left join same table两次慢查询

Question

LEFT JOIN (SELECT xx_coinType AS yy_type, xx_sell AS sell_xx
           FROM xxs
           ORDER BY xx_id DESC) AS xxs
               ON xxs.yy_type = zz.zz_coin
LEFT JOIN (SELECT xx_coinType AS yy_type, xx_sell AS old_sell_xx
           FROM xxs
           WHERE xx_time < UNIX_TIMESTAMP(DATE_SUB(CURDATE(), INTERVAL 24 HOUR))
           ORDER BY xx_id DESC) AS old_xxs
               ON old_xxs.coin_type = zz.zz_coin

所以我的查询中有 2 个左联接，select 来自同一个 table，但其中一个是 where。由于它与查询中的 table I select 相同，因此速度非常慢。我该如何解决这个问题？

Answer 1

首先，不需要在派生的table中使用order by子句。
其次，您的第一个派生 table 只是选择了 table 中的所有记录，所以我看不出它有什么意义。

试试这个：

LEFT JOIN xxs
               ON xxs.yy_type = zz.zz_coin
LEFT JOIN (SELECT xx_coinType AS yy_type, xx_sell AS old_sell_xx
           FROM xxs
           WHERE xx_time < UNIX_TIMESTAMP(DATE_SUB(CURDATE(), INTERVAL 24 HOUR))
           ) AS old_xxs
               ON old_xxs.coin_type = zz.zz_coin

或者这样：

LEFT JOIN (SELECT xx_coinType AS yy_type, xx_sell AS sell_xx
           FROM xxs
           WHERE xx_time >= UNIX_TIMESTAMP(DATE_SUB(CURDATE(), INTERVAL 24 
           ) AS xxs
               ON xxs.yy_type = zz.zz_coin
LEFT JOIN (SELECT xx_coinType AS yy_type, xx_sell AS old_sell_xx
           FROM xxs
           WHERE xx_time < UNIX_TIMESTAMP(DATE_SUB(CURDATE(), INTERVAL 24 HOUR))
           ) AS old_xxs
               ON old_xxs.coin_type = zz.zz_coin

left join same table两次慢查询

left join same table twice slow query

sql

left-join