Question

我有一个以下SQL查询

SELECT `User`.`username` , Permalink.perma_link_id, Permalink.locale, 
    Permalink.title, DATEDIFF( CURDATE( ) , Permalink.created ) AS dtdiff,
    `TargetSegment`.segment_text, TargetSegment.source_segment_id, 
    TargetSegment.perma_link_id ,TargetSegment.created ,
    TargetSegment.updated,  DATEDIFF( CURDATE( ) , 
    TargetSegment.updated ) AS datediff
FROM `users` AS `User` 
RIGHT JOIN perma_links AS `PermaLink` 
    ON ( `PermaLink`.`username` = `User`.`username` ) 
RIGHT JOIN target_segments AS `TargetSegment` 
    ON ( `TargetSegment`.`username` = `User`.`username` ) 
RIGHT JOIN source_segments AS `SourceSegment` 
    ON ( `SourceSegment`.`source_detail_id` = `PermaLink`.`source_detail_id` ) 
LEFT JOIN source_details AS `SourceDetail` 
    ON ( `SourceSegment`.`source_detail_id` = `SourceDetail`.`id` ) 
WHERE `TargetSegment`.`username` = "xxxx"
AND `TargetSegment`.`segment_text` <> ""
AND `Permalink`.`perma_link_id` = `TargetSegment`.`perma_link_id` 
AND `TargetSegment`.`source_segment_id` = `SourceSegment`.`id` 
AND `Permalink`.`source_detail_id` = `SourceDetail`.`id` 
ORDER BY `TargetSegment`.`updated` DESC 
LIMIT 0 , 10

这个SQL正在为我提取正确的结果。我想从哪个表中确定每一行是否来自，具体到哪个结果是由PermaLink表引起的，哪个来自TargetSegment表。

这是可以实现的吗？

Answer 1

行不是来自一个表或另一个表。每行都来自查询中的所有表。每个表的相应数据连接在一起形成一个长行。

User.username                                                -- from User table

Permalink.perma_link_id                                      -- from Permalink
Permalink.locale                                             -- from Permalink
Permalink.title                                              -- from Permalink
DATEDIFF(CURDATE(), Permalink.created) AS dtdiff             -- from Permalink

TargetSegment.segment_text                                   -- from TargetSegment
TargetSegment.source_segment_id                              -- from TargetSegment
TargetSegment.perma_link_id                                  -- from TargetSegment
TargetSegment.created                                        -- from TargetSegment
TargetSegment.updated                                        -- from TargetSegment
DATEDIFF(CURDATE(), TargetSegment.updated) AS datediff       -- from TargetSegment

如果连接条件由于其中一个表中缺少键而失败，并且该表是LEFT联接中的右表或RIGHT JOIN中的左表，那么该表中的所有值都将为NULL。

Answer 2

我想你想使用别名，如下：

    SELECT `User`.`username` , Permalink.perma_link_id AS p_perma_link_id, Permalink.locale AS p_locale, 
    Permalink.title AS p_title, DATEDIFF( CURDATE( ) , Permalink.created ) AS dtdiff,
    `TargetSegment`.segment_text, TargetSegment.source_segment_id AS t_source_segment_id, 
    TargetSegment.perma_link_id AS t_perma_link ,TargetSegment.created AS t_created,
    TargetSegment.updated AS t_updated,  DATEDIFF( CURDATE( ) , 
    TargetSegment.updated ) AS datediff
FROM `users` AS `User` 
RIGHT JOIN perma_links AS `PermaLink` 
    ON ( `PermaLink`.`username` = `User`.`username` ) 
RIGHT JOIN target_segments AS `TargetSegment` 
    ON ( `TargetSegment`.`username` = `User`.`username` ) 
RIGHT JOIN source_segments AS `SourceSegment` 
    ON ( `SourceSegment`.`source_detail_id` = `PermaLink`.`source_detail_id` ) 
LEFT JOIN source_details AS `SourceDetail` 
    ON ( `SourceSegment`.`source_detail_id` = `SourceDetail`.`id` ) 
WHERE `TargetSegment`.`username` = "xxxx"
AND `TargetSegment`.`segment_text` <> ""
AND `Permalink`.`perma_link_id` = `TargetSegment`.`perma_link_id` 
AND `TargetSegment`.`source_segment_id` = `SourceSegment`.`id` 
AND `Permalink`.`source_detail_id` = `SourceDetail`.`id` 
ORDER BY `TargetSegment`.`updated` DESC 
LIMIT 0 , 10

你会看到以p_为前缀的永久链接列和其他带有t _

的列

区分sql右连接的结果

2 个答案: