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我心中的TOP1编程语言

洲行 2023-06-22 阅读 63
sqlmysqlpython大数据数据库

文章目录

聚合函数

-- 创建数据库
DROP DATABASE IF EXISTS mydb;
CREATE DATABASE mydb;
USE mydb;

-- 创建student表
CREATE TABLE student (
    sid CHAR(6),
    sname VARCHAR(50),
    age INT,
    gender VARCHAR(50) DEFAULT 'male'
);

-- 向student表插入数据
INSERT INTO student (sid,sname,age,gender) VALUES ('S_1001', 'lili', 14, 'male');
INSERT INTO student (sid,sname,age,gender) VALUES ('S_1002', 'wang', 15, 'female');
INSERT INTO student (sid,sname,age,gender) VALUES ('S_1003', 'tywd', 16, 'male');
INSERT INTO student (sid,sname,age,gender) VALUES ('S_1004', 'hfgs', 17, 'female');
INSERT INTO student (sid,sname,age,gender) VALUES ('S_1005', 'qwer', 18, 'male');
INSERT INTO student (sid,sname,age,gender) VALUES ('S_1006', 'zxsd', 19, 'female');
INSERT INTO student (sid,sname,age,gender) VALUES ('S_1007', 'hjop', 16, 'male');
INSERT INTO student (sid,sname,age,gender) VALUES ('S_1008', 'tyop', 15, 'female');
INSERT INTO student (sid,sname,age,gender) VALUES ('S_1009', 'nhmk', 13, 'male');
INSERT INTO student (sid,sname,age,gender) VALUES ('S_1010', 'xdfv', 17, 'female');

Count()

  • 示例

    select count(*) from student;

Max()

  • 示例

    select max(age) from student;

Min()

  • 示例

    SELECT MIN(age) FROM student;

Sum()

  • 示例

    select sum(age) from student;

Avg()

  • 示例

    select avg(age) from student;

select avg(sname) from student;

其他常用函数

时间函数

SELECT NOW();																# 2023-06-11 09:38:22
SELECT DAY (NOW());													# 11
SELECT DATE (NOW());												# 2023-06-11
SELECT TIME (NOW());												# 09:38:23
SELECT YEAR (NOW());												# 2023
SELECT MONTH (NOW());												# 6
SELECT CURRENT_DATE();											# 2023-06-11
SELECT CURRENT_TIME();											# 09:38:24
SELECT CURRENT_TIMESTAMP();									# 2023-06-11 09:38:24
SELECT ADDTIME('14:23:12','01:02:01');			# 15:25:13
SELECT DATE_ADD(NOW(),INTERVAL 1 DAY);			# 2023-06-12 09:38:25
SELECT DATE_ADD(NOW(),INTERVAL 1 MONTH);		# 2023-07-11 09:38:25
SELECT DATE_SUB(NOW(),INTERVAL 1 DAY);			# 2023-06-10 09:38:25
SELECT DATE_SUB(NOW(),INTERVAL 1 MONTH);		# 2023-05-11 09:38:26
SELECT DATEDIFF('2019-07-22','2019-05-05');	# 78

字符串函数

SUBSTRING函数:切割字符串([start:end])

  • 语法格式

  • 示例

    -- 建表语句
DROP TABLE IF EXISTS order_detail; 

CREATE TABLE order_detail( 
  order_id VARCHAR(8), 
  order_time VARCHAR(32) 
) ENGINE = InnoDB DEFAULT CHARSET = utf8; 

INSERT INTO 
  order_detail (order_id,order_time) 
  VALUE ('o001','2020-06-15 09:12:33') 
  ,('o002','2020-06-15 13:24:09') 
  ,('o003','2020-06-16 10:44:24') 
  ,('o004','2020-06-17 08:05:52') 
  ,('o005','2020-06-18 18:03:43');
 
