SQL 函数概述
SQL 函数概述
SQL 函数可以分为几大类，不同数据库系统可能有略微不同的实现。以下是主要的 SQL 函数分类：
1. 聚合函数 (Aggregate Functions)
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COUNT()- 计算行数 -
SUM()- 计算总和 -
AVG()- 计算平均值 -
MIN()- 找最小值 -
MAX()- 找最大值 -
GROUP_CONCAT()(MySQL) /STRING_AGG()(SQL Server, PostgreSQL) - 连接字符串 -
STDDEV()/STDDEV_POP()- 计算标准差 -
VARIANCE()/VAR_POP()- 计算方差
2. 标量函数 (Scalar Functions)
字符串函数
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CONCAT()- 连接字符串 -
SUBSTRING()/SUBSTR()- 提取子字符串 -
TRIM()- 去除空格 -
UPPER()/UCASE()- 转为大写 -
LOWER()/LCASE()- 转为小写 -
LENGTH()/LEN()- 字符串长度 -
REPLACE()- 替换字符串 -
LEFT()/RIGHT()- 获取左/右部分字符串
数学函数
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ABS()- 绝对值 -
ROUND()- 四舍五入 -
CEIL()/CEILING()- 向上取整 -
FLOOR()- 向下取整 -
MOD()- 取模 -
POWER()/POW()- 幂运算 -
SQRT()- 平方根 -
RAND()- 随机数
日期和时间函数
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NOW()/CURRENT_TIMESTAMP- 当前日期时间 -
CURDATE()/CURRENT_DATE- 当前日期 -
CURTIME()/CURRENT_TIME- 当前时间 -
DATE()- 提取日期部分 -
TIME()- 提取时间部分 -
YEAR()/MONTH()/DAY()- 提取年/月/日 -
DATEDIFF()- 计算日期差 -
DATE_ADD()/DATE_SUB()- 日期加减 -
DAYNAME()/MONTHNAME()- 获取星期/月份名称
转换函数
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CAST()- 类型转换 -
CONVERT()- 类型转换 -
COALESCE()- 返回第一个非NULL值 -
NULLIF()- 两值相等返回NULL
条件函数
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CASE WHEN...THEN...ELSE...END- 条件表达式 -
IF()(MySQL) - 简单条件判断 -
IIF()(SQL Server) - 简单条件判断 -
DECODE()(Oracle) - 值匹配判断
3. 窗口函数 (Window Functions)
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ROW_NUMBER()- 行号 -
RANK()- 排名(有间隔) -
DENSE_RANK()- 排名(无间隔) -
NTILE()- 分组排名 -
LEAD()/LAG()- 访问前后行数据 -
FIRST_VALUE()/LAST_VALUE()- 窗口首/尾值 -
PERCENT_RANK()- 百分比排名 -
CUME_DIST()- 累积分布
4. 系统函数
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USER()/CURRENT_USER- 当前用户 -
DATABASE()- 当前数据库 -
VERSION()- 数据库版本 -
LAST_INSERT_ID()(MySQL) - 最后插入的ID
完整的SQL函数分类及使用示例
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一、聚合函数 (Aggregate Functions)
1. COUNT()
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-- 计算所有行数 SELECT COUNT(*) FROM products; -- 计算特定列非NULL值 SELECT COUNT(product_name) FROM products; -- 计算不同值的数量 SELECT COUNT(DISTINCT category_id) FROM products;
2. SUM()
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-- 计算总和 SELECT SUM(quantity) FROM inventory; -- 带条件的求和 SELECT SUM(price) FROM orders WHERE order_date > '2023-01-01';
3. AVG()
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-- 计算平均值 SELECT AVG(rating) FROM product_reviews; -- 结合ROUND函数 SELECT ROUND(AVG(salary), 2) FROM employees;
4. MIN()/MAX()
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复制
-- 最小值和最大值 SELECT MIN(price), MAX(price) FROM products; -- 日期最值 SELECT MIN(hire_date), MAX(hire_date) FROM employees;
5. GROUP_CONCAT()/STRING_AGG()
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-- MySQL SELECT department_id, GROUP_CONCAT(employee_name SEPARATOR '; ') FROM employees GROUP BY department_id; -- SQL Server/PostgreSQL SELECT department_id, STRING_AGG(employee_name, '; ') FROM employees GROUP BY department_id;
6. STDDEV()/VARIANCE()
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-- 标准差和方差 SELECT STDDEV(salary), VARIANCE(salary) FROM employees;
二、标量函数 (Scalar Functions)
字符串函数
1. CONCAT()
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SELECT CONCAT(first_name, ' ', last_name) AS full_name FROM customers;
2. SUBSTRING()/SUBSTR()
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SELECT SUBSTRING('Database', 1, 4); -- 返回'Data' SELECT SUBSTR(email, 1, POSITION('@' IN email)-1) AS username FROM users;3. TRIM()
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SELECT TRIM(' SQL '); -- 返回'SQL' SELECT TRIM(BOTH 'x' FROM 'xxSQLxx'); -- 返回'SQL'4. UPPER()/LOWER()
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SELECT UPPER('Hello'); -- 'HELLO' SELECT LOWER('SQL'); -- 'sql'5. LENGTH()/LEN()
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-- MySQL/PostgreSQL SELECT LENGTH('SQL'); -- 3 -- SQL Server SELECT LEN('SQL'); -- 36. REPLACE()
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SELECT REPLACE('Hello World', 'World', 'SQL'); -- 'Hello SQL'7. LEFT()/RIGHT()
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SELECT LEFT('Database', 4); -- 'Data' SELECT RIGHT('Database', 3); -- 'ase'数学函数
1. ABS()
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SELECT ABS(-15); -- 15
2. ROUND()
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SELECT ROUND(123.4567, 2); -- 123.46
3. CEIL()/CEILING()/FLOOR()
