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

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.