2023.9.3
The SQL statement study notes for single-line functions are as follows:
#常见单行函数介绍（部分省略）
#字符函数
#将姓变大写，名变小写，然后拼接。
SELECT CONCAT(UPPER(`last_name`),' ',LOWER(`first_name`)) AS 姓名 FROM `employees`;
# 姓名中首字符大写，其他字符小写然后用_拼接，显示出来
SELECT CONCAT(UPPER(SUBSTR(`last_name`,1,1)),'_',LOWER(SUBSTR(`last_name`,2))) AS 姓名 FROM `employees`;
#查询员工的工资，要求部门号=30，显示的工资为1.1倍 部门号=40，显示的工资为1.2倍 部门号=50，显示的工资为1.3倍 其他部门，显示的工资为原工资
SELECT salary AS 原工资,`department_id`,
CASE `department_id` WHEN 30 THEN salary*1.1 WHEN 40 THEN salary*1.2 WHEN 50 THEN salary*1.3 ELSE salary END AS 新工资 FROM `employees`;
##案例:查询员工的工资的情况 如果工资>20000,显示A级别 如果工资>15000,显示B级别 如果工资>10000，显示c级别 否则，显示D级别
SELECT salary,
CASE
WHEN salary>20000 THEN 'A'
WHEN salary>15000 THEN 'B'
WHEN salary>10000 THEN 'C'
ELSE 'D'
END AS 工资级别
FROM `employees`;Common function test questions:
SELECT NOW();
SELECT `employee_id`,`last_name`,`salary`,`salary`*1.2 AS 'new salary' FROM `employees`;
SELECT LENGTH(`last_name`) AS 长度,SUBSTR(`last_name`,1,1) AS 首字母,`last_name` FROM `employees` ORDER BY 首字母;
SELECT CONCAT(`last_name`,' earns ',salary,' monthly but wants ',salary*3) AS 'dream salary' FROM `employees` WHERE salary=24000;
SELECT `last_name`,`job_id`,
CASE `job_id`
WHEN 'AD_PRES' THEN 'A'
WHEN 'ST_MAN' THEN 'B'
WHEN 'IT_PROG' THEN 'C'
WHEN 'SA_REP' THEN 'D'
WHEN 'ST_CLERK' THEN 'E'
END AS grade FROM `employees`
WHERE `job_id`='AD_PRES';SQL study notes for grouping functions:
#常见分组函数介绍
#功能：用作统计使用，又称为聚合函数或统计函数或组函数
SELECT SUM(salary) FROM `employees`;
SELECT AVG(salary) FROM `employees`;
SELECT MAX(salary) FROM `employees`;
SELECT MIN(salary) FROM `employees`;
SELECT COUNT(salary) FROM `employees`;
SELECT SUM(salary) AS 薪资和,ROUND(AVG(salary),2) AS 平均薪资 FROM `employees`;
SELECT MAX(`last_name`),MIN(`last_name`) FROM `employees`;
SELECT SUM(`commission_pct`),AVG(`commission_pct`) FROM `employees`;#分组函数会忽略null值。
SELECT COUNT(DISTINCT salary),COUNT(salary) FROM `employees`; #搭配distinct使用
SELECT COUNT(*) FROM `employees`;
SELECT COUNT(1) FROM `employees`;#同上Grouping function test questions:
1. Query the maximum, minimum, average, and total wages of the company’s employees
2. Query the number of days (DIFFRENCE) between the maximum entry time and the minimum entry time in the employee table and query the number of employees with the door number 90
3. Query the number of employees with department number 90
# 分组函数测试题
SELECT MAX(salary),MIN(salary),ROUND(AVG(salary),2),SUM(salary) FROM `employees`;
SELECT DATEDIFF(MAX(`hiredate`),MIN(`hiredate`)) FROM `employees`;
SELECT COUNT(*) AS 个数 FROM `employees` WHERE `department_id`=90;