Databases & Data Storage
Join
Definition
A SQL join clause combines columns from one or more tables in a relational database. It creates a set that can be saved as a table or used as is. A join is a means for combining columns from one (self-join) or more tables by using values common to each.
Why It Matters
Joins are fundamental to retrieving meaningful data from a normalized relational database. They allow you to combine data from multiple tables based on their relationships.
Contextual Example
To get a list of orders and the names of the customers who placed them, you would join the `Orders` table with the `Customers` table on the `CustomerID` column: `SELECT Orders.OrderID, Customers.CustomerName FROM Orders JOIN Customers ON Orders.CustomerID = Customers.CustomerID;`.
Common Misunderstandings
- There are different types of joins, including INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL OUTER JOIN, which handle cases where there might not be a matching record in the other table.
- Joins can be computationally expensive, and poorly written joins are a common source of database performance problems.