Nested Query and Join
Jagdish Chandra Patni · 2022
One of the biggest challenges is selecting the join or subquery while we are extracting the result based on multiple tables. Subqueries are used in complex SQL queries. Usually, there is a main outer query and one or more subqueries nested within the outer query. A subquery is a SELECT statement coded within another SELECT statement. A subquery has two parts: the outer query and inner query, and an inner query again can have multiple queries based on the problem. A subquery will return a single value or multiple values based on the problem. A subquery can be used with the select, insert, update, and delete commands. The JOIN clause does not contain additional queries. It connects two or more tables and selects data from them into a single result set. It is most frequently used to join tables with primary and foreign keys. Subqueries and JOINs can be used in a complex query to select data from multiple tables, but they do so differently. Sometimes we have a choice of either, but there are cases in which a subquery is the only real option. This chapter introduces the subquery and various types of JOIN operators used in the database. This chapter also discusses about the uses of subquery in place of JOIN and vice-versa.