使用@rownum:=@rownum+1作为rownum枚举SQL中的每条记录。

Question

我在MySQL中有这个查询

SELECT 
@rownum:=@rownum+1 AS rownum,
notifications_posts.from_user AS source,
notifications_posts.on_post_id AS destination,
notifications_posts.in_group_id,
groups.group_name AS group_name,
notifications_posts.on_post_id AS other_record_id,
user_info.first_name,
user_info.last_name,
user_info.user_id,
notifications_posts.date,
posts.title AS 'title',
user_rights.right AS 'right',
'article' AS notification_type
FROM notifications_posts
INNER
    JOIN user_info
    ON notifications_posts.from_user = user_info.user_id
INNER
    JOIN posts
    ON posts.id = notifications_posts.on_post_id
INNER
    JOIN groups
    ON groups.group_id = notifications_posts.in_group_id
INNER
    JOIN user_rights
    ON user_rights.group_id = notifications_posts.in_group_id AND user_rights.user_id = user_info.user_id

我得到了

​

​

为什么我在rownum上获得NULL

其次，我需要按该id对记录进行分组

GROUP BY rownum;

有谁可以帮我？

Answer 1

我认为更可取的做法是对结果进行排序(在MySQL中)，然后在应用程序中添加行号。如果你坚持这样做：

警告：任何以这种方式使用变量的查询都可能在未来的MySQL版本中被破坏。

SELECT 
    @rownum := @rownum+1 AS rownum,
    notifications_posts.from_user AS source,
    notifications_posts.on_post_id AS destination,
    notifications_posts.in_group_id,
    groups.group_name AS group_name,
    notifications_posts.on_post_id AS other_record_id,
    user_info.first_name,
    user_info.last_name,
    user_info.user_id,
    notifications_posts.date,
    posts.title AS 'title',
    user_rights.right AS 'right',
    'article' AS notification_type
FROM 
    (SELECT @rownum:=0) AS dummy                  -- initial value
CROSS 
    JOIN notifications_posts
INNER
    JOIN user_info
    ON notifications_posts.from_user = user_info.user_id
INNER
    JOIN posts
    ON posts.id = notifications_posts.on_post_id
INNER
    JOIN groups
    ON groups.group_id = notifications_posts.in_group_id
INNER
    JOIN user_rights
    ON user_rights.group_id = notifications_posts.in_group_id 
    AND user_rights.user_id = user_info.user_id 
ORDER BY                                         -- you need 
    whatever ;                                   -- ORDER BY clause

Answer 2

您必须使用(SELECT rownum:=0) r语句初始化变量@rownum

 select  @rownum:=@rownum+1 AS rownum,a.*
     from
     (SELECT 
       notifications_posts.from_user AS source,
       notifications_posts.on_post_id AS destination,
       notifications_posts.in_group_id,
       groups.group_name AS group_name,
       ........
       <remaining query>
       )a,(SELECT @rownum:=0) r 

编辑：所以您的完整查询应该是

SELECT 
@rownum:=@rownum+1 AS rownum,a.*
FROM
(
    SELECT
    notifications_posts.from_user AS source,
    notifications_posts.on_post_id AS destination,
    notifications_posts.in_group_id,
    groups.group_name AS group_name,
    notifications_posts.on_post_id AS other_record_id,
    user_info.first_name,
    user_info.last_name,
    user_info.user_id,
    notifications_posts.date,
    posts.title AS 'title',
    user_rights.right AS 'right',
    'article' AS notification_type
    FROM notifications_posts
    INNER
        JOIN user_info
        ON notifications_posts.from_user = user_info.user_id
    INNER
        JOIN posts
        ON posts.id = notifications_posts.on_post_id
    INNER
        JOIN groups
        ON groups.group_id = notifications_posts.in_group_id
    INNER
        JOIN user_rights
        ON user_rights.group_id = notifications_posts.in_group_id AND user_rights.user_id = user_info.user_id
)a, (SELECT @rownum:=0 )r

Answer 3

在按rownum运行query.

• Grouping之前，

1. 将rownum设置为0不会有任何影响-每行的rownum都不同。