How to make parent process wait till Completion of Child Process (Joining parent with child process)

How to make the parent process wait till the completion of execution of child process. The following C program makes the parent process to wait till the completion of its child process.

#include<stdio.h>
main()
{

Relational Algebra - Selection, Projection, Joins etc

The relational algebra defines a set of operations on relations, paralleling the usual algebraic operations such as addition, subtraction etc which operate on numbers.The relational algebra operations take one or two relations (tables) as input and return a relation as output. This post we discuss the following relational operations:

1. Selection (σ)
2. Projection (π)
3. Cartesian product
4. Joins

Selection Operation (σ)