Lookup Tables Empty?

Databases
1

Hi there,

I’m working my way through Build Your Own Database Driven Website (3rd Edition) and although it’s been excellent thus far, I’ve hit a snag.

I’m at Chapter 5: Relational Database Design, near the end of the chapter, and I’ve just established a lookup table called JOKECATEGORY which is drawing information from JOKE and CATEGORY tables.

When I try to SELECT information from JOKECATEGORY it just gives me an EMPTY SET message. Is this normal? I haven’t inserted any information to JOKECATEGORY, because I was under the impression it was taking its information from those other tables I mentioned. Am I wrong? Should I be inputting information? Perhaps I’ve misinterpreted somewhere.

Any help is greatly appreciated!

2

No they should not be empty. There should be data which ties the two together.

Example:

In Jokes (format is JokeID, Joke, Punchline)
1, Why Did the Chicken Cross the Road, To Get to the Other Side
2, What is a cows favorite subject in school?, Moo-sic!
3, Where do sheep get their coats cut? At the baa-bershop!

In Category (format is CategoryID, Category
1, Classic Jokes
2, Classic Puns

In your JokeCategory Table, you’d have something like (format CategoryID, JokeID)
1, 1
2, 2
2, 3

I think you missed a step somewhere when you created your table - you forgot to populate them.

3

Hmm, perhaps the question I was asking was too bloated. Would anyone be able to tell me this then: is a lookup table meant to be empty?

4

Thanks for the reply. I’ve managed to add the appropriate values to JOKECATEGORY, but I’m still receiving an “empty set” message. This is beginning to get frustrating :frowning:

Here is the MYSQL code I’m trying to execute, if it’s of any use

mysql> SELECT joketext 
    -> FROM joke INNER JOIN jokecategory 
    ->   ON joke.id = jokeid 
    -> INNER JOIN category 
    ->   ON categoryid = category.id 
    -> WHERE name = "Knock-knock";
5

SCRATCH THAT! I dialed back the code a bit, and executed it section by section, and found that the WHERE=“Knock-knock” statement was the culprit. I change that to search for another thing, and it worked :slight_smile:

Thanks very much for the help