请问下面的数据库 关系代数表达式怎么写啊?Employee(EmployeeId,EmployeeName,Block,UnitNo,Street,City)Working(EmployeeId,CompanyId,Salary)Company(CompanyId,CompanyName,City)Managing(EmployeeId,ManagerID)Write relational algebra expressions for the following queries:(1) Find the list of EmployeeIds and EmployeeNames working in the company XYZ.(2) Find all Employees who live in the same city as the company in which they work.(3) Find all employees who work for City Bank and e
问题描述：
请问下面的数据库 关系代数表达式怎么写啊?
Employee(EmployeeId,EmployeeName,Block,UnitNo,Street,City)
Working(EmployeeId,CompanyId,Salary)
Company(CompanyId,CompanyName,City)
Managing(EmployeeId,ManagerID)
Write relational algebra expressions for the following queries:
(1) Find the list of EmployeeIds and EmployeeNames working in the company XYZ.
(2) Find all Employees who live in the same city as the company in which they work.
(3) Find all employees who work for City Bank and earn more than $30000.
(4) Find the number of employees working in each company and the average salary.
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