How to Join with Data in Another Sequence

The basis of any Relational Database Management System (DBMS) is the ability to draw together data from multiple sources (tables generally) as the result of a query; this is called “Joining” data. In relational databases, the joins are most often between primary key and foreign key-related columns, but a join can be achieved on any data as long as both data sources share a common column value. One fundamental reason for splitting data into multiple sources is to avoid duplicating data—this process is called normalization by database developers.

Although the concept of joining in-memory collections isn’t a common pattern today (it’s more likely that you nest a collection inside another object to indicate a parent-child relationship), LINQ to Objects introduces the concept of joining multiple data sources through its powerful Join and GroupJoin operators and their equivalent query expression syntax options. LINQ also enables join-like functionality by using a subquery expression within the Select clause. This subquery style of coding is often simpler to read and helps you avoid having to use any specific Join syntax in the query at all. Both techniques are covered in the examples throughout this chapter.