Mysql 优化连接查询以从A获取数据，条件为B，按B排序
我用以下表格建立了项目间相似性矩阵：Mysql 优化连接查询以从A获取数据，条件为B，按B排序,mysql,join,Mysql,Join,我用以下表格建立了项目间相似性矩阵： items (id, ...) (Primary key `id`) similarities (item1_id, item2_id, similarity) (Index on `item1_id` and `item2_id`) 相似性表包含具有相似性索引的ID对，即： item1_id item2_id similarity 1 2 0.3143 2 3 0.734 为了高效存储
items (id, ...) (Primary key `id`)
similarities (item1_id, item2_id, similarity) (Index on `item1_id` and `item2_id`)
相似性item1_id item2_id similarity
1 2 0.3143
2 3 0.734
item1\u iditem2\u idSELECT `Item`.*
FROM `items` AS `Item`
LEFT JOIN `similarities` AS `Similarity`
ON (`Item`.`id` = `Similarity`.`item1_id`
AND `Similarity`.`item2_id` IN (1, 2, 3, ...))
OR (`Item`.`id` = `Similarity`.`item2_id`
AND `Similarity`.`item1_id` IN (1, 2, ,3, ...))
WHERE `Similarity`.`item1_id` IN (1, 2, 3, ...)
OR `Similarity`.`item2_id` IN (1, 2, 3, ...)
GROUP BY `Item`.`id`
ORDER BY `Similarity`.`similarity` desc
SELECT `Item`.*
FROM `items` AS `Item`
JOIN (
SELECT `item1_id` AS `id`, `similarity`
FROM `similarities`
WHERE `similarities`.`item2_id` IN (1, 2, 3, ...)
UNION
SELECT `item2_id` AS `id`, `similarity`
FROM `similarities`
WHERE `similarities`.`item1_id` IN (1, 2, 3, ...)
) AS `SimilarityUnion` ON `SimilarityUnion`.`id` = `Item`.`id`
GROUP BY `SimilarityUnion`.`id`
ORDER BY `SimilarityUnion`.`similarity` DESC
EXPLAINselect_type table type possible_keys key key_len ref rows Extra
SIMPLE Similarity index_merge item1_id,item2_id item1_id,item2_id 110,110 NULL 31 Using sort_union(item1_id,...
SIMPLE Item ALL PRIMARY NULL NULL NULL 136600 Using where; Using join buffer
我能做些什么来加快速度？最糟糕的情况是，我会在两个单独的查询中执行此操作，但如果可能的话，我更喜欢一个联接查询。我实际上没有尝试此操作，但它可能为您指明了正确的方向。其思想是对（唯一）id的
联合相似性SELECT Item.*, s.other_item_id, s.similarity
FROM items AS Item
JOIN
(
SELECT item1_id AS id, item2_id AS other_item_id, similarity FROM similarities
UNION
SELECT item2_id AS id, item1_id AS other_item_id, similarity FROM similarities
) AS s ON s.id = items.id
WHERE items.id IN (1, 2, 3, ...)
ORDER BY s.similarity DESC;
在原始查询中，您不需要在
JOINWHERE相似性SELECT
CASE WHEN `Item2`.`id` IS NULL THEN
`Item1`.`id`
ELSE `Item2`.`id`
END,
SELECT
CASE WHEN `Item2`.`id` IS NULL THEN
`Item1`.`name`
ELSE `Item2`.`name`
END,
SELECT
CASE WHEN `Item2`.`id` IS NULL THEN
`Item1`.`description`
ELSE `Item2`.`description`
END,
[and so on]
FROM `items` AS `Item1`
LEFT OUTER JOIN `similarities` AS `Similarity`
ON (`Item1`.`id` = `Similarity`.`item1_id`
RIGHT OUTER JOIN `items` AS `Item2`
ON (`Item2`.`id` = `Similarity`.`item2_id`
WHERE `Similarity`.`item1_id` IN (1, 2, 3, ...)
OR `Similarity`.`item2_id` IN (1, 2, 3, ...)
ORDER BY `Similarity`.`similarity` desc
多亏了这些灵感，我最终提出了以下问题：
SELECT `Item`.*
FROM `items` AS `Item`
LEFT JOIN `similarities` AS `Similarity`
ON (`Item`.`id` = `Similarity`.`item1_id`
AND `Similarity`.`item2_id` IN (1, 2, 3, ...))
OR (`Item`.`id` = `Similarity`.`item2_id`
AND `Similarity`.`item1_id` IN (1, 2, ,3, ...))
WHERE `Similarity`.`item1_id` IN (1, 2, 3, ...)
OR `Similarity`.`item2_id` IN (1, 2, 3, ...)
GROUP BY `Item`.`id`
ORDER BY `Similarity`.`similarity` desc
SELECT `Item`.*
FROM `items` AS `Item`
JOIN (
SELECT `item1_id` AS `id`, `similarity`
FROM `similarities`
WHERE `similarities`.`item2_id` IN (1, 2, 3, ...)
UNION
SELECT `item2_id` AS `id`, `similarity`
FROM `similarities`
WHERE `similarities`.`item1_id` IN (1, 2, 3, ...)
) AS `SimilarityUnion` ON `SimilarityUnion`.`id` = `Item`.`id`
GROUP BY `SimilarityUnion`.`id`
ORDER BY `SimilarityUnion`.`similarity` DESC
我真的很想回答这个问题，作为对你帮助的回报。既然点击了，我想我做不到。祝你好运“一组其他项目”是否由（1,2,3，…）表示？谢谢。@Lester是的，这些是我想要获取相似项目的一组项目的ID。所以基本上我的两个独立查询策略合并成了一个查询？：）我试试看。我必须看看如何通过我的DAL来实现这一点。如果你能给出一个期望结果的例子，这会有所帮助。是否要查看给定项目与之类似的其他项目ID？我只想要项目的记录，就像普通的
SELECT*FROM itemsLIMITLIMITSELECTCASEselectitem1.*，Item2.*大小写测试Item1.id ELSE Item2.id END CASE大小写附近出现语法错误…END CASE