Joining Tables: `INNER JOIN`

Today, we'll dive into the world of database joins, specifically the `INNER JOIN`. You'll learn how to combine data from multiple tables to create a more comprehensive and useful dataset, a fundamental skill for any data scientist working with relational databases.

Learning Objectives

  • Define what a database join is and why it's used.
  • Explain the purpose of the `INNER JOIN`.
  • Write `INNER JOIN` queries to retrieve data from two or more tables.
  • Identify the key components of a join condition.

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Lesson Content

What are Database Joins?

Imagine you have information about customers in one table (e.g., Customers) and their orders in another table (e.g., Orders). Each table holds different, but related, pieces of information. A join allows you to bring together related information from these separate tables into a single result set. This is crucial for getting a complete picture of your data and performing meaningful analysis. Joining combines rows from two or more tables based on a related column, often a primary key in one table matching a foreign key in another.

Introducing the `INNER JOIN`

The INNER JOIN is the most common type of join. It returns only the rows where there's a match in both tables based on the specified join condition. Think of it as finding the intersection of the two tables based on the matching column. Rows without a match in the other table are excluded from the result.

Syntax:

SELECT column_names
FROM table1
INNER JOIN table2
ON table1.column_name = table2.column_name;
  • column_names: The columns you want to retrieve from both tables. You can use * to select all columns.
  • table1: The first table.
  • table2: The second table.
  • ON: This is the crucial part! It specifies the join condition. It tells the database how to match rows between the tables. It typically involves comparing a column in table1 (often the primary key) to a column in table2 (the foreign key referencing the primary key).

Example:
Let's say we have two tables:

  • Customers: CustomerID, CustomerName, City
  • Orders: OrderID, CustomerID, OrderDate

To find the customer names and order dates for all customers, we'd use:

SELECT Customers.CustomerName, Orders.OrderDate
FROM Customers
INNER JOIN Orders
ON Customers.CustomerID = Orders.CustomerID;

This query will return a list of customer names and their corresponding order dates, showing only the customers who have placed orders.

More on the Join Condition

The ON clause is critical for the INNER JOIN. It defines how the tables are related. The join condition typically uses an equals sign (=) to compare the values in the joining columns.

Important: The column names in the ON clause should be fully qualified (e.g., Customers.CustomerID) if the column name appears in both tables to avoid ambiguity. If the column names are unique, you can use just the column name (e.g., CustomerID).

Example with alias: To make the query more readable, you can use table aliases:

SELECT c.CustomerName, o.OrderDate
FROM Customers c  -- 'c' is an alias for Customers
INNER JOIN Orders o  -- 'o' is an alias for Orders
ON c.CustomerID = o.CustomerID;

This does the same thing as the previous example but is often easier to read, especially when joining multiple tables or working with complex queries.

Handling Multiple Tables and Multiple Columns

You can join more than two tables in a single query by chaining INNER JOIN clauses.

Example: Suppose we have a third table, Products, with ProductID, ProductName, and Price. We also assume that the Orders table has ProductID.

SELECT c.CustomerName, o.OrderDate, p.ProductName, p.Price
FROM Customers c
INNER JOIN Orders o ON c.CustomerID = o.CustomerID
INNER JOIN Products p ON o.ProductID = p.ProductID;

This query retrieves customer names, order dates, product names, and prices, linking information across three tables. Be mindful of which columns you select and always include the primary/foreign key connections.

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