Write queries using aggregate functions in relational algebra (DBMS)?

The aggregate functions are follows −

• max()

• min()

• sum()

• average()

• count()

Consider the following tables −

Employee − Emp(empid,ename,salary)

Dept − Dept(deptid,dname)

Project − Project(projected,pname)

Worksin − Worksin(empid,depid)

Assign − Assign(empid,projectid)

Example 1

Display the details of the employee who works in the ece department.

Step 1

Use the below mentioned syntax. Here, T1= deptid of Ece

T1=∏deptid(σdname=ece(dept))

Output

Step 2

Here, ?= {T1.deptid=worksin.deptid} and T2= all empid of Ece

T2= ∏empid(T1?worksin)

Output

Step 3

Here, T3=(T2?emp), ?={T2.empid=emp.empid} and T3= details of all employees of Ece

Example 2

Display all names of employees who work on database projects.

Step 1

Use the command mentioned below −

T1=∏projectid(σpname=database(project))

Output

Step 2

Use the command given below −

T2= ∏empid(T1?assign)

Output

Step 3

Use the command given below −

T3= ∏empname(T2?emp)

Output