Question

有一个子查询，它使用GROUP_CONCAT将结果分组到一个字段中。但查询需要3秒才能运行，而没有使用0.05秒。我索引了accessory.actionid和action.actionid是PK。有没有提高绩效的解决方案？

SELECT
  action.actiondate, acc.acc, acc.acccount
FROM
  `action`
LEFT JOIN 
  (SELECT 
    actionid,
    GROUP_CONCAT(accessory) AS acc,
    COUNT(actionid) AS acccount
  FROM
  accessorieslink 
  GROUP BY actionid) AS acc 
  ON acc.actionid = action.actionid

Answer 1

你可能最好使用它：

SELECT
    a.actiondate, 
    GROUP_CONCAT(IFNULL(al.accessory,'') ) as acc, 
    SUM(IF(al.actionid IS NULL,0,1)) as acccount
FROM
  `action` a
LEFT JOIN accessorieslink al
    ON al.actionid = a.actionid
GROUP BY
    a.actionid
ORDER BY 
    NULL

Answer 2

因为你使group_concat的别名与导致mysql思考的表acc相同。

更改其中一个别名。

   GROUP_CONCAT(accessory) AS acc,
                               ^^^--//------ this

和

  GROUP BY actionid) AS acc 
                        ^^--// and this

在子查询中缓慢GROUP_CONCAT

2 个答案: