Mysql多個連接到一個表具有不同的條件 - 他們去哪裏？

Question

我正在查詢wordpress posts帖子中的一些帖子，並通過多次加入postmeta表獲取多個元值。我對帖子欄和元值也有很多條件。查詢似乎正在工作，但我很好奇，如果我已經把條件放在正確的地方。比較以下查詢，這些查詢將最有效/最有效地獲得帖子ID及其相關元數據？

這一個：

SELECT a.ID as `post_id`, b.meta_value as `metaval1`, c.meta_value as `metaval2`, d.meta_value as `metaval3` 
FROM posts AS a 
JOIN postmeta AS b ON (b.post_id = a.ID AND b.meta_key = 'metakey1') 
LEFT JOIN postmeta as c ON (c.post_id = a.ID AND c.meta_key = 'metakey2') 
LEFT JOIN postmeta as d ON (d.post_id = a.ID AND d.meta_key = 'metakey3') 
WHERE 
    b.meta_value != 10 
    AND a.post_status = 'pending' 
    AND a.post_date < '2013-03-07 00:00:00' 
    AND a.post_type = "post" 
ORDER BY a.post_date DESC 
LIMIT 100; 

或這一個：

SELECT a.ID as `post_id`, b.meta_value as `metaval1`, c.meta_value as `metaval2`, d.meta_value as `metaval3` 
FROM posts AS a 
JOIN postmeta AS b ON (b.post_id = a.ID) 
LEFT JOIN postmeta as c ON (c.post_id = a.ID) 
LEFT JOIN postmeta as d ON (d.post_id = a.ID) 
WHERE 
    b.meta_key = 'metakey1' 
    AND b.meta_value != 10 
    AND c.meta_key = 'metakey2' 
    AND d.meta_key = 'metakey3' 
    AND a.post_status = 'pending' 
    AND a.post_date < '2013-03-07 00:00:00' 
    AND a.post_type = "post" 
ORDER BY a.post_date DESC 
LIMIT 100; 

任何意見，將不勝感激:)

Answer 1

有bewteen你的兩個查詢的差異。

第一個查詢將返回帖子的值，而不管匹配的metakey值的數量是多少（至少對於「b」值不是10的帖子）。您在on子句中進行比較，因此left join處理結果。

第二個查詢只會在所有三個鍵都存在時才返回值。該比較在where子句中，因此left join生成的不匹配行將被過濾掉。

你更喜歡哪一種取決於你想要的結果。兩者都需要大致相同的計算量。

如果你想效率，你也應該嘗試：

SELECT p.id, 
     max(case when pm.meta_key = 'metakey1' then pm.meta_value end) as metavalue1, 
     max(case when pm.meta_key = 'metakey2' then pm.meta_value end) as metavalue2, 
     max(case when pm.meta_key = 'metakey3' then pm.meta_value end) as metavalue3 
FROM posts p join 
    postmeta pm 
    on pm.post_id = p.ID 
WHERE pm.meta_key in ('metakey1', 'metakey2', 'metakey3') and 
     (pm.meta_key <> 'metakey1' or pm.meta_value != 10) and 
     p.post_status = 'pending' and 
     p.post_date < '2013-03-07 00:00:00' and 
     p.post_type = 'post' 
group by p.post_date, p.id 
ORDER BY p.post_date DESC 
LIMIT 100; 

這限制連接數，使用聚合帶來的結果放在一起。您的原始查詢必須掃描結果才能執行order by - 聚合應該大致相同，但使用較少的連接。