Why Am I Getting 'Can't create table ... (errno: 150)' When Adding a Foreign Key?

Error Adding Foreign Key: Understanding and Resolving

When attempting to add a foreign key to an existing table named "katalog," users may encounter an error similar to "Can't create table 'mytable.#sql-7fb1_7d3a' (errno: 150)." This error can be resolved by following a specific process.

The query to add the foreign key should be in the following format:

ALTER TABLE <table_name> ADD FOREIGN KEY (<column_name>) REFERENCES <referenced_table_name> (<referenced_column_name>) ON DELETE <delete_action> ON UPDATE <update_action>;

In this case, the correct query would be:

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

Ensure that the following conditions are met:

• The table and columns being referenced exist.
• The data types of the columns match.
• The referenced table has a primary key or unique index.

If these conditions are fulfilled and the error persists, it may indicate other underlying issues that need to be addressed.

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