如果存在或不存在，则表与WHERE子句条件有两个表关系

Question

查询

$query = "SELECT a.title, a.content, SUM(b.rating) as rating FROM posts a, ratings b WHERE a.id = :id AND a.id = b.pid GROUP BY b.author ORDER BY rating";

取决于这些表

                 posts                                       ratings
  [ id  -  title   -   content -  author ]            [ pid  -  rating  -  author ]
   --------------------------------------              ---------------------------
  [ 1  -  title1  -  content1 -  author1 ]            [  1    -   8   -   author1 ]
  [ 2  -  title1  -  content2 -  author2 ]            [  1    -   5   -   author1 ]
  [ 3  -  title1  -  content4 -  author3 ]            [  1    -   9   -   author2 ]
  [ 4  -  title4  -  content4 -  author4 ]            [  1    -   6   -   author4 ]
  [ 5  -  title5  -  content5 -  author5 ]            [  5    -   3   -   author5 ]

现在我正在尝试按ORDER所有评分SUM WHERE a.id = b.pid AND a.author = b.author发帖WHERE a.id = p.id但是它没有按照预期进行排序，如果根本没有评级，则整个查询值变为空，因为子句id为空，我想将其设为SUM = 0，我该如何解决这个问题？

如果在1编辑时echo为author1, author2, author4

，则应显示如下值

ORDER

但它只是随机显示select table1.*, table2.date_in, table2.description from table1 left outer join table2 on table1.id_t1 = table2.id_table1_fk ，如果评级表为空，则根本不会调用任何值。

Answer 1

从不在FROM子句中使用逗号。 始终使用正确的JOIN语法。

要弄清楚你想要什么有点困难。但是你似乎非常关注作者，尽管作者在你的查询中扮演的角色很小。这就是你所描述的：

SELECT p.author, SUM(r.rating) as sumrating
FROM posts p LEFT JOIN
     ratings r
     ON p.id = r.pid AND p.author = r.author
WHERE a.id = :id 
GROUP BY p.author
ORDER BY sumrating;

它对我来说似乎没什么用。