GROUP BY clause

The GROUP BY clause will gather all of the rows together that contain data in the specified column(s) and will allow aggregate functions to be performed on the one or more columns. This can best be explained by an example:

GROUP BY clause syntax:

SELECT column1, 
SUM(column2)
FROM "list-of-tables"
GROUP BY "column-list";

Let’s say you would like to retrieve a list of the highest paid salaries in each dept:

SELECT max(salary), dept
FROM employee
GROUP BY dept;

This statement will select the maximum salary for the people in each unique department. Basically, the salary for the person who makes the most in each department will be displayed. Their salary and their department will be returned.

Multiple Grouping Columns – What if I wanted to display their lastname too?

For example, take a look at the items_ordered table. Let’s say you want to group everything of quantity 1 together, everything of quantity 2 together, everything of quantity 3 together, etc. If you would like to determine what the largest cost item is for each grouped quantity (all quantity 1’s, all quantity 2’s, all quantity 3’s, etc.), you would enter:

SELECT quantity, max(price)
FROM items_ordered
GROUP BY quantity;

Enter the statement in above, and take a look at the results to see if it returned what you were expecting. Verify that the maximum price in each Quantity Group is really the maximum price.

Review Exercises

1. How many people are in each unique state in the customers table? Select the state and display the number of people in each. Hint: count is used to count rows in a column, sum works on numeric data only.

   Show Answer

   SELECT state, count(state)
   FROM customers
   GROUP BY state;

2. From the items_ordered table, select the item, maximum price, and minimum price for each specific item in the table. Hint: The items will need to be broken up into separate groups.

   Show Answer

   SELECT item, max(price), min(price)
   FROM items_ordered
   GROUP BY item;

3. How many orders did each customer make? Use the items_ordered table. Select the customerid, number of orders they made, and the sum of their orders. Click the Group By answers link below if you have any problems.

   Show Answer

   SELECT customerid, count(customerid), sum(price)
   FROM items_ordered
   GROUP BY customerid;

Results will be displayed here