How to get a boolean value into a SELECT operation based on another table?

I have two tables: Message and Attachments (which represent entity types that have a one-to-many relationship) as follows:

CREATE TABLE message (
    messageId ..., 
    [from]..., 
    [to]..., 
    subject..., 
    body...
);

CREATE TABLE attachments (
    messageId..., 
    filename...
);

I want to SELECT the basic message data and include a bit column to denote whether the message has attachments or not. I am trying to use EXISTS in the operation shown below:

SELECT  
M.*,
hasAttachments = EXISTS(SELECT AttachmentId 
                          FROM Attachment 
                         WHERE messageId = M.messageId),
FROM Message M

But this doesn't work - What's the way to do what I'm looking for?

I'd do it with a CASE statement:

select
    m.*,
    hasAttachments = CASE WHEN EXISTS(select * from Attachment where messageId = M.messageId) then 1 else 0 end
from Message M

or

select distinct 
    m.*,
    hasAttachments = CASE WHEN a.AttachmentId is not null then 1 else 0 end
from Message m
left join Attachment a on a.MessageId = m.MessageId

StackExchange Database Administrators Q#2772, answer score: 7