oracle mysql函数的使用方法

-- 字符串连接
SELECT CONCAT('Hello', ' ', 'World') AS result;

-- 字符串截取
SELECT SUBSTRING('Hello World', 1, 5) AS result;

-- 大小写转换
SELECT UPPER('hello world') AS result;
SELECT LOWER('HELLO WORLD') AS result;

-- 求和
SELECT SUM(salary) AS total_salary FROM employees;

-- 平均值
SELECT AVG(salary) AS average_salary FROM employees;

-- 最大值
SELECT MAX(salary) AS max_salary FROM employees;

-- 最小值
SELECT MIN(salary) AS min_salary FROM employees;

-- 获取当前时间
SELECT NOW() AS current_time;

-- 日期加法
SELECT DATE_ADD('2023-01-01', INTERVAL 1 MONTH) AS result;

-- 日期差
SELECT DATEDIFF('2023-02-01', '2023-01-01') AS result;

-- 使用 CASE 语句
SELECT 
    CASE 
        WHEN salary > 5000 THEN 'High'
        WHEN salary > 3000 THEN 'Medium'
        ELSE 'Low'
    END AS salary_level 
FROM employees;