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我制造“文字垃圾”无数，但说到檄文，此篇即使不是绝无仅有，也是独出心裁。我的朋友圈微信昵称扣扣昵称直接称sir的，只有师姐颜丽颜sir，还有就是杨黎涌杨兄杨sir了。莫砺锋老师在《浮生琐忆》中说他反感...
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COunt 计算制定字段在查询结果中出现的个数 SELECT count(employee_id),count(salary) ,count(salary12) from employees; SEL...0693
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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...0250
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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...0212
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SELECT COUNT(employee_id),COUNT(salary),COUNT(salary12) from employees SELECT COUNT(1) from employee...0248
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1.count 计算指定字段在查询结果中出现sele的个数 SELECT COUNT(employee_id),COUNT(salary),COUNT(salary12) from employees...0268 -
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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...0211
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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...0239
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SELECT last_name,e.department_id,department_name from employees e LEFT JOIN departments d on e.depar...0304
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SELECT last_name,e.department_id, department_name FROM employees e LEFT OUTER JOIN departments d ON ...0298
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1.avg/sum:只适用于数值类型的字段或变量 SELECT AVG(salary),SUM(salary) from employees; 字符串求和没有任何意义 SELECT AVG(last_...0288
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练习SELECT e.last_name,d.department_id,d.department_name FROM employees e,departments d WHERE e.depart...0214
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