Database INSERT INTO error - php

Question

I was trying to find out what was wrong with my code.
This is the error I'm recieving
"Cannot add or update a child row: a foreign key constraint fails "
This is my code
<?php
$sql = "INSERT INTO stasjon (navn) VALUES ('skogen', 'voksenlia') ";
$resultat = $kobling->query ($sql);
$sql ="SELECT * FROM stasjon WHERE navn = ('skogen')";
$resultat = $kobling->query ($sql);
while ($rad = $resultat->fetch_assoc()) {
$stasjon_id = $rad['stasjon_id'];
}
$sql = "INSERT INTO linjestasjon (linje_nr, stasjon_id) VALUES ('1','$stasjon_id')";
$resultat = $kobling->query ($sql);
if($kobling->query($sql)) {
echo "Spoerringen $sql ble gjennomfoert.";
} else {
echo "Noe gikk galt med spoerringen $sql ($kobling->error).";
?>
Some of it is in Norwegian because That's the language of the database I'm making. I was trying to add values to two different tables (that had stasjon_id as a foreign key) Thanks in advance

Answer 1

Foreign Key constraints checks in the value that you are inserting or updating to a particular filed exists in some other filed of another table. Suppose I have 2 tables as follows
CREATE TABLE TableA
(
SeqNo INT PRIMARY KEY,
Name VARCHAR(500
)
CREATE TABLE TableB
(
SeqNo INT NULL FOREIGN KEY REFERENCES TableA(SeqNo),
Name VARCHAR(50)
)
So when you insert a new record to the Tableb.SeqNo filed, The value should either be NULL or some value that exists in the TableA.SeqNo.
So Before inserting the values make sure that you are inserting the value that satisfy your foreign key constraint