在mysql查詢中計數不同的原因是什麼？

Question

當我使用SQL_CALC_FOUND_ROWS運行以下查詢或使用IN子句在子查詢中指定所有顏色RED,BLUE,YELLOW時，我得到的記錄數爲198。但是，當我單獨執行每種顏色時，我會得到一個COUNT，合計爲248 (72 + 59 + 118)。額外的50行從哪裏來？我會想，如果我單獨做了他們，然後加上他們，我會得到198，但我得到248。我是否查看IN子句如何正確工作？我寧願用COUNT代替SQL_CALC_FOUND_ROWS因爲我發現SQL_CALC_FOUND_ROWS要慢得多（+3秒），

這裏有一些數據：

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

像這樣的事情正在發生：

Persons表

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

Colors表

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

現在查詢Persons.*爲John Doe同時加入到Colors：

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

你的結果會是下李四一個p.*每種顏色行：

​​

如果應用COUNT DISTINCT對此，您將以1結束。

如果分別計算Red和Yellow，則每個查詢都會得到一個。把它們加起來，你會得到2。

---

看看這樣說：如果你扔出去的COUNT，只是SELECT DISTINCT Persons.*你得到這些結果：

紅色或黃色的李四：

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 

只紅對於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 

只爲李四黃色：

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

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