SQL专栏

SQL数据库基础知识汇总

SQL数据库高级知识汇总

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.com', 3);

从字符串的第 4 个字符位置开始取，直到结束

SELECT substring('example.com', 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.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

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;

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

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