Beginning SQL

Chapter 6
This section covers queries that provide more of a summary, or aggregation, of results. You learn how to
use SQL to answer questions such as “How many members come from each state?” “What’s the average
film rating of all the films in the Film Club database?” and “How old is the oldest person in the club?”
The section begins by looking at the COUNT() aggregate function.
Counting Results
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You can use the COUNT() function to count the number of records in the results. It’s used in the SELECT
statement along with the column list. Inside the brackets, insert the name of the column you want counted.
The value returned in the results set is the number of non-NULL values in that column. Alternatively, you
can insert an asterisk (*), in which case all columns for all records in the results set are counted regardless
of whether the value is NULL or not. The COUNT() function can also accept expressions, for example
COUNT(MemberId + CategoryId).
Execute the following SQL:
SELECT COUNT(*)
FROM MemberDetails;
You get the answer 14, which is how many records the SQL returns, which, given the lack of a WHERE
clause, represents all the records in the MemberDetails table. However, if you execute the following SQL,
you get the answer 13:
SELECT COUNT(Street)
FROM MemberDetails;
Why 13 and not 14? After all, it’s the same SQL with no WHERE clause, so surely it has the same number
of records regardless of which column is counted. The difference is that when a column name is specified
in the COUNT() function’s arguments, only the columns where the value is not NULL are counted. As
you can see from the following table, which is the result of SELECT MemberId, Street,City,State
FROM MemberDetails, one of the records in the Street, City, and State columns contains a NULL value:
MemberId Street City State
1 Main Road Townsville Mega State
4 45 Upper Road New Town New State
5 Newish Lane Orange Town New State
6 Newish Lane Orange Town New State
7 Long Lane Orange Town New State
8 Main Street Big City Mega State
9 Long Lane Windy Village Golden State
10 Main Road Townsville Mega State