記兩則親身經歷的sql優化技巧：

一、主表數據不到100萬，以下查詢結果集約200左右，第一條sql執行效率為40ms，第二條為200ms，使用上面方法，其效率明顯優于left join：

sql1：

SELECT SQL_NO_CACHE usersr_id ,businessunit_id,ifnull((SELECT name FROM sync_businessunit WHERE id= obj.businessunit_id),'無名' ) businessunit_name,ifnull((SELECT fullname FROM sync_usersys WHERE id= obj.usersr_id),'無名' ) sr_name,SUM(price) price,IFNULL((SELECT target FROM analysis_target WHERE year=2017 and month=4 and usersr_id = obj.usersr_id ),0) target FROM analysis_cusorder obj WHERE send_time >= '2017-02-01' AND send_time < '2017-03-01' #BETWEEN '2017-02-01' and '2017-02-28' GROUP BY usersr_id ORDER BY usersr_id;

sql2：

SELECT SQL_NO_CACHE o.usersr_id,o.businessunit_id,u.`name` as businessunit_name,us.fullname,t.target as zongzhibiao,SUM(o.price) as zongdacheng FROM analysis_cusorder o LEFT JOIN analysis_target t ON o.usersr_id = t.usersr_id and t.`year`=2017 and t.`month` = 4 LEFT JOIN sync_businessunit u ON o.businessunit_id = u.id LEFT JOIN sync_usersys us ON o.usersr_id = us.id WHERE o.send_time BETWEEN '2017-02-01' and '2017-02-28' GROUP BY o.usersr_id;

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二、為針對limit的優化，一般表數據超過1000萬，limit基本就廢了，需采用sql1的方法進行優化，效率相關極為明顯，以下語句為使用php框架后的寫法：

sql1：

$query = $this->db->select('id,third_id,recommend_menus')-> where("id > $maxid and recommend_menus != ''")-> order_by("id asc")-> limit($perpage)-> get('crawler_merchant');

sql2：

$query = $this->db->select('id,third_id,recommend_menus')-> limit($perpage, $offset)-> get('crawler_merchant');

總結

以上是生活随笔為你收集整理的【一起去大厂系列】针对left join以及limit的两条优化小技巧的全部內容，希望文章能夠幫你解決所遇到的問題。

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