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Free SQL interview practice question

Let's get started with a free practice question to test your SQL skills:

Cocoa Confections is a small bakery that sells brownies, cookies, pies, and other delicious treats to customers online. It keeps records of all of its online sales in an SQL database that is automatically populated as customers place orders on its site.

In its database, Cocoa Confections has a customers table to keep track of customer contact information, and an orders table to keep track of various orders that those customers have placed. The schema of these tables is as follows:

CREATE TABLE `customers` (
  `customer_id` int(11) NOT NULL AUTO_INCREMENT,
  `first_name` varchar(255) NOT NULL,
  `last_name` varchar(255) NOT NULL,
  `email` varchar(255) NOT NULL,
  `address` varchar(255) DEFAULT NULL,
  `city` varchar(255) DEFAULT NULL,
  `state` varchar(2) DEFAULT NULL,
  `zip_code` varchar(5) DEFAULT NULL,
  PRIMARY KEY (`customer_id`)
);

CREATE TABLE `orders` (
  `order_id` int(11) NOT NULL AUTO_INCREMENT,
  `customer_id` int(11) NOT NULL,
  `order_placed_date` date NOT NULL,
  PRIMARY KEY (`order_id`),
  KEY `customer_id` (`customer_id`),
  FOREIGN KEY (`customer_id`) REFERENCES `customers` (`customer_id`)
);

It's the end of 2016, and the owner of Cocoa Confections wants to write an SQL query that finds the COUNT of orders placed in 2016 by customer e-mail address. She wants to ORDER the results by the COUNT of orders placed in 2016, descending, so that she can personally send thank-you e-mails to Cocoa Confection's top customers by order volume.

Can you write a query that will help the owner of Cocoa Confections find the COUNT of all orders placed in 2016, by customer e-mail address, sorted descending?

Hint: Combine tables

The owner of Cocoa Confections wants to find the COUNT of orders placed by e-mail address. But the email field is located within the customers table, not the orders table. What type of statement do we need to use to combine the two tables together?

Hint: Use a JOIN

To combine our two tables together, we'll use a JOIN. Since the orders table has a customer_id field, we can match that against the customer_id in the customers table to link up the two separate data sources. We'll use an INNER JOIN so that data is only included if the customer_id listed matches both of our input tables. Try the following to get you started — it'll compile a list of all orders in our database with all requisite customer information appended:

SELECT
	*
FROM
	orders
INNER JOIN
	customers ON orders.customer_id = customers.customer_id;
Hint: Use COUNT(*)

Now that we have a complete list of orders with e-mail addresses included, we can use the COUNT(*) and GROUP BY functions to get a list of orders by e-mail address. This will return a COUNT of all orders in our database by e-mail address. We'll also restrict our SELECT clause so that it only pulls the e-mail address and the COUNT of orders from our database, excluding all other fields.

SELECT
	email, COUNT(*)
FROM
	orders
INNER JOIN
	customers on orders.customer_id = customers.customer_id
GROUP BY
	email;
Hint: Use WHERE and ORDER BY

We're not done yet! We still need to restrict the dates of our pull to 2016 only. How do we filter for particular results in SQL? Using a WHERE clause! We'll also need to use an ORDER BY clause to ensure that our results are sorted descending by number of orders. These two clauses, combined, should look something like this:

WHERE
	order_placed_date >= '2016-01-01'
	AND order_placed_date <= '2016-12-31'
ORDER BY
	2016_num_orders DESC;
		
Does your answer use a JOIN?

Your solution will need to use a JOIN statement to combine the customers table with the orders table. Is a JOIN statement part of your solution?

Does your answer use COUNT(*)?

In order to find the total number of orders placed by e-mail address, you'll need to use SQL's COUNT(*) or SUM functions, which also necessitate a GROUP BY clause in your query. Does your solution contain those features?

The solution

Great! Sounds like you're on the right track, so let's move on to the solution.

Here's our full solution to the problem:

SELECT
	customers.email, COUNT(*) AS 2016_num_orders
FROM
	orders
INNER JOIN
	customers on orders.customer_id = customers.customer_id
WHERE
	orders.order_placed_date >= '2016-01-01'
	AND orders.order_placed_date <= '2016-12-31'
GROUP BY
	customers.email
ORDER BY
	2016_num_orders DESC;

Check out what's happening above:

First, we use a SELECT statement to pull specific fields from our database. We'll pull the email field, which is located on the customers table, and the aggregated COUNT(*) of records, using the AS clause to refer to this column as 2016_num_orders.

We'll use a FROM orders clause to note that we're pulling this data from our orders table. But since the email field isn't located on the orders table, we'll also need to perform an INNER JOIN linking the orders.customer_id field to the customers.customer_id field.

We'll use a WHERE clause to restrict our data range to 2016 only using two criteria: >= '2016-01-01' and <= '2016-12-31'.

Since we're using a COUNT(*) function to sum records, we'll need to use a GROUP BY clause to tell SQL that we want to group our results by email address.

Finally, we'll tell SQL that we want our results arranged by number of orders, from greatest to least, using ORDER BY.

Our query is complete!

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