Beginning SQL

Chapter 6
190
Begin by looking at how GROUP BY can answer the question, “Which states do members of the film club
live in?”
The answer doesn’t require a list of every member and the state they live in; you simply want a list of
the specific different states. Use the GROUP BY clause to answer this question, even though strictly
speaking SELECT DISTINCT would work just as well:
SELECT State
FROM MemberDetails
GROUP BY State;
The GROUP BY clause must go after any FROM or WHERE clauses in the SELECT statement. All the columns
you want to be grouped must be listed in the GROUP BY column list. For example, the preceding code
groups by the State column. If you want to include more than one column in the GROUP BY clause, then
separate the columns with commas, in the same way that you would separate columns in a SELECT
statement’s column list.
The preceding SQL produces the results shown in the following table. One of the values in the table is
NULL, so you end up with one group that is NULL:
State
NULL
Golden State
Mega State
New State
When dealing with a GROUP BY clause, the database system first creates a temporary results set based on
the FROM and WHERE clauses. So, in the preceding example, the results set is created from the following
SELECT statement:
SELECT State
FROM MemberDetails;
The DBMS then uses these results and looks for groups of identical records based on the column or
columns specified in the GROUP BY clause. In this case, State is the only column, so it’s just a matter of
grouping all the identical states into one row. If your query includes a WHERE clause, like the one shown
below, the grouping is based on the results returned by the WHERE clause:
SELECT State
FROM MemberDetails
WHERE State IN (‘Mega State’, ‘Golden State’,’New State’)
GROUP BY State;