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在SQL中,聚合函数用于对一组值执行计算,并返回单个值。以下是一些常用的聚合函数及其用法:
COUNT()
SELECT COUNT(*) FROM employees;
SUM()
SELECT SUM(salary) FROM employees;
AVG()
SELECT AVG(salary) FROM employees;
MAX()
SELECT MAX(salary) FROM employees;
MIN()
SELECT MIN(salary) FROM employees;
GROUP BY
SELECT department, COUNT(*) AS employee_count
FROM employees
GROUP BY department;
HAVING
GROUP BY
一起使用,用于过滤分组后的结果。SELECT department, COUNT(*) AS employee_count
FROM employees
GROUP BY department
HAVING COUNT(*) > 10;
ORDER BY
SELECT name, salary
FROM employees
ORDER BY salary DESC;
GROUP BY
子句一起使用,以便对每个组进行计算。HAVING
子句用于过滤聚合后的结果,而WHERE
子句用于过滤原始数据。SUM()
、AVG()
、MAX()
和MIN()
),或者是可以被计数的非空值(对于COUNT()
)。通过这些基本的聚合函数和子句,你可以执行各种复杂的数据分析和报告任务。
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