Beginning SQL

Chapter 4
144
The results of the query are shown in the following table:
MeetingDate MemberAttended MemberId LocationId
2004-01-01 Y 1 2
2004-01-01 N 4 2
2004-01-01 Y 5 2
2004-01-01 Y 6 2
2004-03-01 Y 1 1
2004-03-01 Y 4 1
2004-03-01 N 5 1
2004-03-01 N 6 1
For each meeting, the Attendance table stores each film club member and their attendance status. Rather
than store a record regardless of whether a member attended or not, you can save space by storing a
record only if a member actually attends a meeting. Getting a list of members who attended a meeting is
simple enough. The SQL to find out who didn’t attend, however, is a little more involved and is covered
in greater detail in Chapter 7.
To make the changes, delete all the records where the member didn’t attend, that is, where
MemberAttended equals N. Then delete the MemberAttended column.
Use this SQL to delete all the records where MemberAttended equals N:
DELETE FROM Attendance
WHERE MemberAttended = ‘N’;
Once you delete those records, you can drop the whole column:
ALTER TABLE Attendance
DROP COLUMN MemberAttended;
Dropping the column deletes all the data stored in that column for all rows. Unfortunately, IBM’s DB2
doesn’t support dropping a column. The only option, as you saw earlier, is to delete the whole table and
re-create it from scratch, having first saved the data:
CREATE TABLE TempAttendance
(
MeetingDate date,
LocationId integer,
MemberId integer
);
INSERT INTO TempAttendance (MeetingDate, LocationId, MemberId)
SELECT MeetingDate, LocationId, MemberId FROM Attendance