How can the sql query efficiency be improved?

We can used temporary tables or variables and use them instead of bigger tables and that can improve the efficiency of sql query.

What is multi threading and its importance in object oriented programming?

When one wants to improve the speed of a program, multi threading i.e using multiple threads of programs can be applied so that the speed of cpu is enhanced and the output of program comes out faster.

What is the ER diagram and the importance of entity,attributes and relationship?

ER stands for entity relationship diagram. They are used for making the scope of databases, high level, so that one can understand the entities, its properties( i.e. attributes) and the relationships among them as in one to one or one to many.