In SQL, a many-to-many relationship occurs when multiple records in one table can be associated with multiple records in another table.
To represent this type of relationship, a third table—often referred to as a “junction table” or “associative entity”—is used. This junction table typically contains foreign keys that reference the primary keys of the two related tables, allowing for the connection between multiple records.
Continue readingIn SQL Server, ON DELETE NO ACTION is an option that we can apply to foreign key constraints to prevent the deletion of a row in the parent table if there are related rows in the child table.
Unlike ON DELETE CASCADE, which would delete the related rows in the child table, NO ACTION enforces that if a deletion would result in orphaned records in the child table, the deletion operation is prohibited, and an error is raised.
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.
There are various key types in SQL, each with their own characteristics. Some of these can be enforced with a constraint of one kind or another. Others are not necessarily enforced with a constraint, but are more of an abstract notion, albeit, an important one. These key types consist of one or more columns with certain characteristics that qualify it as the particular key.
One such key type is the superkey.
Continue readingIn relational databases, a foreign key is typically a field that is linked to another table‘s primary key field in a relationship between two tables.
However, there’s also a type of foreign key we call the self-referencing foreign key. The self-referencing foreign key refers to a field within the same table, creating a relationship between rows in the same table.
Continue readingThere are many different “key” types in SQL. We have the primary key, the foreign key, and even the unique key.
But have you heard about the candidate key?
Continue readingIn SQL Server, foreign keys are essential for maintaining referential integrity between tables. When setting up a foreign key, we have various options for handling actions when the referenced data is deleted. One of these options is ON DELETE SET DEFAULT.
This feature can be particularly useful in scenarios where it’s important to maintain the relationship while ensuring that the foreign key column doesn’t end up with invalid or null values.
Continue readingWhen 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.
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.
If you’re new to relational databases, and you’re trying to get your head around this concept of a “relationship” in your database, I hope this article helps.
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