使用CONCAT的MySQL查询

Question

I have two tables:

categories => Category_ID , Title , Description , Default_Points , Groups

transactions => Transaction_ID , Datetime , Giver_ID , Recipient_ID , Points , Category_ID , Reason

Teachers award points, choosing a category (like "Positive Attitude & Behaviour") and a reason (like "Excellent work today") which puts an entry into the transactions table.

A typical categories row may be:

INSERT INTO `categories` (`Category_ID`, `Title`, `Description`, `Default_Points`, `Groups`) VALUES
(17, 'Olympic Values', 'Please clearly state the correct Olympic Value that''s being used currently in the REASON box.', 5, '');

A typical transactions row may be:

INSERT INTO `transactions` (`Transaction_ID`, `Datetime`, `Giver_ID`, `Recipient_ID`, `Points`, `Category_ID`, `Reason`) VALUES
(50, '2011-09-07', 35023, 90236, 5, 17, 'Excellent work during PE');

What I'd like to try and do using MySQL is produce a list of total points (ie SUM(transactions.Points) for EACH category, with a few sample Reasons too.

I'd imagine this will have to use a CONCAT?

I need:

• SUM(transactions.Points) per category
• categories.title
• 5 unique transactions.reason per category

This might look like...

Points      Title                   Sample
14252       Olympic Values          Excellent work in PE!|Great display of friendship|Well done!
15532       Outstanding Effort      Amazing work!|Worked so hard|Great piece!

Is this possible?

Thanks in advance,

Answer 1

It's GROUP_CONCAT you want.

You'll need to do something like this:

SELECT SUM(t.Points), 
    c.title, 
    SUBSTRING_INDEX(GROUP_CONCAT(transactions.reasons SEPERATOR '|'), '|', 5)
FROM transactions t JOIN categories c ON (t.Category_ID=c.Category_ID)
GROUP BY c.Category_ID

Answer 2

Thanks to @ChrisPatrick 's answer, this is the code I used:

SELECT
 SUM(t.Points) AS Total_Points, 
    c.Title, 
    SUBSTRING_INDEX(GROUP_CONCAT(DISTINCT t.reason SEPARATOR '|'), '|', 5) AS Sample_Reasons
FROM
 transactions t JOIN categories c ON (t.Category_ID=c.Category_ID)
GROUP BY c.Category_ID
ORDER BY c.Title ASC