-- 查询语句
SELECT * ,
SUBSTRING(order_time,1,4) AS order_year,
SUBSTRING(order_time,6,2) AS order_month,
SUBSTRING(order_time,9,2) AS order_day 
FROM order_detail;

SUBSTRING_INDEX函数:切割字符串(split)

  • 语法格式

  • 示例演示

    -- 案例1
SELECT SUBSTRING_INDEX('a*b','*',1) -- a
 
-- 案例2
SELECT SUBSTRING_INDEX('a*b','*',-1) -- b
 
-- 案例3
SELECT SUBSTRING_INDEX(SUBSTRING_INDEX('a*b*c*d*e','*',3),'*',-1) -- c
-- 案例3可拆解成以下两个步骤
SELECT SUBSTRING_INDEX('a*b*c*d*e','*',3); -- a*b*c 
SELECT SUBSTRING_INDEX('a*b*c','*',-1) -- c
 
-- 案例4
-- 如果任一输入参数为NULL，则返回NULL
SELECT SUBSTRING_INDEX('https://www.google.com', null, 2);
SELECT SUBSTRING_INDEX('https://www.google.com', '.', null);
SELECT SUBSTRING_INDEX(null, '.', 2);
 
-- 案例5
-- 如果number超过了实际分隔符的个数，则返回实际个数的字符串
-- https://www.google.com
SELECT SUBSTRING_INDEX('https://www.google.com', '.', 4);

  • 示例实战

    drop table if exists user_submit;
CREATE TABLE `user_submit` (
`id` int NOT NULL,
`device_id` int NOT NULL,
`profile` varchar(100) NOT NULL,
`blog_url` varchar(100) NOT NULL
);
INSERT INTO user_submit VALUES(1,2138,'180cm,75kg,27,male','http:/url/bisdgboy777');
INSERT INTO user_submit VALUES(2,3214,'165cm,45kg,26,female','http:/url/dkittycc');
INSERT INTO user_submit VALUES(3,6543,'178cm,65kg,25,male','http:/url/tigaer');
INSERT INTO user_submit VALUES(4,4321,'171cm,55kg,23,female','http:/url/uhsksd');
INSERT INTO user_submit VALUES(5,2131,'168cm,45kg,22,female','http:/url/sysdney');


SELECT device_id, 
       SUBSTRING_INDEX(blog_url,'/',-1)AS user_name
FROM user_submit;

LOWER&UPPER函数:大小写转化

  • 语法格式

  • 示例

    DROP TABLE IF EXISTS `vendors`;
CREATE TABLE `vendors`  (
  `id` int(11) NOT NULL,
  `vend_name` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Dynamic;
 
INSERT INTO `vendors` VALUES (1, 'Acme');
INSERT INTO `vendors` VALUES (2, 'Anvils R Us');
INSERT INTO `vendors` VALUES (3, 'Furball Inc.');
INSERT INTO `vendors` VALUES (4, 'Jet Set');
INSERT INTO `vendors` VALUES (5, 'Jouets Ft Ours');
INSERT INTO `vendors` VALUES (6, 'LT Supplies');

    SELECT vend_name, 
       UPPER(vend_name)AS vend_name_upper,
       LOWER(vend_name)AS vend_name_lower
FROM vendors
ORDER BY vend_name;

CONCAT&CONCAT_WS函数:连接字符串

  • CONCAT语法格式

  • 注意事项

  • 示例

    -- 结果是MySQL
SELECT CONCAT('My', 'S', 'QL');
 
-- 结果是SQL Runoob Gooogle Facebook
SELECT CONCAT("SQL ", "Runoob ", "Gooogle ", "Facebook");
 
-- 结果是Null
SELECT CONCAT('My', NULL, 'QL'); 
 
-- 一个数字参数被变换为等价的字符串形式
-- 结果是143
SELECT CONCAT(14,3);

  • CONCAT_WS语法格式

  • 注意事项

  • 示例

    -- 结果是data_frog_study
SELECT CONCAT_WS("_", "data", "frog", "study");
 
