What is the reason the counts are different in the mysql queries?

Question

When I run the following queries with SQL_CALC_FOUND_ROWS or specify all the colors RED,BLUE,YELLOW in the sub-query using an IN clause, I get a record count of 198. But when I do each color individually, I get a COUNT that adds up to 248 (72 + 59 + 118). Where are the extra 50 rows coming from? I would have thought that if I did them individually and then added them up, I would get 198, but I am getting 248. Am I viewing how the IN clause works incorrectly? I would prefer to use COUNT instead of SQL_CALC_FOUND_ROWS because I have found SQL_CALC_FOUND_ROWS to be much slower (+3 seconds)

Here are few stats:

1. I am running MySQL Version: 5.5.2 
2. The tables engine is INNODB.
3. All the `CHECK TABLE .... EXTENDED` return OK.

-- Returns 198
SELECT SQL_CALC_FOUND_ROWS DISTINCT p.* FROM Persons AS p
INNER JOIN Colors AS c ON c.PersonId = p.Id
WHERE p.Name = 'John Doe' AND c.ColorName IN ('YELLOW','RED','BLUE');
SELECT FOUND_ROWS();

-- Returns 198
SELECT COUNT(1) as MyCount FROM (SELECT DISTINCT p.* FROM Persons AS p
INNER JOIN Colors AS c ON c.PersonId = p.Id
WHERE p.Name = 'John Doe' AND c.ColorName IN ('YELLOW','RED','BLUE')) all;

-- Returns 72
SELECT COUNT(1) as MyCount FROM (SELECT DISTINCT p.* FROM Persons AS p
INNER JOIN Colors AS c ON c.PersonId = p.Id
WHERE p.Name = 'John Doe' AND c.ColorName IN ('RED')) red;

-- Returns 59
SELECT COUNT(1) as MyCount FROM (SELECT DISTINCT p.* FROM Persons AS p
INNER JOIN Colors AS c ON c.PersonId = p.Id
WHERE p.Name = 'John Doe' AND c.ColorName IN ('BLUE')) blue;

-- Returns 118
SELECT COUNT(1) as MyCount FROM (SELECT DISTINCT p.* FROM Persons AS p
INNER JOIN Colors AS c ON c.PersonId = p.Id
WHERE p.Name = 'John Doe' AND c.ColorName IN ('YELLOW')) yellow;

Answer 1

Something like this is happening:

Persons table

ID  Name
--  ------------
 1  John Doe
 2  Xaisoft

Colors table

PersonID  ColorName
--------  ---------
       1  Red
       1  Yellow
       2  Yellow

Now query Persons.* for John Doe while joining to Colors:

 SELECT p.*
 FROM Persons p
 INNER JOIN Colors C ON c.PersonID = p.ID
 WHERE p.Name = 'John Doe';

Your result will be one p.* row for each color under John Doe:

ID  Name
--  ------------
 1  John Doe
 1  John Doe

If you apply a COUNT DISTINCT against this you'll end up with 1.

If you count Red and Yellow separately, you'll get one for each query. Add them up and you'll get 2.

---

Look at it this way: if you throw out the COUNT and just SELECT DISTINCT Persons.* you get these results:

Red or Yellow for John Doe:

 SELECT DISTINCT p.*
 FROM Persons p
 INNER JOIN Colors C ON c.PersonID = p.ID
 WHERE p.Name = 'John Doe'
   AND c.ColorName IN ('RED', 'YELLOW');

ID  Name
--  ----------
 1  John Doe

Red only for John Doe:

 SELECT DISTINCT p.*
 FROM Persons p
 INNER JOIN Colors C ON c.PersonID = p.ID
 WHERE p.Name = 'John Doe'
   AND c.ColorName IN ('RED');

ID  Name
--  ----------
 1  John Doe

Yellow only for John Doe:

 SELECT DISTINCT p.*
 FROM Persons p
 INNER JOIN Colors C ON c.PersonID = p.ID
 WHERE p.Name = 'John Doe'
   AND c.ColorName IN ('YELLOW');

ID  Name
--  ----------
 1  John Doe