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: relationships
Understanding the Different Types of Keys in SQL
Probably the most widely known key type in SQL is the primary key, which is chosen to uniquely identify each row in a table. Perhaps next is the foreign key, which is used to establish a relationship between tables.
But there are more key types than this, and the differences between them can be subtle, but important. Here we’ll look at nine of the various key types in SQL.
Continue readingUnderstanding the Various ON DELETE Options in SQL Server Foreign Keys
It’s widely understood among SQL Server developers that foreign keys can prevent us from deleting a row from the parent table if there’s child table that references the key in that row.
But did you know that we can specify a different course of action in such cases?
For example, we could configure our foreign key to delete the child as well. Or we could set it to NULL, or to its default value.
These options are all available, thanks to the ON DELETE clause.
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 readingUsing ON UPDATE SET DEFAULT in SQL Server
In SQL Server, foreign keys are essential for maintaining referential integrity between tables. When creating foreign keys, we have a number options available to us when it comes to defining what should happen in the event that the related data in the parent table changes. One such option is ON UPDATE SET DEFAULT.
This article will explain what this option does, provide an example of its usage, and offer a practical scenario where this feature can be useful.
Continue readingWhat is an ER Diagram?
An entity relationship (ER) diagram (also known as an ERD for Entity-Relationship Diagram) is a visual representation of the relationships between entities within a system. ER Diagrams are often used when designing relational databases. An ERD serves as a blueprint for designing a database by depicting entities, their attributes, and the relationships between them.
ER diagrams help in conceptualizing the structure of a database and serve as a communication tool between database designers, developers, and stakeholders.
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.
How to Truncate Tables with Foreign Key Relationships in PostgreSQL
When working with PostgreSQL databases, you may sometimes need to clear out all the data from tables that have foreign key relationships. This process, known as truncation, can be tricky when dealing with interconnected tables.
By default, if we try to truncate a table that is referenced by another table via a foreign key constraint, we’ll get an error that looks something like this: “ERROR: cannot truncate a table referenced in a foreign key constraint“.
You may have encountered this before finding this article. However, all is not lost. Below are two options for overcoming this issue.
Continue readingWhat is a One-To-One Relationship?
In relational databases, a one-to-one relationship is a relationship between two tables where each record in the first table corresponds to one and only one record in the second table, and vice versa.
This kind of relationship is less common compared to other relationship types, but it can be useful in certain scenarios. The one-to-many and many-to-many relationships are much more common than the one-to-one relationship in SQL.
Continue readingWhat is a One-To-Many Relationship?
When working with relational databases, we tend to create a lot of relationships between tables. But not all relationships are created equal – there are different types of relationships. The one-to-many relationship is probably the most common type of relationship in relational database design.
A one-to-many relationship is a type of association between two tables where a record in one table (the “one” side) can be related to multiple records in another table (the “many” side). For example, in an e-commerce database, a single customer can place many orders. In this scenario, the Customers table would have a one-to-many relationship with the Orders table.