SQL

What is an ER Diagram?

by

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.

Read more

What Does Deterministic Mean in SQL?

by

In SQL, a deterministic expression or function always returns the same result when given the same input. Conversely, a nondeterministic expression or function may return different results even with the same input due to factors like system time, randomization, or external data changes.

Below is a quick overview of the differences between deterministic and nondeterministic functions/expressions.

Read more

How SQL Constraints Work: A Beginner’s Overview

by

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.

Read more

What is a One-To-One Relationship?

by

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.

Read more

Understanding the NOT NULL Constraint in SQL

by

In 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.

Read more

What is a One-To-Many Relationship?

by

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.

Read more

What is a Many-To-Many Relationship?

by

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.

Read more

Compound Keys Explained

by

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.

Read more