Beginning SQL

MemberId Street City State
11 Newish Lane Orange Town New State
12 Long Lane Windy Village Golden State
13 Winding Road Big City Mega State
14 White Cliff Street Dover Golden State
15 NULL NULL NULL
You can also see that the MemberId column contains no NULLs, so if you change the column counted to
MemberId (by including MemberId inside parentheses), you get the value 13, because 13 records in the
MemberId column contain values that are not NULL.
You can include more than one aggregate function in the SELECT statement. Consider the following
statement:
SELECT COUNT(City), COUNT(LastName)
FROM MemberDetails;
Executing the preceding SQL returns the following results:
COUNT(City) COUNT(MemberId)
12 13
Remember, the City column does contain one record with a NULL value; hence the value above is 12. The
MemberId column contains no NULLs in any of the records, so its value is 13, or all the records in the
results set.
Trying to combine an aggregate function and a nonaggregate column is not allowed by the rules of SQL.
For example, the following SQL results in an error:
SELECT City, COUNT(MemberId)
FROM MemberDetails;
The reason for this rule is that City might return more than one row, whereas COUNT() only ever returns
one row.
In this situation, the GROUP BY clause comes in quite handy. For example, if you want to know how
many members live in each state based on information from the MemberDetails table, run the following
query:
SELECT State, COUNT(LastName)
FROM MemberDetails
GROUP BY State;
Grouping and Aggregating Data
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