Count Statement in mysql select

First step is to re-write your query using JOIN syntax (you need to learn this)

SELECT t.title
     , m.userName
     , COUNT(p.postID) As Expr1
     , t.threadID 
FROM   thread As t
 INNER
  JOIN post As p
    ON p.threadID = t.threadID
 INNER
  JOIN person As m
    ON m.PersonID = t.PersonID 
WHERE  t.categories = "Announcement"
GROUP
    BY t.title
     , m.PersonName

This uses INNER joins. This join type indicates that every record must have a match on either side of the join.

To get the result you're looking for you need an OUTER join. This will return all records from one side of the join and match up, where possible, the records from the outer table.

SELECT t.title
     , m.userName
     , COUNT(p.postID) As Expr1
     , t.threadID 
FROM   thread As t
 LEFT
  JOIN post As p
    ON p.threadID = t.threadID
 INNER
  JOIN person As m
    ON m.PersonID = t.PersonID 
WHERE  t.categories = "Announcement"
GROUP
    BY t.title
     , m.PersonName

Note the very subtle change in the second query. I have changed one join to be a LEFT join. It will take all records in the first(left) table in the join, thread, and match up the post where possible.

Use LEFT JOIN instead, something like this:

SELECT 
  t.title, m.userName, t.threadID, 
  COUNT(COALESCE(p.postID, 0)) AS Expr1
FROM 
(
   SELECT * 
   FROM  thread 
   WHERE categories = 'Announcement'
) AS t
LEFT JOIN post p ON t.threadID = p.threadID
LEFT JOIN person m ON m.PersonID = t.PersonID 
GROUP BY t.title, m.PersonName, t.threadID ;

Then if a thread doesn't have any posts, it will be included in the result set with COUNT = 0.

SELECT t.title, m.userName, COUNT(p.postID) AS Expr1, t.threadID 
FROM post p  left join thread t on t.threadID = p.threadID 
left join person m  on  m.PersonID = t.PersonID 
Where t.categories = "Announcement"
GROUP BY t.title, m.PersonName