Difference between a theta join, equijoin and natural join

Question

I\'m having trouble understanding relational algebra when it comes to theta joins, equijoins and natural joins. Could someone please help me better understand it? If I use t

Answer 1

Cartesian product of two tables gives all the possible combinations of tuples like the example in mathematics the cross product of two sets . since many a times there are some junk values which occupy unnecessary space in the memory too so here joins comes to rescue which give the combination of only those attribute values which are required and are meaningful.

inner join gives the repeated field in the table twice whereas natural join here solves the problem by just filtering the repeated columns and displaying it only once.else, both works the same. natural join is more efficient since it preserves the memory .Also , redundancies are removed in natural join .

equi join of two tables are such that they display only those tuples which matches the value in other table . for example : let new1 and new2 be two tables . if sql query select * from new1 join new2 on new1.id = new.id (id is the same column in two tables) then start from new2 table and join which matches the id in second table . besides , non equi join do not have equality operator they have <,>,and between operator .

theta join consists of all the comparison operator including equality and others < , > comparison operator. when it uses equality(=) operator it is known as equi join .