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SELECT CEIL(123.45); -- 124 SELECT FLOOR(123.45); -- 123
4. MOD()
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SELECT MOD(10, 3); -- 1 (10除以3的余数)
5. POWER()/POW()
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SELECT POWER(2, 3); -- 8 (2的3次方)
6. SQRT()
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SELECT SQRT(25); -- 5
7. RAND()
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-- 生成0-1之间的随机数 SELECT RAND(); -- 生成1-100的随机整数(MySQL) SELECT FLOOR(1 + RAND() * 100);
日期和时间函数
1. NOW()/CURRENT_TIMESTAMP
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SELECT NOW(); -- 返回当前日期时间 '2023-10-25 15:30:45'
2. CURDATE()/CURRENT_DATE
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SELECT CURDATE(); -- '2023-10-25'
3. CURTIME()/CURRENT_TIME
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SELECT CURTIME(); -- '15:30:45'
4. DATE()/TIME()
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SELECT DATE('2023-10-25 15:30:45'); -- '2023-10-25' SELECT TIME('2023-10-25 15:30:45'); -- '15:30:45'5. YEAR()/MONTH()/DAY()
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SELECT YEAR('2023-10-25'); -- 2023 SELECT MONTH('2023-10-25'); -- 10 SELECT DAY('2023-10-25'); -- 256. DATEDIFF()
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-- MySQL SELECT DATEDIFF('2023-12-31', '2023-10-25'); -- 67 -- SQL Server SELECT DATEDIFF(day, '2023-10-25', '2023-12-31');7. DATE_ADD()/DATE_SUB()
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-- MySQL SELECT DATE_ADD('2023-10-25', INTERVAL 7 DAY); -- '2023-11-01' -- SQL Server SELECT DATEADD(day, 7, '2023-10-25');8. DAYNAME()/MONTHNAME()
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SELECT DAYNAME('2023-10-25'); -- 'Wednesday' SELECT MONTHNAME('2023-10-25'); -- 'October'转换函数
1. CAST()/CONVERT()
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SELECT CAST('123' AS SIGNED); -- 字符串转整数 SELECT CONVERT('2023-10-25', DATE); -- 字符串转日期2. COALESCE()
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SELECT COALESCE(NULL, NULL, 'SQL', NULL, 'Example'); -- 'SQL'
3. NULLIF()
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SELECT NULLIF(10, 10); -- NULL SELECT NULLIF(10, 20); -- 10
条件函数
1. CASE WHEN
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SELECT product_id, price, CASE WHEN price > 100 THEN 'Expensive' WHEN price > 50 THEN 'Moderate' ELSE 'Cheap' END AS price_category FROM products;2. IF() (MySQL)
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SELECT IF(1 > 0, 'True', 'False'); -- 'True'
3. IIF() (SQL Server)
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SELECT IIF(1 > 0, 'True', 'False'); -- 'True'
4. DECODE() (Oracle)
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SELECT DECODE(status, 1, 'Active', 0, 'Inactive', 'Unknown') FROM users;
三、窗口函数 (Window Functions)
1. ROW_NUMBER()
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SELECT employee_id, salary, ROW_NUMBER() OVER (ORDER BY salary DESC) AS rank FROM employees;2. RANK()/DENSE_RANK()
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SELECT product_id, sales, RANK() OVER (ORDER BY sales DESC) AS rank, DENSE_RANK() OVER (ORDER BY sales DESC) AS dense_rank FROM products;3. NTILE()
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SELECT customer_id, total_purchases, NTILE(4) OVER (ORDER BY total_purchases DESC) AS quartile FROM customers;4. LEAD()/LAG()
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SELECT date, revenue, LAG(revenue, 1) OVER (ORDER BY date) AS prev_day_revenue, LEAD(revenue, 1) OVER (ORDER BY date) AS next_day_revenue FROM daily_sales;5. FIRST_VALUE()/LAST_VALUE()
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SELECT department_id, employee_id, salary, FIRST_VALUE(employee_id) OVER (PARTITION BY department_id ORDER BY salary DESC) AS highest_paid FROM employees;6. PERCENT_RANK()/CUME_DIST()
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SELECT student_id, score, PERCENT_RANK() OVER (ORDER BY score) AS percentile, CUME_DIST() OVER (ORDER BY score) AS cumulative_dist FROM exam_results;四、系统函数
1. USER()/CURRENT_USER
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SELECT USER(); -- 'root@localhost'
2. DATABASE()
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SELECT DATABASE(); -- 返回当前数据库名
3. VERSION()
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SELECT VERSION(); -- '8.0.26'
4. LAST_INSERT_ID() (MySQL)
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-- 获取最后插入的自动增量ID SELECT LAST_INSERT_ID();