MySQL 8.0常用内置函数超详细完整指南

-- 假设有一张 products 表
-- 总产品数
SELECT COUNT(*) AS total_products FROM products;
​
-- 有效库存记录数（stock 列非 NULL）
SELECT COUNT(stock) AS stock_count FROM products;
​
-- 不同类别的数量
SELECT COUNT(DISTINCT category) AS category_count FROM products;
​
-- 总库存
SELECT SUM(stock) AS total_stock FROM products;
​
-- 平均价格
SELECT AVG(price) AS avg_price FROM products;
​
-- 最高价格
SELECT MAX(price) AS max_price FROM products;
​
-- 最低价格
SELECT MIN(price) AS min_price FROM products;

-- 示例对比
SELECT 
    COUNT(1)         AS all_rows,       -- 所有行
    COUNT(*)         AS all_rows_too,   -- 所有行（同上）
    COUNT(email)     AS has_email,      -- 邮件不为空的行
    COUNT(DISTINCT department) AS dept_count  -- 部门数量
FROM employees;

-- 将每个部门的人员姓名合并
SELECT 
    department,
    GROUP_CONCAT(name ORDER BY name SEPARATOR ', ') AS members
FROM employees
GROUP BY department;
​
-- 结果类似：IT | Alice, Bob, Charlie

-- 将分组数据聚合为 JSON 数组
SELECT department, JSON_ARRAYAGG(name) AS names FROM employees GROUP BY department;
​
-- 将分组数据聚合为 JSON 对象
SELECT department, JSON_OBJECTAGG(emp_id, name) AS emp_map FROM employees GROUP BY department;

-- 拼接姓名
SELECT CONCAT(first_name, ' ', last_name) AS full_name FROM users;
-- 结果：Alice Smith
​
-- 用分隔符拼接
SELECT CONCAT_WS('-', '2024', '01', '15') AS date_str;
-- 结果：2024-01-15
​
-- 截取子字符串（pos 从 1 开始）
SELECT SUBSTRING('Hello World', 1, 5) AS result;
-- 结果：Hello
​
-- 手机号脱敏
SELECT CONCAT(LEFT(phone, 3), '****', RIGHT(phone, 4)) AS masked_phone FROM users;
-- 结果：138****1234

SELECT UPPER('mysql') AS result;         -- MYSQL
SELECT LOWER('MYSQL') AS result;         -- mysql
SELECT TRIM('  hello  ') AS result;      -- hello
SELECT TRIM(LEADING '0' FROM '00123') AS result;  -- 123
SELECT TRIM(TRAILING 'x' FROM 'abcxxx') AS result; -- abc

-- 左侧补零（常用于生成工号）
SELECT LPAD('123', 8, '0') AS emp_id;
-- 结果：00000123
​
-- 右侧填充
SELECT RPAD('MySQL', 10, '*') AS result;
-- 结果：MySQL*****
​
-- 重复
SELECT REPEAT('Go', 3) AS result;
-- 结果：GoGoGo

-- 查找位置（不区分大小写）
SELECT LOCATE('world', 'Hello World') AS position;  -- 7
SELECT INSTR('Hello World', 'world') AS position;    -- 7
​
-- SQL 标准写法
SELECT POSITION('world' IN 'Hello World') AS position; -- 7
​
-- 替换
SELECT REPLACE('Hello World', 'World', 'MySQL') AS result;
-- Hello MySQL
​
-- 指定位置替换
SELECT INSERT('Hello World', 7, 5, 'CSDN') AS result;
-- Hello CSDN

SELECT 
    CHAR_LENGTH('Hello')    AS char_len,   -- 5（字符数）
    LENGTH('Hello')         AS byte_len,   -- 5（字节数）
    CHAR_LENGTH('你好')      AS cn_char_len,-- 2（字符数）
    LENGTH('你好')           AS cn_byte_len; -- 6（UTF-8 每中文字符 3 字节）

SELECT NOW();          -- 2024-01-15 10:30:45
SELECT CURDATE();      -- 2024-01-15
SELECT CURTIME();      -- 10:30:45
SELECT UTC_TIMESTAMP();-- 2024-01-15 02:30:45（UTC 时间）

-- 加 3 天
SELECT DATE_ADD('2024-01-15', INTERVAL 3 DAY) AS result;
-- 2024-01-18
​
-- 减 1 个月
SELECT DATE_SUB('2024-01-15', INTERVAL 1 MONTH) AS result;
-- 2023-12-15
​
-- 日期差
SELECT DATEDIFF('2024-03-01', '2024-02-15') AS days_diff;
-- 15
​
-- 时间差
SELECT TIMESTAMPDIFF(HOUR, '2024-01-15 08:00:00', '2024-01-15 17:30:00') AS hours;
-- 9
​
-- 计算年龄
SELECT TIMESTAMPDIFF(YEAR, '1990-05-20', CURDATE()) AS age;

