1.LEFT function

LEFT function Used to extract from a given string left Extract a specified number of characters

Grammatical structure

LEFT(str,len)

str: Given string , The character... Will be extracted from its left

len: Number of characters to extract , If this parameter is greater than the number of characters in the string , Then this function will return the actual string

Be careful :  If any parameter is Null, The result returned to Null

Example  

--  The result is empty. 
SELECT LEFT('abcdefg',0);
-- abc
SELECT LEFT('abcdefg',3);
-- abcdefg
SELECT LEFT('abcdefg',10);
-- Null
SELECT LEFT('abcd',NULL);
-- LEFT Function can intercept numbers 
-- 12
SELECT LEFT(123456,2);

2.RIGHT function

RIGHT function Used to extract from a given string On the right side Extract a specified number of characters

Grammatical structure

RIGHT(str,len)

str: Given string , The character... Will be extracted from its right

len: Number of characters to extract , If this parameter is greater than the number of characters in the string , Then this function will return the actual string

Be careful :  If any parameter is Null, The result returned to Null

Example  

--  The result is empty. 
SELECT RIGHT('abcdefg',0);
-- efg
SELECT RIGHT('abcdefg',3);
-- abcdefg
SELECT RIGHT('abcdefg',10);
-- Null
SELECT RIGHT('abcd',NULL);
-- RIGHT Function can intercept numbers 
-- 56
SELECT RIGHT(123456,2);

Practice cases  

Import data

DROP TABLE IF EXISTS `employee_info`;
CREATE TABLE `employee_info` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) DEFAULT NULL,
  `phone` varchar(255) DEFAULT NULL,
  `salary` int(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8mb4;

INSERT INTO `employee_info` VALUES ('1', 'Odin', '13810809401', '10000');
INSERT INTO `employee_info` VALUES ('2', 'Kacky', '15820126789', '12000');
INSERT INTO `employee_info` VALUES ('3', 'Jerry', '15815810158', '11000');
INSERT INTO `employee_info` VALUES ('4', 'Harry', '15820176889', '13000');
INSERT INTO `employee_info` VALUES ('5', 'Sun', '13578945621', '9000');

employee_info surface  

problem :  Encrypt the employee's mobile phone number ( for example :138****9401) , By salary ascending order name,phone,salary Format output data

SELECT name, CONCAT(LEFT(phone,3),'****',RIGHT(phone,4))AS phone, salary
FROM employee_info
ORDER BY salary ASC;

Result display :