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COUNT 作用：计算指定字段在查询结果中出现的个数 SELECT COUNT(employee_id),COUNT(salary),COUNT(salary12) FROM employees;...030 -
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count 作用：计算指定字段在查询结果中出现的个数 SELECT count(employee_id),count(salary),count(salary12) FROM employees;...014
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SELECT COUNT(employee_id),COUNT(salary),COUNT(salary12) FROM employees; SELECT COUNT(1) FROM employe...013
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GROUP BY 查询各个部门的平均工资和最高工资 SELECT department_id,AVG(salary),MAX(salary) FROM employees GROUP BY dep...021
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count 作用：计算指定字段在查询结果中出现sele的个数 SELECT COUNT(employee_id),COUNT(salary) FROM employees; salary是2000还是...019
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1显示所有员工的姓名，部门号和部门名称 SELECT last_name,e.department_id,department_name from employees e LEFT JOIN dep...023
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七、聚合函数 1.常见的聚合函数 1. AVG / SUM AVG 求平均数 SELECT AVG (salary) FROM employees; 以下两个是字符串， 它们在求平均数和求和时...027
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