GROUP BY clause - XI

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 you ALSO want to display their lastname for the query below:

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

What you'll need to do is:

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

This is a called "multiple grouping columns".

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.