Section 1.1 What is SQL JOIN and Why is It Used?

The SQL JOIN command is utilized to merge data from two or more tables based on shared columns. When the information you need is spread across different tables, SQL JOIN enables you to consolidate these tables using one or more common columns. This allows you to select records, some of which may or may not include columns from both tables.

Section 1.2 Different Types of SQL JOINs

JOINs can be categorized into four main types depending on how you want to combine two or more tables and select records:

1. INNER JOIN: This merges two tables such that only the rows with matching values in the specified columns are included. It retrieves records common to both tables.
2. LEFT JOIN: This retrieves all records from the left table along with matching records from the right table.
3. RIGHT JOIN: This brings back all records from the right table and the matching records from the left table.
4. FULL JOIN: As the name suggests, this retrieves all records from both tables.

Instead of merely listing the types of JOINs, providing a brief explanation can illustrate your deeper understanding of the subject matter.

Section 1.3 Writing a SQL Query to Join Two Tables

When an interviewer provides you with two tables, they often share one or more common columns. For example, the OrderID serves as the common column in both tables, necessitating a JOIN operation on this field.

To retrieve all rows from the first table, the most straightforward solution is to use a LEFT JOIN:

SELECT t1.OrderID,

t1.orderDate,

t1.orderStatus,

t2.salesManager,

t2.salesUSD

FROM orders AS t1

LEFT JOIN sales AS t2

ON t1.OrderID = t2.OrderID

This checks how well you can specify individual column names in the SELECT statement and whether you can effectively use table aliases.

Section 1.4 What is a CROSS JOIN in SQL?

CROSS JOIN returns all possible combinations of two tables, meaning each row from the first table pairs with every row from the second table. The total number of records returned is the product of the number of records in each table, assuming no WHERE clause is applied. This output is also referred to as the Cartesian Product of the two tables.

For example, if you have two tables and wish to find all combinations of orders and sales while selecting all columns from both, you would write:

SELECT orders.*, sales.*

FROM orders

CROSS JOIN sales

Note that unlike other JOIN types, no ON clause is necessary for CROSS JOIN.

Section 1.5 Understanding SELF JOIN in SQL

SELF JOIN allows you to merge a table with itself. While this may seem perplexing at first, it is extremely valuable for comparing rows within the same table.

For instance, if you have a dataset containing student names, countries, and other details, and you want to identify students from the same country, you would write:

SELECT t1.country,

t1.name AS student1,

t2.name AS student2

FROM students AS t1

JOIN students AS t2

ON t1.studentID > t2.studentID

AND t1.country = t2.country

This query quickly returns pairs of students from the same country while ensuring different students are compared.

Section 1.6 Joining More Than Two Tables

Joining multiple tables functions similarly to joining two tables. You perform consecutive JOIN operations, first merging the first and second tables to produce an intermediate result, and then joining another table to this result.

For example, to retrieve orderID, sales in USD, and quantity from three distinct tables, you might structure your query as follows:

SELECT orders.orderID,

sales.salesUSD,

products.quantity

FROM orders

INNER JOIN sales

ON orders.orderID = sales.orderID

INNER JOIN products

ON orders.orderID = products.orderID

This will yield a single row output if only one orderID is present across all tables.

Section 1.7 Equi JOIN vs. Non-Equi JOIN

Equi JOIN merges tables based on matching values in shared columns, using the equality operator (=) in the ON clause. In contrast, Non-Equi JOIN can utilize a variety of operators such as <, >, ≤, ≥, and <> in the ON clause.

A classic example of Non-Equi JOIN is the Rising Temperature problem from LeetCode, which can provide valuable insights into its application.

Section 1.8 LEFT JOIN vs. INNER JOIN Row Outcomes

When posed with two tables of varying sizes, interviewers often assess your conceptual understanding rather than expecting you to execute a query to determine the row count.

For instance, given two tables, orders and sales, consider the following queries:

-- INNER JOIN

SELECT orders.orderID,

orders.orderStatus,

orders.orderDate,

sales.salesManager,

sales.salesUSD

FROM orders

INNER JOIN sales

ON orders.orderID = sales.orderID

-- LEFT JOIN

SELECT orders.orderID,

orders.orderStatus,

orders.orderDate,

sales.salesManager,

sales.salesUSD

FROM orders

LEFT JOIN sales

ON orders.orderID = sales.orderID

An ideal response would explain the definitions of INNER and LEFT JOINs, indicating your solid grasp of the material.

Section 1.9 LEFT JOIN vs. RIGHT JOIN Outcomes

The answer here is a resounding YES! However, a simple affirmative isn't sufficient during an interview; you need to elaborate with examples.

Using the same tables from the earlier example, to obtain orderID and salesUSD for all entries in the orders table, you could use a LEFT JOIN:

SELECT orders.orderID,

sales.salesUSD

FROM orders

LEFT JOIN sales

ON orders.orderID = sales.orderID

To achieve the same outcome using RIGHT JOIN, simply reverse the order of the tables in the JOIN clause:

SELECT orders.orderID,

sales.salesUSD

FROM sales

RIGHT JOIN orders

ON orders.orderID = sales.orderID

This demonstrates that the RIGHT JOIN can be rewritten as a LEFT JOIN.

Section 1.10 Differences Between UNION and JOIN

Both UNION and JOIN facilitate the combination of data from two tables, yet they do so in fundamentally different ways. JOINs merge tables horizontally based on shared columns, while UNION stacks the results of two SELECT statements vertically.

To combine two tables using INNER JOIN:

SELECT orders.*, sales.*

FROM orders

INNER JOIN sales

ON orders.orderID = sales.orderID

This brings together the two tables side by side. Conversely, with UNION, the resulting tables must possess the same column names:

SELECT orderID, orderStatus FROM orders

UNION

SELECT orderID, salesManager FROM sales

That's a wrap! I hope you found this guide immensely helpful for preparing for SQL JOIN interview questions. Drawing from my personal interview experiences—including those at FAANG—I believe that providing detailed explanations during interviews showcases your comprehensive understanding of these concepts.

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