When a foreign key is defined in a table, it ensures that the values in the column(s) correspond to values in a primary key or unique key in another table. While PostgreSQL automatically creates an index for primary keys (because these need to be fast for lookups and enforcing uniqueness), it does not automatically create an index for foreign keys.
Continue readingTag: create relationship
What is a Junction Table in SQL?
A junction table, also known as an associative table or a bridge table, is a table used to manage many-to-many relationships between two other tables in a relational database.
In SQL, many-to-many relationships cannot be directly implemented using just two tables because each table’s foreign key can only relate to one other table’s primary key. Instead, a junction table is created to “link” the two tables together.
Continue readingImplementing ON UPDATE CASCADE for SQL Server Foreign Keys
ON UPDATE CASCADE is a referential integrity constraint option that we can use in SQL Server when creating foreign keys. It automatically updates foreign key values in child tables when the corresponding primary key in the parent table is updated.
Understanding the ON UPDATE SET NULL Option for Foreign Keys in SQL Server
In SQL Server (and relational databases in general), a foreign key is a field (or collection of fields) in one table that uniquely identifies a row of another table.
This relationship is a cornerstone of relational database design, ensuring referential integrity between the related tables. When designing databases, it’s crucial to understand how changes to primary keys in parent tables can affect the foreign keys in child tables.
One of the various options available in SQL Server for handling such changes is the ON UPDATE SET NULL action.
Using ON DELETE SET NULL for Foreign Keys in SQL Server
When creating a foreign key constraint in SQL Server, we have the option of including ON DELETE SET NULL in the definition. When we use this option in a foreign key, it tells SQL Server to automatically set the foreign key column values to NULL in the child table when the corresponding primary key record in the parent table is deleted.
Using ON DELETE CASCADE When Creating a Foreign Key in SQL Server
By default, if we try to delete a row in the parent table of a referential relationship in SQL Server, we’ll get an error. That’s because the default action is NO ACTION. This means that the delete doesn’t happen, and an error is raised.
But we’re not quite doomed yet. We can use the ON DELETE CASCADE option to ensure that the delete operation does happen, and that no error is returned. This option automatically deletes related records in the child table when a record in the parent table is deleted.
If we’re going to use this option, we need to define it when creating the foreign key. That’s because ON DELETE CASCADE is an optional argument that we can provide when creating the foreign key.
Create a Relationship in SQL
In SQL, you create a relationship by creating a foreign key constraint.
More specifically, you have a parent table and a child table. The parent contains the primary key, and the child table contains a foreign key that references the primary key of the parent table.
When you use SQL to create a relationship, you can create the relationship at the time you create the table, or you can create it later (by altering the table). This article covers both scenarios.
Continue readingCreate a Foreign Key in SQLite
When you create a table in SQLite, you can also create a foreign key in order to establish a relationship with another table.
This article provides an example of creating a foreign key when creating a table in SQLite.
Continue readingHow to Enable Foreign Key Support in SQLite
In SQLite, foreign key constraints are not enforced unless foreign key support has been enabled.
Enabling foreign keys involves the following:
- Enable foreign keys when compiling SQLite.
- Enable foreign keys at runtime.
How to Add a Foreign Key Constraint to an Existing Table in SQL Server (T-SQL)
In database terms, a foreign key is a column that is linked to another table‘s primary key field in a relationship between two tables.
A foreign key is a type of constraint, and so if you want to create a foreign key in SQL Server, you’ll need to create a foreign key constraint.
This article demonstrates how to create a foreign key constraint in SQL Server, using Transact-SQL.