Question

此查询正常工作：

$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";

如果我想加入第三个MySQL表（称为“注释”），它会工作吗？

$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";

提前致谢，

约翰

Answer 1

你应该真正使用连接，它更清晰，它可以使你的连接条件超出你的where子句。此外，当您想要添加第3个表时，它更加一致：

这是您的原始代码，使用显式连接语法：

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

然后，如果你想添加第三个表，那很简单：

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，这是一个技巧。如果您加入的两列名称相同，则可以使用USING语法：

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

Answer 2

几乎。

你的语法是第三个表是正确的（虽然旧式;它使用隐式JOIN）。

但是要使COUNT（c.comment）工作，你必须（或者我应该说“应该”，因为MySQL在某些语法上是松散的）在选择中添加GROUP BY子句。该子句应该为SELECT中的每个其他列命名：

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

你必须添加一个带有一个或多个字段的GROUP BY子句（我在下面的例子中只设置了loginid）以及你在那里使用COUNT（）函数，所以SQL会是这样的：

$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";

将第三个表添加到Join

3 个答案: