SQL: How to select a count of multiple records from one table, based on records in a different table?

Question

Let's say I have two tables:

TABLE A

MessageID | Message

1 | Hello
2 | Bonjour

etc..

TABLE B

CommentID | MessageID | Comment

1 | 2 | This is a comment to someone saying Bonjour
2 | 2 | This is another comment to Bonjour

What I'm trying to do is run one query that pulls all the records from Table A ("the messages") along with a count of all the comments for each message from Table B.

The result would be:

Hello - 0 comments
Bonjour - 2 comments

I know this is probably some combination of using a join with a count(*), but I can't seem to hit on just the right syntax.

Any ideas?

Answer 1

For a message based approach:

SELECT message, count(commentID) 
FROM tableA LEFT JOIN tableB ON tableA.messageID = tableB.messageID 
GROUP BY message

You'll want a LEFT JOIN to include records in Table A that don't have any comments in Table B.

Answer 2

Give this a try:

SELECT a.MessageID, COUNT(*)
FROM TABLEA a
JOIN TABlEB b
ON b.MessageID = a.MessageID
GROUP BY a.MessageID

Answer 3

Something like this:

SELECT MessageID, COUNT(CommentID)
FROM "TABLE B"
GROUP BY MessageID

If you need Message in addition to MessageID, you should be able to do something like this:

SELECT MessageID, MIN(Message), COUNT(CommentID)
FROM "TABLE A" LEFT JOIN "TABLE B" USING (MessageID)
GROUP BY MessageID

(Oracle syntax, probably same for MSSQL)

Answer 4

Just an addendum to what's already posted since, if you're using this for a scripting language, you'll likely need named columns for everything in the SELECT list:

SELECT tableA.messageID, message, count(commentID) AS commentCount
FROM tableA LEFT JOIN tableB ON tableA.messageID = tableB.messageID 
GROUP BY message