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分组函数
# 功能:用作统计,又称为聚合函数或者统计函数或者组函数
# 分类: sum 求和,avg 平均值,max 最大值,min最小值,count 计算个数
SELECT SUM(salary) FROM employees;
SELECT AVG(salary) FROM employees;
SELECT MAX(salary) FROM employees;
SELECT MIN(salary) FROM employees;
SELECT COUNT(salary) FROM employees;
# 分组函数的特点:
# sum avg 一般处理数值型;忽略null值;
# max min count 可以处理任何类型;忽略null值;
# 可以和distinct(去重复) 搭配使用
SELECT SUM(DISTINCT salary),SUM(salary) FROM employees;
SELECT COUNT(DISTINCT salary) FROM employees;
# count 函数的详细介绍
SELECT COUNT(salary) FROM employees;
# 统计行数 效率高于count(salary)
SELECT COUNT(*) FROM employees;
SELECT COUNT(1) FROM employees;