-- 结果是SQL-Tutorial-is-fun!
SELECT CONCAT_WS("-", "SQL", "Tutorial", "is", "fun!")AS ConcatenatedString;

-- 结果是hello-word!
SELECT CONCAT_WS("-", "hello", null, "word!")AS ConcatenatedString;

REPLACE函数:字符串替换

  • 语法格式

  • 注意事项

  • 示例

    -- 结果是http:www.google.com.cn
SELECT REPLACE('http://www.google.com.cn/','/','');
 
-- 将字符串abc中的字符a替换为字符x
SELECT REPLACE('abc','a','x') -- xbc

-- 13922221111
SELECT REPLACE('13922221111','136','138');

--
SELECT INSTR ();

--统计长度
SELECT LENGTH();

数学函数

ROUND函数:四舍五入

  • 语法格式

  • 示例一:ROUND(X)

    -- ROUND(X)函数将值X四舍五入之后保留整数部分
-- -7
SELECT ROUND(-6.6);
-- -8
SELECT ROUND(-8.49);
-- -9
SELECT ROUND(-8.50);
 
-- 1
SELECT ROUND(1);
-- 2
SELECT ROUND(1.58);
-- 3
SELECT ROUND(3.49);
-- 4
SELECT ROUND(3.50);
 
 
-- ROUND(X)与ROUND(X,0)所得结果相同
-- 1123
SELECT ROUND(1123.26723);
-- 1123
SELECT ROUND(1123.26723,0);

  • 示例二:ROUND(X,D)

    -- 1123.27
SELECT ROUND(1123.26723,2);
-- 1123.3
SELECT ROUND(1123.26723,1);
-- 1123
SELECT ROUND(1123.26723,0);
-- 1120
SELECT ROUND(1123.26723,-1);
-- 1100
SELECT ROUND(1123.26723,-2);
-- 1000
SELECT ROUND(1123.26723,-3);
 
-- 根据参数D值，将参数X四舍五入后得到保留小数点后D位的值，X值的小数位不够D位的补零
-- 如果D为负值，则保留小数点左边D位，先进行四舍五入操作，再将相应的位数值取零
-- -6.7
SELECT ROUND(-6.66,1);
-- 3.33
SELECT ROUND(3.33,3);
-- 90
SELECT ROUND(88.66,-1);
-- 100
SELECT ROUND(88.46,-2);

其他函数

IFNULL函数:判断是否为NULL(类比与dict.get())

  • 语法格式

  • 扩展

  • 示例

    -- 第一个参数为NULL
SELECT IFNULL(NULL, "World"); -- World
 
-- 第一个参数不为NULL
SELECT IFNULL("Hello", "World"); -- Hello
 
-- 1不为NULL
SELECT IFNULL(1,0); -- 1
-- ''字符串不为NULL
SELECT IFNULL('',1); -- ''
 
SELECT IFNULL(NULL,'IFNULL function'); -- IFNULL function

    CREATE TABLE IF NOT EXISTS contacts (
    contact_id INT AUTO_INCREMENT PRIMARY KEY,
    contact_name VARCHAR(20) NOT NULL,
    business_phone VARCHAR(15),
    home_phone VARCHAR(15)
);
 
INSERT INTO contacts(contact_name,business_phone,home_phone)
VALUES('John Doe','(541) 754-3009',NULL),
      ('Cindy Minsu',NULL,'(541) 754-3110'),
      ('Sue Greenspan','(541) 754-3010','(541) 754-3011'),
      ('Lily Bush',NULL,'(541) 754-3111');
      
      
# 如果联系人的商务电话不可用，可以通过查询获得联系人的家庭电话也是一个获取联系方式的办法
SELECT contact_name, IFNULL(business_phone, home_phone)AS phone FROM contacts;


-- 绝对值
SELECT ABS(-136);														# 136
-- 向下取整
SELECT FLOOR(3.14);													# 3
-- 向上取整
SELECT CEILING(3.14);												# 4
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