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. However, depending on the database system you’re working with and the specific database schema, it can be easy to miss or misdiagnose a foreign key data type mismatch. You may get an error message such as Foreign key mismatch or Could not create constraint when you try to create the table. If you’re lucky, it’ll be easy to diagnose when you’ve made this mistake. For example, if the referencing column where you’re declaring the foreign key uses the INT data type, then the referenced column must be INT as well. When you’re using a foreign key to reference a column in another table, the datatypes of both tables have to be the same.
#SQLITE FOREIGN KEY CASCADE EXAMPLE HOW TO#
In all honesty, I don't think turning on foreign keys (as shown in my code) can be useful inside Qt until Qt has data-aware components.Here are three of the most common mistakes made with foreign keys, and a little advice on how to avoid them! 1.
#SQLITE FOREIGN KEY CASCADE EXAMPLE CODE#
I've been messing around with this for a while, long before even helping Noah drive nails in the Ark! But I can't get it to fire up.Īnd while it's not as elegant, I can hard code all deletes and updates. "FOREIGN KEY(recipe_id) REFERENCES recipes(recipe_id) "
"ingredient_id INTEGER PRIMARY KEY AUTOINCREMENT, " "recipe_id INTEGER PRIMARY KEY AUTOINCREMENT,"ĭetail table: query2 = "CREATE TABLE IF NOT EXISTS recipe_ingredients(" Master table: query1 = "CREATE TABLE IF NOT EXISTS recipes(" But just a simple delete in the master table fails to delete across the detail tables. In all the posts on this topic that I've read, they're all fairly consistent in their remedies to the elusive turning on the SQLite3 Foreign Key apparatus. MainWindow::MainWindow(QWidget *parent) :ĭb = QSqlDatabase::addDatabase("QSQLITE") Īnd I got that from Asimov, here on the Forum. At the beginning of my program, I have this: #include "mainwindow.h"
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