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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...0215 -
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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...0214
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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...0241
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SELECT last_name,e.department_id, department_name FROM employees e LEFT OUTER JOIN departments d ON ...0301
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练习SELECT e.last_name,d.department_id,d.department_name FROM employees e,departments d WHERE e.depart...0215
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select 122106; Mysql 不支持full outer join 做满外查询 select employee_id,department_name from employees e fu...0237
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（一）SELECT last_name, e.department_id, department_name FROM employees e LEFT OUTER JOIN departments d...0910 -
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select employee_id,department_name from employees e right join departments d on e.department_id d.d...0248
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SELECT employee_id,department_name from employees e left join departments d on e.department_id d.de...0226
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中图：内连接 SELECT employee_id,department_name FROM employees e JOIN departments d ON e.department_id d....0218