-- 格式化
SELECT DATE_FORMAT(NOW(), '%Y年%m月%d日 %H:%i:%s') AS formatted;
-- 2024年01月15日 10:30:45
​
-- 字符串转日期
SELECT STR_TO_DATE('2024-01-15', '%Y-%m-%d') AS date_val;
-- 2024-01-15
​
-- 时间格式化
SELECT TIME_FORMAT('10:30:45', '%H时%i分%s秒') AS time_str;
-- 10时30分45秒

-- 假设有 scores 表：student, subject, score
SELECT 
    student, subject, score,
    ROW_NUMBER() OVER (ORDER BY score DESC) AS row_num,
    RANK()       OVER (ORDER BY score DESC) AS rank_val,
    DENSE_RANK() OVER (ORDER BY score DESC) AS dense_rank_val
FROM scores;
​
-- 结果对比（分数相同时）：
-- student | score | row_num | rank_val | dense_rank_val
-- Alice   | 95    | 1       | 1        | 1
-- Bob     | 95    | 2       | 1        | 1
-- Charlie | 90    | 3       | 3        | 2

-- LAG/LEAD 计算环比
SELECT 
    order_date,
    daily_amount,
    LAG(daily_amount, 1) OVER (ORDER BY order_date) AS prev_day_amount,
    daily_amount - LAG(daily_amount, 1) OVER (ORDER BY order_date) AS diff
FROM daily_sales;

SELECT 
    student, score,
    NTILE(4) OVER (ORDER BY score DESC) AS quartile,
    PERCENT_RANK() OVER (ORDER BY score DESC) AS pct_rank
FROM scores;

function_name([expr]) OVER (
    [PARTITION BY expr, ...]    -- 分区
    [ORDER BY expr [ASC|DESC]]  -- 排序
    [frame_clause]              -- 窗口帧范围
)

ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW   -- 默认
ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING            -- 前后各一行
ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING    -- 当前行到末尾
RANGE BETWEEN INTERVAL 1 DAY PRECEDING AND CURRENT ROW -- 时间范围

SELECT ROUND(88.234, -1);   -- 90
SELECT ROUND(88.234, -2);   -- 100

-- [a, b) 之间的随机数：RAND() * (b - a) + a
-- 生成 [5, 10) 的随机数
SELECT RAND() * 5 + 5;
​
-- 生成 [1, 100] 的随机整数
SELECT FLOOR(RAND() * 100) + 1;

-- IF 简单判断
SELECT name, IF(score >= 60, '通过', '未通过') AS result FROM exams;
​
-- IFNULL 处理 NULL
SELECT IFNULL(avatar_url, '/images/default.png') AS avatar FROM users;
​
-- NULLIF 避免除零错误
SELECT amount / NULLIF(quantity, 0) AS unit_price FROM orders;
​
-- COALESCE 优先取值
SELECT COALESCE(mobile, phone, email, '无联系方式') AS contact FROM users;
​
-- CASE 多条件判断
SELECT name, score,
    CASE 
        WHEN score >= 90 THEN '优秀'
        WHEN score >= 80 THEN '良好'
        WHEN score >= 60 THEN '及格'
        ELSE '不及格'
    END AS grade
FROM students;

-- 假设 data 列存储：{"name":"Alice","address":{"city":"Beijing"}}
​
-- 提取字段
SELECT data->'$.name' AS name FROM users;         -- "Alice"（带引号）
SELECT data->>'$.name' AS name FROM users;        -- Alice（文本）
​
-- 嵌套取值
SELECT data->>'$.address.city' AS city FROM users; -- Beijing
​
-- 提取数组元素
SELECT JSON_EXTRACT('[1, 2, 3]', '$[0]');         -- 1

-- 将 JSON 数组转为表格数据
SELECT * FROM JSON_TABLE(
    '[{"id":1,"name":"Alice"},{"id":2,"name":"Bob"}]',
    '$[*]' COLUMNS (
        id INT PATH '$.id',
        name VARCHAR(50) PATH '$.name'
    )
) AS jt;
​
-- 结果：
-- +------+-------+
-- | id   | name  |
-- +------+-------+
-- | 1    | Alice |
-- | 2    | Bob   |
-- +------+-------+

-- 字符串转整数
SELECT CAST('123' AS SIGNED) + 1;          -- 124
​
-- 整数转字符串
SELECT CONCAT('编号：', CAST(123 AS CHAR)); -- 编号：123
​
-- 字符串转日期
SELECT CAST('2024-01-15' AS DATE);          -- 2024-01-15
​
-- 转为 JSON
SELECT CAST('{"id":1}' AS JSON);
​
-- 大小写敏感比较
SELECT 'ABC' = 'abc';                        -- 1（不区分大小写）
SELECT BINARY 'ABC' = 'abc';                 -- 0（区分大小写）

-- 如果 quantity 为 0，NULLIF 返回 NULL，除 NULL 结果为 NULL
SELECT amount / NULLIF(quantity, 0) AS unit_price FROM orders;