嵌套选择数据库sql查询

Question

如何订购从数据库中获取的值？

SELECT * FROM comment;
------------------------------------
| commentid | content | read_unread|
------------------------------------
1              hello      Unread
2              hi         read    


Select * From replies;
//commentid Fk from table "  comment  " so it means row 1 from table comments has 2 replies
------------------------------------
| repnum | rep_content | commentid |
------------------------------------
1              see ya        1
2              ok            1

我想像这样在表格html / php中显示

---

comment num |  Content | replies count
--------------------------------------------------   
 1              hello    2
 2              hi       0

//“回复数2”来自表格回复，回复来自表格评论的评论ID 1

我如何通过答复计数的计数在单个查询sql顺序中显示此内容？？？

Answer 1

您可以对SQL查询使用JOIN ：

SELECT c.commentid, c.content, COUNT(r.replies) FROM comment c JOIN replies r ON r.commentid = c.commentID GROUP BY c.commentid, c.content;

此处提供更多信息： http : //dev.mysql.com/doc/refman/5.7/en/join.html

Answer 2

试试吧。

SELECT a.commentid, a.content, count(*) as replies_count
FROM comment a
LEFT JOIN replies b on a.commentid=b.commentid
GROUP BY a.commentid, a.content

Answer 3

离开联接并从答复表计数是您的伙伴。 内部联接将删除尚未得到任何replies comments因此此处需要左联接。 根据您的要求来计算replies数，因此count（*）将不起作用。

SELECT A.commentid AS comment_num, A.content AS Content, count(B.commentid) as replies_count
FROM comment A
LEFT JOIN replies B on A.commentid=B.commentid
GROUP BY A.commentid, A.content

Answer 4

尝试这个 ：

SELECT a.commentid, a.content, count(b.repnum) as replies_count 
FROM comment a left join replies b on a.commentid = b.commentid 
GROUP by a.commentid