导入
看清全局：SUM、AVG 和 COUNT
学习如何统计行数，以及对数据进行求和与求平均值。
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1SELECT SUM(salary) AS "Total Salary Cost" 2FROM employees;
1SELECT AVG(salary) AS "Average Salary", 2 COUNT(*) AS "Number of Employees" 3FROM employees;
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1SELECT AVG(salary) AS "Average Engineering Salary" 2FROM employees 3WHERE department = 'Engineering';
1SELECT COUNT(*) AS "Number of Marketing Employees", 2 SUM(salary) AS "Total Marketing Salary" 3FROM employees 4WHERE department = 'Marketing';
Table
| 函数 | 作用 | 示例 |
|---|---|---|
| SUM(column) | 将所有值相加 | SUM(salary) |
| AVG(column) | 计算平均值 | AVG(salary) |
| COUNT(*) | 统计行数 | COUNT(*) |
| WHERE + 聚合函数 | 汇总表中一部分数据 | WHERE department = 'Sales' |