Question

We want to remove member 37, who has never made a booking, from our database. How can we achieve that?
Schema reminder
DB schema

Expected Results

memid surname firstname address zipcode telephone recommendedby joindate
0 GUEST GUEST GUEST 0 (000) 000-0000 2012-07-01 00:00:00
1 Smith Darren 8 Bloomsbury Close, Boston 4321 555-555-5555 2012-07-02 12:02:05
2 Smith Tracy 8 Bloomsbury Close, New York 4321 555-555-5555 2012-07-02 12:08:23
3 Rownam Tim 23 Highway Way, Boston 23423 (844) 693-0723 2012-07-03 09:32:15
4 Joplette Janice 20 Crossing Road, New York 234 (833) 942-4710 1 2012-07-03 10:25:05
5 Butters Gerald 1065 Huntingdon Avenue, Boston 56754 (844) 078-4130 1 2012-07-09 10:44:09
6 Tracy Burton 3 Tunisia Drive, Boston 45678 (822) 354-9973 2012-07-15 08:52:55
7 Dare Nancy 6 Hunting Lodge Way, Boston 10383 (833) 776-4001 4 2012-07-25 08:59:12
8 Boothe Tim 3 Bloomsbury Close, Reading, 00234 234 (811) 433-2547 3 2012-07-25 16:02:35
9 Stibbons Ponder 5 Dragons Way, Winchester 87630 (833) 160-3900 6 2012-07-25 17:09:05
10 Owen Charles 52 Cheshire Grove, Winchester, 28563 28563 (855) 542-5251 1 2012-08-03 19:42:37
11 Jones David 976 Gnats Close, Reading 33862 (844) 536-8036 4 2012-08-06 16:32:55
12 Baker Anne 55 Powdery Street, Boston 80743 844-076-5141 9 2012-08-10 14:23:22
13 Farrell Jemima 103 Firth Avenue, North Reading 57392 (855) 016-0163 2012-08-10 14:28:01
14 Smith Jack 252 Binkington Way, Boston 69302 (822) 163-3254 1 2012-08-10 16:22:05
15 Bader Florence 264 Ursula Drive, Westford 84923 (833) 499-3527 9 2012-08-10 17:52:03
16 Baker Timothy 329 James Street, Reading 58393 833-941-0824 13 2012-08-15 10:34:25
17 Pinker David 5 Impreza Road, Boston 65332 811 409-6734 13 2012-08-16 11:32:47
20 Genting Matthew 4 Nunnington Place, Wingfield, Boston 52365 (811) 972-1377 5 2012-08-19 14:55:55
21 Mackenzie Anna 64 Perkington Lane, Reading 64577 (822) 661-2898 1 2012-08-26 09:32:05
22 Coplin Joan 85 Bard Street, Bloomington, Boston 43533 (822) 499-2232 16 2012-08-29 08:32:41
24 Sarwin Ramnaresh 12 Bullington Lane, Boston 65464 (822) 413-1470 15 2012-09-01 08:44:42
26 Jones Douglas 976 Gnats Close, Reading 11986 844 536-8036 11 2012-09-02 18:43:05
27 Rumney Henrietta 3 Burkington Plaza, Boston 78533 (822) 989-8876 20 2012-09-05 08:42:35
28 Farrell David 437 Granite Farm Road, Westford 43532 (855) 755-9876 2012-09-15 08:22:05
29 Worthington-Smyth Henry 55 Jagbi Way, North Reading 97676 (855) 894-3758 2 2012-09-17 12:27:15
30 Purview Millicent 641 Drudgery Close, Burnington, Boston 34232 (855) 941-9786 2 2012-09-18 19:04:01
33 Tupperware Hyacinth 33 Cheerful Plaza, Drake Road, Westford 68666 (822) 665-5327 2012-09-18 19:32:05
35 Hunt John 5 Bullington Lane, Boston 54333 (899) 720-6978 30 2012-09-19 11:32:45
36 Crumpet Erica Crimson Road, North Reading 75655 (811) 732-4816 2 2012-09-22 08:36:38

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Answers and Discussion Show

delete from cd.members where memid = 37;          

This exercise is a small increment on our previous one. Instead of deleting all bookings, this time we want to be a bit more targeted, and delete a single member that has never made a booking. To do this, we simply have to add a WHERE clause to our command, specifying the member we want to delete. You can see the parallels with SELECT and UPDATE statements here.

There's one interesting wrinkle here. Try this command out, but substituting in member id 0 instead. This member has made many bookings, and you'll find that the delete fails with an error about a foreign key constraint violation. This is an important concept in relational databases, so let's explore a little further.

Foreign keys are a mechanism for defining relationships between columns of different tables. In our case we use them to specify that the memid column of the bookings table is related to the memid column of the members table. The relationship (or 'constraint') specifies that for a given booking, the member specified in the booking must exist in the members table. It's useful to have this guarantee enforced by the database: it means that code using the database can rely on the presence of the member. It's hard (even impossible) to enforce this at higher levels: concurrent operations can interfere and leave your database in a broken state.

PostgreSQL supports various different kinds of constraints that allow you to enforce structure upon your data. For more information on constraints, check out the PostgreSQL documentation on foreign keys

Take a look at the DELETE statement in the PostgreSQL docs.

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