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
代码语言：javascript复制SELECT * FROM t_user
Where (username,password) in
(
SELECT username,password
FROM t_user
group by username,password
having count(username)>1
);查找去重记录查找id最大的记录
代码语言：javascript复制SELECT * FROM t_user
WHERE id in
(SELECT max(id) FROM t_user
group by username,password
having count(username)>1
);删除重复记录只保留id值最小的记录
代码语言：javascript复制DELETE c1
FROM customer c1,customer c2
WHERE c1.cust_email=c2.cust_email
AND c1.id>c2.id;DELETE FROM user Where (username,password) in
(SELECT username,password FROM
(SELECT username,password FROM t_user
group by username,password
having count(username)>1) as tmp1
)
and id not in
(SELECT id FROM
(SELECT min(id) id FROM t_user
group by username,password
having count(username)>1) as tmp2
);2 缺失值处理查找缺失值记录
代码语言：javascript复制SELECT * FROM customer
WHERE cust_email IS NULL;更新列填充空值
代码语言：javascript复制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
代码语言：javascript复制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
代码语言：javascript复制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
代码语言：javascript复制SELECT * FROM orderitems
ORDER BY price_new DESC,quantity;查询排名前几的记录
代码语言：javascript复制SELECT * FROM orderitems
ORDER BY price_new DESC LIMIT 5;查询第10大的值
代码语言：javascript复制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
代码语言：javascript复制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
代码语言：javascript复制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()
left：截取字符串左边的3个字符
代码语言：javascript复制SELECT left('example.com', 3);substring：从字符串的第 4 个字符位置开始取，直到结束
代码语言：javascript复制SELECT substring('example.com', 4);substring：从字符串的第 4 个字符位置开始取，只取 2 个字符
代码语言：javascript复制SELECT substring('example.com', 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
代码语言：javascript复制SELECT substring_index('www.google.com','.',1);取倒数第二个分隔符之后的所有字符，结果是google.com;
代码语言：javascript复制SELECT substring_index('www.google.com','.',-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
代码语言：javascript复制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% _ [] ^
代码语言：javascript复制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
代码语言：javascript复制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
代码语言：javascript复制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
代码语言：javascript复制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
