聚合函数通常用于对数据进行统计和聚合操作。以下是一些常见数据库系统（如 MySQL、PostgreSQL、Oracle、SQL Server 等）中常用的聚合函数：
常见的数据库聚合函数：
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COUNT()：计算指定列中非空值的数量
SELECT COUNT(*) FROM table_name; -
SUM()：计算指定列值的总和
SELECT SUM(column_name) FROM table_name; -
AVG()：计算指定列值的平均值
SELECT AVG(column_name) FROM table_name; -
MAX()：找出指定列中的最大值。
SELECT MAX(column_name) FROM table_name; -
MIN()：找出指定列中的最小值
SELECT MIN(column_name) FROM table_name;
这些函数在不同的数据库系统中可能会有细微的语法或功能差异，但基本功能是一致的。它们通常与 GROUP BY 语句一起使用，以便对分组后的数据进行聚合操作。
SQL 查询中常用的关键词可以根据功能分为以下几类：
数据定义语言（DDL）
用于定义和管理数据库结构：
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CREATE：创建数据库或数据库对象（如表、视图、索引等）。 -
ALTER：修改数据库或数据库对象的结构。 -
DROP：删除数据库或数据库对象。 -
TRUNCATE：删除表中的所有数据，但保留表结构。
数据操作语言（DML）
用于操作数据库中的数据：
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SELECT：查询数据。 -
INSERT：插入新数据。 -
UPDATE：更新现有数据。 -
DELETE：删除数据。
数据控制语言（DCL）
用于管理数据库的访问权限：
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GRANT：授予用户对数据库的访问权限。 -
REVOKE：撤销用户的访问权限。
数据库事务控制语言（TCL）
用于管理数据库事务：
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COMMIT：提交事务，使所有更改永久生效。 -
ROLLBACK：回滚事务，撤销所有未提交的更改。 -
SAVEPOINT：设置事务的保存点，以便在发生错误时可以回滚到该点。
其他常见关键词
1. 数据查询与过滤
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SELECT：查询数据-- 查询所有列 SELECT * FROM employees; -- 查询特定列 SELECT employee_id, first_name, last_name, salary FROM employees; -
WHERE：条件过滤-- 等值查询 SELECT * FROM employees WHERE department_id = 5; -- 范围查询 SELECT * FROM employees WHERE salary BETWEEN 5000 AND 10000; -- 模糊查询 SELECT * FROM employees WHERE last_name LIKE 'A%'; -- 多条件查询 SELECT * FROM employees WHERE department_id = 5 AND salary > 5000; -
HAVING：聚合条件过滤SELECT department_id, AVG(salary) AS avg_salary FROM employees GROUP BY department_id HAVING AVG(salary) > 5000;
2. 数据排序
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ORDER BY：排序-- 单列排序 SELECT * FROM employees ORDER BY salary DESC; -- 多列排序 SELECT * FROM employees ORDER BY department_id ASC, salary DESC;
3. 数据分组
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GROUP BY：分组-- 按部门分组，计算每个部门的员工数 SELECT department_id, COUNT(*) AS employee_count FROM employees GROUP BY department_id; -- 按部门分组，计算每个部门的平均工资 SELECT department_id, AVG(salary) AS avg_salary FROM employees GROUP BY department_id;
4. 连接操作
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INNER JOIN：内连接SELECT employees.employee_id, employees.first_name, departments.department_name FROM employees INNER JOIN departments ON employees.department_id = departments.department_id; -
LEFT JOIN：左连接SELECT employees.employee_id, employees.first_name, departments.department_name FROM employees LEFT JOIN departments ON employees.department_id = departments.department_id; -
RIGHT JOIN：右连接SELECT employees.employee_id, employees.first_name, departments.department_name FROM employees RIGHT JOIN departments ON employees.department_id = departments.department_id; -
FULL JOIN：全连接SELECT employees.employee_id, employees.first_name, departments.department_name FROM employees FULL JOIN departments ON employees.department_id = departments.department_id;
5. 子查询
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IN：值在子查询结果集中SELECT employee_id, first_name, last_name FROM employees WHERE department_id IN (SELECT department_id FROM departments WHERE location_id = 1001); -
EXISTS：子查询返回至少一行SELECT employee_id, first_name, last_name FROM employees WHERE EXISTS (SELECT 1 FROM departments WHERE department_id = employees.department_id AND location_id = 1001); -
ANY：与子查询返回的值进行比较，满足任意一个条件SELECT employee_id, first_name, last_name FROM employees WHERE salary > ANY (SELECT salary FROM employees WHERE department_id = 5); -
ALL：与子查询返回的值进行比较，满足所有条件SELECT employee_id, first_name, last_name FROM employees WHERE salary > ALL (SELECT salary FROM employees WHERE department_id = 5);
6. 表达式和条件判断
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AS：指定别名SELECT employee_id AS id, first_name AS fname, last_name AS lname, salary * 12 AS annual_salary FROM employees; -
CASE：条件表达式SELECT employee_id, first_name, last_name, salary, CASE WHEN salary > 10000 THEN 'High' WHEN salary BETWEEN 5000 AND 10000 THEN 'Medium' ELSE 'Low' END AS salary_level FROM employees;
7. 事务控制
-
COMMIT：提交事务-- 提交事务，使所有更改永久生效 COMMIT; -
ROLLBACK：回滚事务-- 回滚事务，撤销所有未提交的更改 ROLLBACK; -
SAVEPOINT：设置事务的保存点-- 设置保存点 SAVEPOINT sp1; -- 执行一些操作 UPDATE employees SET salary = salary * 1.1 WHERE department_id = 5; -- 回滚到保存点 ROLLBACK TO SAVEPOINT sp1;
8. 其他常用操作
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UNION：合并两个或多个SELECT语句的结果集（去重）SELECT employee_id, first_name, last_name FROM employees WHERE department_id = 5 UNION SELECT employee_id, first_name, last_name FROM employees WHERE salary > 10000; -
UNION ALL：合并两个或多个SELECT语句的结果集（不去重）SELECT employee_id, first_name, last_name FROM employees WHERE department_id = 5 UNION ALL SELECT employee_id, first_name, last_name FROM employees WHERE salary > 10000; -
VIEW：创建或替换视图-- 创建视图 CREATE VIEW high_salary_employees AS SELECT employee_id, first_name, last_name, salary FROM employees WHERE salary > 10000; -- 查询视图 SELECT * FROM high_salary_employees; -
INDEX：创建或删除索引-- 创建索引 CREATE INDEX idx_employee_last_name ON employees (last_name); -- 删除索引 DROP INDEX idx_employee_last_name; -
EXPLAIN：显示 SQL 语句的执行计划EXPLAIN SELECT * FROM employees WHERE last_name = 'Smith';
