php - MYSQL JOIN所有子行，父行只显示一次

Question

我有一个简单的文章和评论系统，包含以下表格:

文章表:

id | writer | text
 1 | Bob    | first article
 2 | Marley | second article

评论表:

id | article_id | comment
 1 |      1     |  i love this article
 2 |      1     |  good one
 3 |      2     |  waiting for more

我想选择每篇文章及其下方的评论。我使用以下查询:

SELECT * FROM articles LEFT JOIN comments ON articles.id = comments.article_id 

我得到的结果:

 articles.id   | articles.writer | articles.text  | comments.id | comments.article_id | comments.comment
 1             | Bob             | first article  |    1        |         1           | i love this article
 1             | Bob             | first article  |    2        |         1           | good one  
 2             | Marley          | second article |    3        |         2           | waiting for more   

我想要的:

articles.id   | articles.writer | articles.text  | comments.id | comments.article_id | comments.comment
 1            | Bob             | first article  |    1        |         1           | i love this article
NULL          | NULL            |    NULL        |    2        |         1           | good one  
 2            | Marley          | second article |    3        |         2           | waiting for more 

那么我如何选择每篇文章及其评论并仅显示一次文章而不是每条评论

谢谢

Answer 1

您可以在 MySQL 中使用用户变量来执行此操作:

SELECT 
  case when rownum =1 then id else null end id,
  case when rownum =1 then writer else null end writer,
  case when rownum =1 then text else null end text,
  comment
FROM
(
  SELECT a.id, a.writer, a.text,
    c.article_id, c.comment, 
    @rownum := case
                when @prev = a.id
                  and @prev_art = c.article_id
                then @rownum+1 else 1 end rownum,
    @prev := a.id p_id,
    @prev_art := c.article_id p_art
  FROM articles a
  LEFT JOIN comments c
    ON a.id = c.article_id 
  ORDER BY a.id, article_id
) src

参见 SQL Fiddle with Demo .

结果是:

|     ID | WRITER |           TEXT |             COMMENT |
----------------------------------------------------------
|      1 |    Bob |  first article | i love this article |
| (null) | (null) |         (null) |            good one |
|      2 | Marley | second article |    waiting for more |

编辑，如果你需要comment.id，那么你可以将它添加到结果中:

SELECT 
  case when rownum =1 then id else null end id,
  case when rownum =1 then writer else null end writer,
  case when rownum =1 then text else null end text,
  commentid,
  article_id,
  comment
FROM
(
  SELECT a.id, a.writer, a.text,
    c.article_id, c.comment, c.id commentid,
    @rownum := case
                when @prev = a.id
                  and @prev_art = c.article_id
                then @rownum+1 else 1 end rownum,
    @prev := a.id p_id,
    @prev_art := c.article_id p_art
  FROM articles a
  LEFT JOIN comments c
    ON a.id = c.article_id 
  ORDER BY a.id, article_id
) src

参见 SQL Fiddle with Demo .结果是:

|     ID | WRITER |           TEXT | COMMENTID | ARTICLE_ID |             COMMENT |
-----------------------------------------------------------------------------------
|      1 |    Bob |  first article |         1 |          1 | i love this article |
| (null) | (null) |         (null) |         2 |          1 |            good one |
|      2 | Marley | second article |         3 |          2 |    waiting for more |