SQL: Get all records from one table AND a count of records from a second table?

Question

Say there are two tables:

TABLE A

messageID / Message                     / More..
1         / This is the first message   / Etc..
2         


        

Answer 1

You can use CTE for the same.

;WITH CTE_MessageCount (MessageId, Count)
AS
(
SELECT MessageId, Count(*) FROM TableB GROUP BY MessageId
) 

SELECT A.*, T.*
FROM tableA A JOIN CTE_MessageCount T ON A.messageID = T.MessageID

Answer 2

Try this query:

SELECT a.*, b.msgCount
  FROM tableA a LEFT JOIN 
    ( SELECT messageID, COUNT(1) AS msgCount FROM tableB b GROUP BY messageID) b
        ON a.messageID =  b.messageID 

Answer 3

Scalar subquery will work:

SELECT tableA.*
    ,(SELECT count(commentID) FROM tableB WHERE tableA.messageID = tableB.messageID) as commentcount 
FROM tableA

As usual, there are a lot of ways to skin this cat, with varying performance profiles.

When using a GROUP BY, all columns in the output either need to be in the GROUP BY or in aggregate functions - even though there is no variation in the other columns within a messageID, they still would need to be in the GROUP BY.