What are foreign key constraints in SQLite?
Table of Contents
What are foreign key constraints in SQLite?
SQLite Foreign key constraint. A FOREIGN KEY in one table points to a PRIMARY KEY in another table. It is a referential constraint between two tables. The foreign key identifies a column or a set of columns in one (referencing) table that refers to a column or set of columns in another (referenced) table.
Does SQLite enforce foreign key constraints?
SQLite has supported foreign key constraint since version 3.6. 19.
How do I add a foreign key constraint in SQLite?
How to Add a Foreign Key to an Existing Table. You can not use the ALTER TABLE statement to add a foreign key in SQLite. Instead you will need to rename the table, create a new table with the foreign key, and then copy the data into the new table.
Which constraints work on foreign key?
The FOREIGN KEY constraint is used to prevent actions that would destroy links between tables. A FOREIGN KEY is a field (or collection of fields) in one table, that refers to the PRIMARY KEY in another table.
What happens to foreign key references when a table is dropped SQLite?
If a foreign key constraint violation occurs, SQLite issues an error message and will not drop the table. Notice that the DROP TABLE statement deletes the table from the database and the file on disk completely. You will not be able to undo or recover from this action.
Can foreign keys be null SQLite?
A foreign key with a “set null on delete” can only be defined in a CREATE TABLE statement. TIP: You can not add a foreign key with “set null on delete” to a table using ALTER TABLE because SQLite does not support ADD CONSTRAINT in the ALTER TABLE statement.
Why are foreign key constraints used?
Although the main purpose of a foreign key constraint is to control the data that can be stored in the foreign key table, it also controls changes to data in the primary key table. For example, if the row for a salesperson is deleted from the Sales.
What does foreign key constraint failed mean?
The error message itself showing there is a foreign key constraint error, which means you are deleting a parent table where the child table contains the Primary table identifier as a foreign key. To avoid this error, you need to delete child table records first and after that the parent table record.
Can a table have 2 foreign keys?
A table can have multiple foreign keys and no composite keys. A composite key simply means that there are two or more columns making up the key value. The set of columns in a foreign key references the values in a set of columns in another table (or, exceptionally, of another set of columns in the same table).
What are foreign key restraints?
A foreign key is a column (or combination of columns) in a table whose values must match values of a column in some other table. FOREIGN KEY constraints enforce referential integrity, which essentially says that if column value A refers to column value B, then column value B must exist.
How do foreign keys work?
Foreign Keys A foreign key column in a table points to a column with unique values in another table (often the primary key column) to create a way of cross-referencing the two tables. If a column is assigned a foreign key, each row of that column must contain a value that exists in the ‘foreign’ column it references.
Can we delete constraints in primary or foreign key?
To successfully change or delete a row in a foreign key constraint, you must first either delete the foreign key data in the foreign key table or change the foreign key data in the foreign key table, which links the foreign key to different primary key data.
How do you remove a foreign key constraint?
To delete a foreign key constraint
- In Object Explorer, expand the table with the constraint and then expand Keys.
- Right-click the constraint and then click Delete.
- In the Delete Object dialog box, click OK.
