RELATED Performance Benefits of NOT NULL Constraints on Foreign Key Reference Columns If you’re attempting to drop a column or table but repeatedly getting an error, it could be because you’re attempting to remove something that’s a foreign key constraint in another table. Other database systems may throw errors that are a bit tougher to parse. In CockroachDB, for example, attempting to drop a table that’s referenced by another table’s foreign key will result in the following error: ERROR: 'table_1' is referenced by foreign key from table 'table_2'. Some database management systems will allow this, leaving a dangling foreign key, while others will throw errors. More commonly, dangling foreign keys crop up when you remove a table or column that was being referenced elsewhere in the database. Many database systems, including CockroachDB, prevent this by making it impossible to create a table that references a nonexistent table or column – if you try, the CREATE TABLE statement will fail with an error such as ERROR: relation 'referenced_table' does not exist. If you subsequently forget to create that referenced table or column, you’ve got a dangling foreign key. With some database management systems, you can create a dangling foreign key simply by creating the referencing table first, and including a foreign key constraint that links it to a column in the next table you’re planning to create. This is obviously bad practice, but it happens – mostly because it’s possible to create dangling foreign keys accidentally. Dangling foreign keysĪ dangling foreign key is a foreign key that links to a nonexistent table or column. If you’re altering an existing table with a new foreign key constraint, your database system will likely return an error if you attempt to create a foreign key constraint that links columns with different data types. The easiest way to avoid this issue is to ensure that all columns linked to each other with foreign keys share the same data type when you’re creating the tables in the first place.

This mismatch can then create other errors further down the line. With a SQLite database, for example, it is possible to successfully create a table with a foreign key data type mismatch. RELATED What is a foreign key? (with SQL examples)

Sqlite foreign key mismatch referencing