W3. SQL Group By

Question 1

Q: What does the SQL GROUP BY statement do?

Answer 1

A: It groups rows that have the same values into summary rows, often used with aggregate functions.

Question 2

Q: When is the GROUP BY statement commonly used?

Answer 2

A: It’s used with aggregate functions like COUNT(), MAX(), MIN(), SUM(), and AVG() to group results by one or more columns.

Question 3

Q: Write the syntax for a GROUP BY statement.

Answer 3

SELECT column_name(s)
FROM table_name
WHERE condition
GROUP BY column_name(s)
ORDER BY column_name(s);

Question 4

Q: Write a query to count the number of customers in each country.

Answer 4

SELECT COUNT(CustomerID), Country
FROM Customers
GROUP BY Country;

Question 5

Q: How do you sort a GROUP BY result in descending order by count?

Answer 5

A: Use ORDER BY COUNT(column_name) DESC.

Question 6

Q: Write a query to list the number of customers in each country, sorted high to low.

Answer 6

SELECT COUNT(CustomerID), Country
FROM Customers
GROUP BY Country
ORDER BY COUNT(CustomerID) DESC;

Question 7

Q: Can you use GROUP BY with JOIN statements?

Answer 7

A: Yes, GROUP BY can be used with JOIN to group results based on related data from multiple tables.

Question 8

Q: Write a query to list the number of orders sent by each shipper.

Answer 8

SELECT Shippers.ShipperName, COUNT(Orders.OrderID) AS NumberOfOrders
FROM Orders
LEFT JOIN Shippers ON Orders.ShipperID = Shippers.ShipperID
GROUP BY ShipperName;