In PostgreSQL, both UNIQUE indexes and UNIQUE constraints can be used to enforce uniqueness on columns.
So you may be wondering; if that’s true, why have both? And are there any circumstances that we should choose one over the other?
Let’s find out.
Continue readingA UNIQUE constraint is a rule that we can apply to one or more columns in a database table to ensure that the values in those columns are unique across all rows.
In SQL Server we have a few options when it comes to creating a UNIQUE constraint. But it’s usually done when we create the table or alter it. That is, we include the constraint code in the CREATE TABLE statement or the ALTER TABLE statement.
We can also create a UNIQUE index, which can be used in place of the previous methods, but can have the same effect.
Let’s check out several ways to create a UNIQUE constraint in SQL Server.
Constraints are an important concept in relational database management systems (RDBMSs). Whenever we design a database, we need to ensure that it will be able to enforce data integrity, by checking or restricting what the user can enter to only data that conforms to the rules of the database. That’s where a constraint can help.
This article explores what SQL constraints are, the various types available, their importance, and how they are implemented.
Continue readingWhen 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 readingIn SQL, constraints are essential for maintaining data integrity and ensuring the accuracy and reliability of the data stored within a database. One of the most commonly used constraints in SQL databases is the NOT NULL constraint.
In this article, I provide a quick overview of the NOT NULL constraint, its purpose, syntax, usage, and practical examples.
The DEFAULT constraint is a nifty little feature in SQL databases that simplifies data management by assigning default values to columns in a table. This can help us maintain data integrity in our databases.
Let’s take a look at the DEFAULT constraint.
In 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 databases, a compound key is a type of primary key that consists of two or more columns combined to uniquely identify each row in a table. The key columns are used together as a single unit to ensure uniqueness.
Some within the SQL community define compound keys as composite primary keys comprising of foreign keys from other tables, so there doesn’t seem to be an agreed consensus on the precise definition.
With that in mind, let’s explore these definitions of compound keys.
Continue readingIn SQL, a composite key is a key that consists of two or more columns used together to uniquely identify a record in a table. The combination of these columns creates a unique identifier, even though the individual columns might not be unique by themselves.
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 reading