2023.9.3

        单行函数的SQL语句学习笔记如下：

#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`;

常见函数测试题：

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';

         分组函数的SQL学习笔记：

#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

分组函数测试题：

1、查询公司员工工资的最大值，最小值，平均值，总和

2、查询员工表中的最大入职时间和最小入职时间的相差天数(DIFFRENCE)查询都门编号为90的员工个数

3、查询部门编号为90的员工个数

# 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;