MySQL - How to insert into multiple tables with foreign keys

Question

I'am new to MySQL, so please be nice :)

I would like to insert data from a php form into 3 different tables, which all have foreign keys. How can I write an insert command which updates all 3 tables at once, because if I try to update a table manually then I get an error because of the missing references. Do I have to deal with "NULL" entries and update every table after another or is it possible to solve this with one single command? Like MySQLi_Multi_Query?

Thank you very much!

Answer 1

You can do it in 3 Methods:

First & Recommended. USING SELECT IN THE INSERT VALUE:

  INSERT INTO user (name) VALUES ('John Smith'); INSERT INTO user_details (id, weight, height) VALUES ((SELECT id FROM user WHERE name='John Smith'), 83, 185); 

Second. USING LAST_INSERT_ID IN THE INSERT VALUE:

INSERT INTO a (id)
     VALUES ('anything');
INSERT INTO user_details (id, weight, height)
     VALUES (LAST_INSERT_ID(),83, 185);

Third. USING PHP SCRIPT

<?php
// Connecting to database
$link = mysql_connect($wgScriptsDBServerIP, $wgScriptsDBServerUsername, $wgScriptsDBServerPassword, true);
if(!$link || !@mysql_SELECT_db($wgScriptsDBName, $link)) {
echo("Cant connect to server");
    exit;
}

// Values to insert
$name = 'John Smith';
$weight = 83;
$height = 185;

// insertion to user table
$sql = "INSERT INTO user (name) VALUES ('$name')";
$result = mysql_query( $sql,$conn );
// retrieve last id
$user_id = mysql_insert_id( $conn );
mysql_free_result( $result );

// insertion to user_details table
$sql = "INSERT INTO user_details (id, weight, height) VALUES ($user_id, $weight, $height)";
$result = mysql_query( $sql,$conn );
mysql_free_result( $result );
?>

Answer 2

You're most likely going to have to insert things in order of their dependence. So if you have three tables (A, B, and C) we'll assume C depends on B and B depends on A. We'll also assume each table has primary keys AID, BID, and CID respectively.

1. You'd insert your row into A and get AID.
2. Then you'd insert your row into B using the AID you got from step 1.
3. Then you'd insert your row into C using the BID (and perhaps AID) you got from step 2 (and perhaps 1)

Answer 3

There is an error in your syntax for the method 1.

INSERT INTO user_details (id, weight, height)
     VALUES (SELECT(id FROM user WHERE name='John Smith'), 83, 185);

should be

INSERT INTO user_details (id, weight, height)
     VALUES ((SELECT id FROM user WHERE name='John Smith'), 83, 185);

Answer 4

1) The sense of foreign_key is to associate a value in a field with a pre_existing value somewhere else. So you should make your inserts in a logic order.

2) If you want to avoid logical restrictions you should

SET foreign_key_checks = 0  // disable key checks in server
INSERT ... // any order
INSERT ...
...
SET foreign_key_checks = 1