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;UPDATE sale set city = "未知"
WHERE city IS NULL;
UPDATE orderitems set
price_new=IFNULL(price_new,5.74);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);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
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大家好，我是老表
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