Ordering
Limiting the output
Eliminating duplicate results
Aggregation
Grouping
HAVING: filtering and ordering groups
Let's practice
Instruction
Very good! We'll now focus on another topic. By default, the database returns every row that matches the given criteria. This isn't always ideal, especially when it comes to duplicate rows.
Imagine the following situation: we want to get the IDs of all customers who have ever placed an order. We might use the following code:
SELECT CustomerId FROM Order;
What's wrong with the code in this case? Well, try to do the exercise to find out.
Exercise
Select the Year column for all rows in the Employee table. Then, examine the result carefully.
Stuck? Here's a hint!
Type:
SELECT Year FROM Employee;
