左加入同一个表，但具有不同的值？

表-B具有相关ENTITY_ID与attribute_id = 1

表-B具有相关ENTITY_ID与attribute_id = 2

SELECT Table_B.value 
    FROM Table_A 
    LEFT JOIN Table_B 
    ON Table_A.entity_id = Table_B.entity_id 
    WHERE Table_A.date = '2012-7-30 00:00:00' 
    AND Table_B.attribute_id = 1

我想将attribute_id = 1和attribute_id = 2合并到一个查询中。

来源

2012-07-30 Gershon Herczeg

这是一对多的关系吗？我的意思是，这两个属性是在同一列？（这意味着，不同的行）？在这种情况下，Table_b.Value会显示哪个值？从哪一行？ – 2012-07-30 22:14:26

您可以再次加入Table_B，并使用AS给它一个新名称（别名）。

SELECT Table_B.value, Table_B2.value 
FROM Table_A 
LEFT JOIN Table_B 
ON Table_A.entity_id = Table_B.entity_id 
LEFT JOIN Table_B AS Table_B2 
ON Table_A.entity_id = Table_B2.entity_id 
WHERE Table_A.date = '2012-7-30 00:00:00' 
AND Table_B.attribute_id = 1 
AND Table_B2.attribute_id = 2

您还可以将属性的标准，使之更清晰一点，你在做什么：

SELECT Table_B.value, Table_B2.value 
FROM Table_A 
LEFT JOIN Table_B 
ON Table_A.entity_id = Table_B.entity_id AND Table_B.attribute_id = 1 
LEFT JOIN Table_B AS Table_B2 
ON Table_A.entity_id = Table_B2.entity_id AND Table_B2.attribute_id = 2 
WHERE Table_A.date = '2012-7-30 00:00:00'

来源

2012-07-30 22:16:08 Andrew

这些比较不应该是“Table_B2.entity_id”吗？ – 2012-07-30 22:34:19

是的 - 好赶上！固定。 – Andrew 2012-07-31 04:06:30

看起来不错，我会尝试这一点以后谢谢！ – 2012-07-31 13:13:03

那么你可以简单地用两种简单的方法

select 
    t1.entity_id, 
    t2.attribute 
from t1 
left join t2 on t2.entity_id = t1.entity_id;

做

或

select 
    t1.entity_id, 
    group_concat(t2.attribute) as Attr 
from t1 
left join t2 on t2.entity_id = t1.entity_id 
group by t1.entity_id

来源

2012-07-31 03:23:34

为什么如果你想同时获得attribute_id = 1和2，那么你不是使用OR作为条件吗？

SELECT Table_B.value 
FROM Table_A 
LEFT JOIN Table_B 
ON Table_A.entity_id = Table_B.entity_id 
WHERE Table_A.date = '2012-7-30 00:00:00' 
AND (Table_B.attribute_id = 1 OR Table_B.attribute_id = 2)

来源

2012-07-31 04:20:42

左加入同一个表，但具有不同的值？

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