Php 当不存在左联接时，计数表上的行数返回1

例如，我有两张桌子：

发布：

id | author | content | date
1  | Lucas  | Hello!  | 2016
2  | Igor   | Hi!     | 2016

id | post_id | content | date
1  | 2       | hehehe  | 2016
2  | 1       | hahaha  | 2016
3  | 2       | huhuhu  | 2016

评论：

id | author | content | date
1  | Lucas  | Hello!  | 2016
2  | Igor   | Hi!     | 2016

id | post_id | content | date
1  | 2       | hehehe  | 2016
2  | 1       | hahaha  | 2016
3  | 2       | huhuhu  | 2016

我要做一个

选择

，返回所有帖子，并用

post.id=comment.id

对所有评论行进行

计数

所以，我试着：

SELECT p.id, p.author, p.content, p.date, COUNT(*) AS numComments FROM post p LEFT JOIN comment ON p.id = post_id WHERE p.author = '$author' GROUP BY p.id DESC LIMIT 12

SELECT p.id, p.author, p.content, p.date, CASE WHEN COUNT(*) < 1 THEN '0' ELSE COUNT(*) END AS numComments FROM post p LEFT JOIN comment ON p.id = post_id WHERE p.author = '$author' GROUP BY p.id DESC LIMIT 12

我得去做。但是，即使不存在带有
p.id=post\u id
的注释，他也会返回1

所以，我试着：

SELECT p.id, p.author, p.content, p.date, COUNT(*) AS numComments FROM post p LEFT JOIN comment ON p.id = post_id WHERE p.author = '$author' GROUP BY p.id DESC LIMIT 12

SELECT p.id, p.author, p.content, p.date, CASE WHEN COUNT(*) < 1 THEN '0' ELSE COUNT(*) END AS numComments FROM post p LEFT JOIN comment ON p.id = post_id WHERE p.author = '$author' GROUP BY p.id DESC LIMIT 12

选择p.id、p.author、p.content、p.date、CASE WHEN COUNT（*）<1，然后选择“0”ELSE COUNT（*）作为numComments从p.id=post_id的post p LEFT JOIN comment中结束，其中p.author=“$author”GROUP BY p.id DESC LIMIT 12

但结果是一样的。如何做到这一点？
您可以通过这种方式获得计数，最后一种是按订单而不是按分组：

SELECT p.id, p.author, p.content, p.date, 
(select COUNT(*) from comment where p.id = comment.post_id) AS numComments FROM post p 
WHERE p.author = '$author' 
ORDER BY p.id DESC LIMIT 12

您可以通过这种方式获得计数，最后一种是按订单而不是按分组：

SELECT p.id, p.author, p.content, p.date, 
(select COUNT(*) from comment where p.id = comment.post_id) AS numComments FROM post p 
WHERE p.author = '$author' 
ORDER BY p.id DESC LIMIT 12

由于外部联接返回一行，即使没有匹配的数据，您也需要从内部表中计算一列，通常是联接中使用的列：

SELECT p.id, p.author, p.content, p.date, COUNT(post_id) AS numComments
FROM post p LEFT JOIN comment ON p.id = post_id 
WHERE p.author = '$author' 
GROUP BY p.id -- seems to be mysql, otherwise you need to add more columns to the list

如果不想显示计数为零的行，只需切换到
内部联接。
由于外部联接返回一行，即使没有匹配的数据，您也需要从内部表中计算列，通常是联接中使用的列：

SELECT p.id, p.author, p.content, p.date, COUNT(post_id) AS numComments
FROM post p LEFT JOIN comment ON p.id = post_id 
WHERE p.author = '$author' 
GROUP BY p.id -- seems to be mysql, otherwise you need to add more columns to the list

如果不想显示计数为零的行，只需切换到
内部连接