Excel是数据分析中最常用的工具 ，利用Excel可以完成数据清洗，预处理，以及最常见的数据分类，数据筛选，分类汇总，以及数据透视等操作，而这些操作用SQL一样可以实现。SQL不仅可以从数据库中读取数据，还能通过不同的SQL函数语句直接返回所需要的结果，从而大大提高了自己在客户端应用程序中计算的效率。
1 重复数据处理
查找重复记录
SELECT * FROM user Where (nick_name, password ) in ( SELECT nick_name, password FROM user group by nick_name, password having count (nick_name)>1 );查找去重记录
查找id最大的记录
SELECT * FROM user WHERE id in ( SELECT max (id) FROM user group by nick_name, password having count (nick_name)>1 );删除重复记录
只保留id值最小的记录
DELETE c1 FROM customer c1,customer c2 WHERE c1.cust_email=c2.cust_email AND c1.id>c2.id; DELETE FROM user Where (nick_name, password ) in ( SELECT nick_name, password FROM ( SELECT nick_name, password FROM user group by nick_name, password having count (nick_name)>1) as tmp1 ) and id not in ( SELECT id FROM ( SELECT min (id) id FROM user group by nick_name, password having count (nick_name)>1) as tmp2 );2 缺失值处理
查找缺失值记录
SELECT * FROM customer WHERE cust_email IS NULL ;更新列填充空值
UPDATE sale set city = "未知" WHERE city IS NULL ; UPDATE orderitems set price_new=IFNULL(price_new,5.74);查询并填充空值列
SELECT AVG (price_new) FROM orderitems; SELECT IFNULL(price_new,5.74) AS bus_ifnull FROM orderitems;3 计算列
更新表添加计算列
ALTER TABLE orderitems ADD price_new DECIMAL (8,2) NOT NULL ; UPDATE orderitems set price_new= item_price* count ;查询计算列
SELECT item_price* count as sales FROM orderitems;4 排序
多列排序
SELECT * FROM orderitems ORDER BY price_new DESC ,quantity;查询排名前几的记录
SELECT * FROM orderitems ORDER BY price_new DESC LIMIT 5;查询第10大的值
SELECT DISTINCT price_new FROM orderitems ORDER BY price_new DESC LIMIT 9,1;排名
数值相同的排名相同且排名连续
SELECT prod_price, ( SELECT COUNT ( DISTINCT prod_price) FROM products WHERE prod_price>=a.prod_price ) AS rank FROM products AS a ORDER BY rank ;5 字符串处理
字符串替换
UPDATE data1 SET city= REPLACE (city, 'SH' , 'shanghai' ); SELECT city FROM data1;按位置字符串截取
字符串截取可用于数据分列
MySQL 字符串截取函数：left(), right(), substring(), substring_index()
SELECT left ( 'example测试数据' , 3);从字符串的第 4 个字符位置开始取，直到结束
SELECT substring ( 'example测试数据' , 4);从字符串的第 4 个字符位置开始取，只取 2 个字符
SELECT substring ( 'example测试数据' , 4, 2);按关键字截取字符串
取第一个分隔符之前的所有字符，结果是www
SELECT substring_index( 'HdhCmsTestgoogle测试数据' , '.' ,1);取倒数第二个分隔符之后的所有字符，结果是google测试数据;
SELECT substring_index( 'HdhCmsTestgoogle测试数据' , '.' ,-2);6 筛选
通过操作符实现高级筛选
使用 AND OR IN NOT 等操作符实现高级筛选过滤
SELECT prod_name，prod_price FROM Products WHERE vend_id IN ( 'DLL01' , 'BRS01' ); SELECT prod_name FROM Products WHERE NOT vend_id= 'DLL01' ;通配符筛选
常用通配符有% _ [] ^
SELECT * from customers WHERE country LIKE "CH%" ;7 表联结
SQL表连接可以实现类似于Excel中的Vlookup函数的功能
SELECT vend_id,prod_name,prod_price FROM Vendors INNER JOIN Products ON Vendors.vend_id=Products.vend_id; SELECT prod_name,vend_name,prod_price,quantity FROM OderItems,Products,Vendors WHERE Products.vend_id=Vendors.vend_id AND OrderItems.prod_id=Products.prod_id AND order_num=20007;自联结 在一条SELECT语句中多次使用相同的表
SELECT c1.cust_od,c1.cust_name,c1.cust_contact FROM Customers as c1,Customers as c2 WHERE c1.cust_name=c2.cust_name AND c2.cust_contact= 'Jim Jones' ;8 数据透视
数据分组可以实现Excel中数据透视表的功能
数据分组
group by 用于数据分组 having 用于分组后数据的过滤
SELECT order_num, COUNT (*) as items FROM OrderItems GROUP BY order_num HAVING COUNT (*)>=3;交叉表
通过CASE WHEN函数实现
SELECT data1.city, CASE WHEN colour = "A" THEN price END AS A, CASE WHEN colour = "B" THEN price END AS B, CASE WHEN colour = "C" THEN price END AS C, CASE WHEN colour = "F" THEN price END AS F FROM data1原文链接：https://mp.weixin.qq测试数据/s?__biz=MzA3MTg4NjY4Mw==&mid=2457324446&idx=2&sn=9cbf3a9184e66e115c867a3326ca88e9&chksm=88a5d9aabfd250bc3f12aca480686547379692d25efc1e3b223670f27c1a8473f2329729dc9f&mpshare=1&
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