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 readingSQL subqueries are like little helpers that fetch data for our main query. They’re super handy when we need to filter our results based on some other data in our database.
Probably the most common placement of a subquery in SQL is in the WHERE clause. Placing a subquery in the WHERE clause allows us to compare a column against a set of rows that match a certain criteria.
Let’s look at three quick examples of subqueries in the WHERE clause.
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 readingIn 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.
Continue readingConstraints 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 reading“That query’s not sargable” says one developer to another.
“I know it’s not sargable… would you love it more if it had sargability?” replies the other developer.
“What does sargable mean?” you wonder in silence.
Continue readingIn 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 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.
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.