Define key columns in queries

One of the most important aspects of a relational database is defining key columns. The key columns are the columns used to define a relationship between two tables. The key column(s) in a table guarantees the uniqueness of each row of data. The key column guaranteeing the uniqueness of each row within a table is the primary key. The primary key of a table appearing in another table is called a foreign key. In many cases, a single column does not guarantee the uniqueness of rows, but the so-called Composite Key does by considering multiple columns as key columns. The xVelocity engine in the data model layer, as we discuss in more detail in Chapter 8, Data Modeling Components, does not support composite keys, but the Power Query engine does. By defining the key columns in the Power Query Editor, we get some performance gains in refreshing the data, especially when merging two tables. We discussed merging tables in Chapter 5, Common Data Preparation Steps...