mysql查询中计数不同的原因是什么？

Question

当我使用SQL_CALC_FOUND_ROWS 运行以下查询或使用IN 子句在子查询中指定所有颜色RED,BLUE,YELLOW 时，我得到198 的记录计数。但是当我单独做每种颜色时，我得到一个COUNT，加起来就是248 (72 + 59 + 118)。额外的 50 行从何而来？我原以为如果我单独完成它们然后将它们相加，我会得到198，但我得到的是248。我是否正在查看 IN 子句如何不正确地工作？我更喜欢使用COUNT 而不是SQL_CALC_FOUND_ROWS，因为我发现SQL_CALC_FOUND_ROWS 要慢得多（+3 秒）

以下是一些统计数据：

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

这样的事情正在发生：

Persons表

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

Colors表

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

现在在加入Colors 时查询Persons.* 以获得John Doe：

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

您的结果将是一个p.* 行对于 John Doe 下的每种颜色：

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

如果您对此应用COUNT DISTINCT，您最终会得到1。

如果您分别计算Red 和Yellow，则每个查询都会得到一个。把它们加起来，你会得到2。

---

这样看：如果你扔掉 COUNT 而只是 SELECT DISTINCT Persons.* 你会得到这些结果：

John Doe 的红色或黄色：

 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

红色仅适用于 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

黄色只代表 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