Mysql中，這21個寫SQL的好習慣，你值得擁有呀

{"type":"doc","content":[{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"前言"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"每一個好習慣都是一筆財富，本文分SQL後悔藥， SQL性能優化，SQL規範優雅三個方向，分享寫SQL的21個好習慣，謝謝閱讀，加油哈~"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"公衆號："},{"type":"text","marks":[{"type":"strong"}],"text":"撿田螺的小男孩"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"1. 寫完SQL先explain查看執行計劃（SQL性能優化）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"日常開發寫SQL的時候，儘量養成這個好習慣呀：寫完SQL後，用explain分析一下，尤其注意走不走索引。"}]},{"type":"codeblock","attrs":{"lang":""},"content":[{"type":"text","text":"explain select * from user where userid =10086 or age =18;"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/16/1687a0e98511faa3d25be6e6d3a87c73.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"2、操作delete或者update語句，加個limit(SQL後悔藥）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在執行刪除或者更新語句，儘量加上limit，以下面的這條 SQL 爲例吧："}]},{"type":"codeblock","attrs":{"lang":""},"content":[{"type":"text","text":"delete from euser where age > 30 limit 200;"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"因爲加了limit 主要有這些好處："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/f1/f1d20d30dc0cc40d5b1944495f1c244f.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"降低寫錯SQL的代價"},{"type":"text","text":", 你在命令行執行這個SQL的時候，如果不加limit，執行的時候一個"},{"type":"text","marks":[{"type":"strong"}],"text":"不小心手抖"},{"type":"text","text":"，可能數據全刪掉了，如果"},{"type":"text","marks":[{"type":"strong"}],"text":"刪錯"},{"type":"text","text":"了呢？加了limit 200，就不一樣了。刪錯也只是丟失200條數據，可以通過binlog日誌快速恢復的。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"SQL效率很可能更高"},{"type":"text","text":"，你在SQL行中，加了limit 1，如果第一條就命中目標return， 沒有limit的話，還會繼續執行掃描表。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"避免了長事務"},{"type":"text","text":"，delete執行時,如果age加了索引，MySQL會將所有相關的行加寫鎖和間隙鎖，所有執行相關行會被鎖住，如果刪除數量大，會直接影響相關業務無法使用。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"數據量大的話，容易把CPU打滿"},{"type":"text","text":" ,如果你刪除數據量很大時，不加 limit限制一下記錄數，容易把cpu打滿，導致越刪越慢的。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"3. 設計表的時候，所有表和字段都添加相應的註釋（SQL規範優雅）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這個好習慣一定要養成啦，設計數據庫表的時候，所有表和字段都添加相應的註釋，後面更容易維護。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"正例："}]},{"type":"codeblock","attrs":{"lang":""},"content":[{"type":"text","text":"CREATE TABLE `account` (\n `id` int(11) NOT NULL AUTO_INCREMENT COMMENT '主鍵Id',\n `name` varchar(255) DEFAULT NULL COMMENT '賬戶名',\n `balance` int(11) DEFAULT NULL COMMENT '餘額',\n `create_time` datetime NOT NULL COMMENT '創建時間',\n `update_time` datetime NOT NULL ON UPDATE CURRENT_TIMESTAMP COMMENT '更新時間',\n PRIMARY KEY (`id`),\n KEY `idx_name` (`name`) USING BTREE\n) ENGINE=InnoDB AUTO_INCREMENT=1570068 DEFAULT CHARSET=utf8 ROW_FORMAT=REDUNDANT COMMENT='賬戶表';"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"反例："}]},{"type":"codeblock","attrs":{"lang":""},"content":[{"type":"text","text":"CREATE TABLE `account` (\n `id` int(11) NOT NULL AUTO_INCREMENT,\n `name` varchar(255) DEFAULT NULL,\n `balance` int(11) DEFAULT NULL,\n `create_time` datetime NOT NULL ,\n `update_time` datetime NOT NULL ON UPDATE CURRENT_TIMESTAMP,\n PRIMARY KEY (`id`),\n KEY `idx_name` (`name`) USING BTREE\n) ENGINE=InnoDB AUTO_INCREMENT=1570068 DEFAULT CHARSET=utf8;"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"4. SQL書寫格式，關鍵字大小保持一致，使用縮進。（SQL規範優雅）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"正例："}]},{"type":"codeblock","attrs":{"lang":""},"content":[{"type":"text","text":"SELECT stu.name, sum(stu.score)\nFROM Student stu\nWHERE stu.classNo = '1班'\nGROUP BY stu.name"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"反例："}]},{"type":"codeblock","attrs":{"lang":""},"content":[{"type":"text","text":"SELECT stu.name, sum(stu.score) from Student stu WHERE stu.classNo = '1班' group by stu.name."}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"顯然，統一關鍵字大小寫一致，使用縮進對齊，會使你的SQL看起來更優雅~"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"5. INSERT語句標明對應的字段名稱（SQL規範優雅）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"反例："}]},{"type":"codeblock","attrs":{"lang":""},"content":[{"type":"text","text":"insert into Student values ('666','撿田螺的小男孩','100');"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"正例："}]},{"type":"codeblock","attrs":{"lang":""},"content":[{"type":"text","text":"insert into Student(student_id,name,score) values ('666','撿田螺的小男孩','100');"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"6. 變更SQL操作先在測試環境執行，寫明詳細的操作步驟以及回滾方案，並在上生產前review。