Add Foreign Key to existing table

How to Add a Foreign Key to an Existing Table in MySQL

So, you want to add a Foreign Key to an existing table called "katalog" in MySQL, huh? I feel you, it can be a bit tricky sometimes. But worry not, I’m here to guide you through it!

The Error Message

Let’s start by addressing the problem. When you try to add the Foreign Key using the code snippet mentioned, you encounter the following error message:

Error Code: 1005. Can't create table 'mytable.#sql-7fb1_7d3a' (errno: 150)

This error usually occurs due to some issue with the foreign key constraint.

Analyzing the Error

Now, let's dive into the details of the error. MySQL also provides additional information about the issue in the INNODB Status. In this case, the error message states:

FOREIGN KEY (`Sprache`)
REFERENCES `Sprache` (`ID`)
ON DELETE SET NULL
ON UPDATE SET NULL:
Cannot resolve table name close to:
(`ID`)

Identifying the Cause

From the provided error message, it seems that MySQL is unable to resolve the table name close to the "ID" field. This could be due to multiple reasons, but it's most likely a case sensitivity issue.

Easy Solution

To fix this problem, you can modify your code snippet as follows:

ALTER TABLE katalog 
ADD CONSTRAINT fk_katalog_sprache
FOREIGN KEY (Sprache)
REFERENCES sprache(ID)
ON DELETE SET NULL
ON UPDATE SET NULL;

Removing the backticks and matching the letter casing of the table and column names should resolve the error.

Double-check: Table Structures

Just to ensure we cover all bases, let’s quickly review the structure of both tables involved: "katalog" and "sprache".

CREATE TABLE `katalog` (
    `ID` int(11) unsigned NOT NULL AUTO_INCREMENT,
    `Name` varchar(50) COLLATE utf8_unicode_ci NOT NULL,
    `AnzahlSeiten` int(4) unsigned NOT NULL,
    `Sprache` int(11) NOT NULL,
    PRIMARY KEY (`ID`),
    UNIQUE KEY `katalogname_uq` (`Name`)
) ENGINE=InnoDB AUTO_INCREMENT=12 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci ROW_FORMAT=DYNAMIC$$

CREATE TABLE `sprache` (
    `ID` int(11) NOT NULL AUTO_INCREMENT,
    `Bezeichnung` varchar(45) NOT NULL,
    PRIMARY KEY (`ID`),
    UNIQUE KEY `Bezeichnung_UNIQUE` (`Bezeichnung`),
    KEY `ix_sprache_id` (`ID`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8

Additional Tips

Here are a few additional tips that might come in handy:

1. Ensure that the data types of the foreign key column and the referenced column match exactly.

2. Verify that both the "katalog" and "sprache" tables are using the InnoDB storage engine.

3. If you still encounter issues, check if there are any duplicate foreign key constraints previously defined on the same table.

🎉 Wrapping Up

With the modified code and the additional tips, you should be able to add the Foreign Key to your "katalog" table without any further issues. Now, go ahead and give it a shot, and let me know how it goes! Don't forget to share this guide with your fellow developers who might be struggling with the same problem.

If you have any further questions or need more assistance, feel free to leave a comment below. Happy coding! 💻🚀