta_id"] + 1);
if (!$result_2 = $connection -> query($query_2)) {
die ("There was an error running query[" . $connection -> error . "]");
}
while ($row_2 = $result_2 -> fetch_assoc()) {
$i++;
echo "<tr>";
echo "<td width=100>" . sprintf('%03d', $i) . "</td>";
echo "<td width=100 style='background:#aaa;color:#fff;font-weight:bold;'>" . $row_2["meta_id"] . "</td>";
echo "<td width=100>" . $row_2["post_id"] . "</td>";
echo "<td width=100>" . $row_2["meta_key"] . "</td>";
echo "<td width=500>" . $row_2["meta_value"] . "</td>";
echo "</tr>";
}
mysqli_free_result($result_2);
}
mysqli_free_result($result_1);
echo "</table>";
} else {
echo "No entries found.";
}
?>
- edit -
This PHP code is better, I guess... It uses only 2 queries and iterates over the first one...
It works perfectly, but still isn't pure SQL - I want to get rid of the PHP...
<?php
/* --------------------------------------------------- */
$connection = new mysqli(DB_HOST, DB_USER, DB_PASSWORD, DB_NAME);
if ($connection -> connect_errno > 0) {
die ("Unable to connect to database [" . $connection->connect_error . "]");
}
/* --------------------------------------------------- */
$query = "SELECT `meta_id` FROM `wp_postmeta` WHERE `meta_key` LIKE '%ACF%__' AND `meta_value` LIKE ''";
if (!$result = $connection -> query($query)) {
die ("There was an error running query[" . $connection -> error . "]");
}
if ($result) {
$array = array();
while ($row = $result -> fetch_assoc()) {
$array[] = $row["meta_id"];
$array[] = $row["meta_id"] + 1;
}
mysqli_free_result($result);
/* --------------------------------------------------- */
$query = "SELECT * FROM `wp_postmeta` WHERE `meta_id` IN (" . implode(",", array_map("intval", $array)) . ")";
if (!$result = $connection -> query($query)) {
die ("There was an error running query[" . $connection -> error . "]");
}
if ($result) {
$i = 0;
echo "<h2>ACF - empty fields</h2>";
echo "<table>";
echo "<tr><td>record</td><td>meta_id</td><td>post_id</td><td>meta_key</td><td>meta_value</td></tr>";
while ($row = $result -> fetch_assoc()) {
$i++;
echo "<tr>";
echo "<td>" . sprintf('%03d', $i) . "</td>";
echo "<td>" . $row["meta_id"] . "</td>";
echo "<td>" . $row["post_id"] . "</td>";
echo "<td>" . $row["meta_key"] . "</td>";
echo "<td>" . $row["meta_value"] . "</td>";
echo "</tr>";
}
mysqli_free_result($result);
echo "</table>";
if (isset($_GET["delete"])) {
echo "<span>all records deleted</span>";
} else {
echo "<a href='?delete'>delete empty records</a>";
}
}
/* --------------------------------------------------- */
} else {
echo "No entries found.";
}
?>