（SQL後悔藥）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"變更SQL操作先在測試環境測試，避免有語法錯誤就放到生產上了。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"變更Sql操作需要寫明詳細操作步驟，尤其有依賴關係的時候，如：先修改表結構再補充對應的數據。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"變更Sql操作有回滾方案，並在上生產前，review對應變更SQL。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"7.設計數據庫表的時候，加上三個字段：主鍵，create"},{"type":"text","marks":[{"type":"italic"}],"text":"time,update"},{"type":"text","text":"time。（SQL規範優雅）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"反例："}]},{"type":"codeblock","attrs":{"lang":""},"content":[{"type":"text","text":"CREATE TABLE `account` (\n `name` varchar(255) DEFAULT NULL COMMENT '賬戶名',\n `balance` int(11) DEFAULT NULL COMMENT '餘額',\n) ENGINE=InnoDB AUTO_INCREMENT=1570068 DEFAULT CHARSET=utf8 ROW_FORMAT=REDUNDANT COMMENT='賬戶表';"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"正例："}]},{"type":"codeblock","attrs":{"lang":""},"content":[{"type":"text","text":"CREATE TABLE `account` (\n `id` int(11) NOT NULL AUTO_INCREMENT COMMENT '主鍵Id',\n `name` varchar(255) DEFAULT NULL COMMENT '賬戶名',\n `balance` int(11) DEFAULT NULL COMMENT '餘額',\n `create_time` datetime NOT NULL COMMENT '創建時間',\n `update_time` datetime NOT NULL ON UPDATE CURRENT_TIMESTAMP COMMENT '更新時間',\n PRIMARY KEY (`id`),\n KEY `idx_name` (`name`) USING BTREE\n) ENGINE=InnoDB AUTO_INCREMENT=1570068 DEFAULT CHARSET=utf8 ROW_FORMAT=REDUNDANT COMMENT='賬戶表';"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"理由："}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"主鍵一定要加上的，沒有主鍵的表是沒有靈魂的"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"創建時間和更新時間的話，還是建議加上吧，詳細審計、跟蹤記錄，都是有用的。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"阿里開發手冊也提到這個點，如圖"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/44/44a366ff10071b4f73aa6e07e8c51f1d.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"8. 寫完SQL語句，檢查where,order by,group by後面的列，多表關聯的列是否已加索引，優先考慮組合索引。（SQL性能優化）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"反例："}]},{"type":"codeblock","attrs":{"lang":""},"content":[{"type":"text","text":"select * from user where address ='深圳' order by age ;"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/51/5160b3e48a0fcedfa92fac90c0c1716a.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"正例："}]},{"type":"codeblock","attrs":{"lang":""},"content":[{"type":"text","text":"添加索引\nalter table user add index idx_address_age (address,age)"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/ec/ec9bc739f772a43af59104fdf888b158.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"9.修改或刪除重要數據前，要先備份，先備份，先備份（SQL後悔藥）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果要修改或刪除數據，在執行SQL前一定要先備份要修改的數據，萬一誤操作，還能喫口"},{"type":"text","marks":[{"type":"strong"}],"text":"後悔藥"},{"type":"text","text":"~"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"10. where後面的字段，留意其數據類型的隱式轉換（SQL性能優化）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"反例："}]},{"type":"codeblock","attrs":{"lang":""},"content":[{"type":"text","text":"//userid 是varchar字符串類型\nselect * from user where userid =123;"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/b5/b5c8ee27ad79d4c7b73eb3682e47a298.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"正例："}]},{"type":"codeblock","attrs":{"lang":""},"content":[{"type":"text","text":"select * from user where userid ='123';"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/27/2756500a2374bb348222ecefc640e935.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"理由："}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"因爲不加單引號時，是字符串跟數字的比較，它們類型不匹配，MySQL會做隱式的類型轉換，把它們轉換爲浮點數再做比較，最後導致索引失效"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":" 11. 儘量把所有列定義爲NOT NULL（SQL規範優雅）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"NOT NULL列更節省空間"},{"type":"text","text":"，NULL列需要一個額外字節作爲判斷是否爲 NULL 的標誌位。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"NULL列需要注意空指針問題"},{"type":"text","text":"，NULL列在計算和比較的時候，需要注意空指針問題。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"12.修改或者刪除SQL，先寫WHERE查一下，確認後再補充 delete 或 update（SQL後悔藥）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"尤其在操作生產的數據時，遇到修改或者刪除的SQL，先加個where查詢一下，確認OK之後，再執行update或者delete操作"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"13.減少不必要的字段返回，如使用select 代替 select * （SQL性能優化）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"反例："}]},{"type":"codeblock","attrs":{"lang":""},"content":[{"type":"text","text":"select * from employee;"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"正例："}]},{"type":"codeblock","attrs":{"lang":""},"content":[{"type":"text","text":"select id，name from 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