添加第三個表的加入

Question

此查詢工作正常：

$sqlStr = "SELECT s.loginid, s.title, s.url, s.displayurl, l.username 
      FROM submission AS s, 
        login AS l 
      WHERE s.loginid = l.loginid 
     ORDER BY s.datesubmitted DESC 
      LIMIT 10"; 

將這項工作，如果我想加入第三MySQL表（稱爲「評論」）呢？

$sqlStr = "SELECT s.loginid, s.submissionid s.title, s.url, s.displayurl, l.username, count(c.comment) countComments 
      FROM submission AS s, 
        login AS l, 
        comment AS c, 
      WHERE s.loginid = l.loginid 
       AND s.submissionid = c.submissionid 
     ORDER BY s.datesubmitted DESC 
      LIMIT 10"; 

由於提前，

約翰

Answer 1

你真的應該使用連接，它更清晰，它讓你的加入條件，你的where子句中。此外，當你想在添加第三個表，它更是一致的：

這是你的原代碼，使用顯式聯接語法：

SELECT  s.loginid, s.title, s.url, s.displayurl, l.username 
FROM  submission s 
INNER JOIN login l ON l.loginid = s.loginid 
ORDER BY s.datesubmitted DESC 
LIMIT  10 

然後，如果你想添加第三個表，很容易：

SELECT  s.loginid, s.title, s.url, s.displayurl, l.username 
FROM  submission s 
INNER JOIN login l ON l.loginid = s.loginid 
INNER JOIN comment c ON s.submissionid = c.submissionid 
ORDER BY s.datesubmitted DESC 
LIMIT  10 

另外，這裏有一個技巧。如果你加入的是同一個名字的兩列，您可以使用USING語法：

INNER JOIN login l USING (loginid) 
INNER JOIN comment c USING(submissionid) 

Answer 2

差不多。

您的語法是爲第三個表是正確的（雖然是舊式;它使用隱式聯接）。

但是爲了讓COUNT（c.comment）能夠工作，您必須（或者我應該說「應該」，因爲MySQL在某些語法上是鬆散的）爲選擇添加一個GROUP BY子句。這條款應命名在SELECT每隔一列：

SELECT s.loginid, s.submissionid s.title, s.url, 
     s.displayurl, l.username, count(c.comment) AS countComments 
FROM submission AS s, 
    login AS l, 
    comment AS c, 
WHERE s.loginid = l.loginid AND s.submissionid = c.submissionid 
GROUP BY s.loginid, s.submissionid s.title, s.url, s.displayurl, l.username 
ORDER BY s.datesubmitted DESC 
LIMIT 10 

Answer 3

你將不得不增加一個GROUP BY子句與它（我只是設置在下面的例子中的登錄ID）一個或多個字段以及你在那裏使用COUNT（）函數，所以SQL會像這樣：

$sqlStr = "SELECT s.loginid, s.submissionid s.title, s.url, s.displayurl, l.username, count(c.comment) countComments 
      FROM submission AS s, 
        login AS l, 
        comment AS c 
      WHERE s.loginid = l.loginid 
       AND s.submissionid = c.submissionid 
     GROUP BY s.loginid 
     ORDER BY s.datesubmitted DESC 
      LIMIT 10";