Question

I have a MySQL database, in which I have a table of monkeys:

id     name
1      Alice
2      Bill
3      Donkey Kong
4      Edna
5      Feefee

I also have a table of bananas and where they were picked from.

id     where_from
1      USA
2      Botswana
3      Banana-land
4      USA

Finally, I have a table matches that describes which bananas belong to which monkeys. Each monkey can only have one banana, and no monkeys can share a banana. Some monkeys may lack a banana.

id     monkey_id     banana_id
1      3             4 
2      4             1
3      5             2

How can I use a single SQL statement to retrieve all the matches? For each match, I want the name of the monkey as well as where the banana is from.

I have tried the following 3 SQL statements, which work:

1. SELECT * FROM matches
2. SELECT * FROM monkeys WHERE id=[monkey_id from 1st SQL query]
3. SELECT * FROM bananas WHERE id=[banana_id from 1st SQL query]

I feel that 3 SQL statements is cumbersome though. Any ideas on how I can just use a single SQL statement? I am just learning SQL and am monkeying around with the basics. Thanks!

Answer 1

Since some monkeys may lack a banana, that implies a LEFT JOIN between matches and monkeys. That will ensure all monkeys are listed, even if they have no bananas in matches.

SELECT
  monkeys.name,
  bananas.where_from
FROM
  monkeys
  /* List all monkeys, even if they have no match */
  LEFT JOIN matches ON monkeys.id = matches.monkey_id
  /* And another LEFT JOIN to link matches to bananas */
  LEFT JOIN bananas ON bananas.id = matches.banana_id

Here is an example on SQLfiddle.com

I very highly recommend reading over Jeff Atwood’s (co-founder of Stack Overflow) excellent article explaining SQL joins.