Data Science with SQL Server Quick Start Guide

Joining multiple tables

In a relational database, you typically have data spread in multiple tables. Each table represents a set of entities of the same kind, such as employees in the examples you have seen so far. In order to make result sets meaningful for the business your database supports, most of the time you need to retrieve data from multiple tables in the same query. You need to join two or more tables based on some conditions. The most frequent kind of a join is the inner join. An inner join returns only rows for which the condition in the join predicate for the two joined tables evaluates to true. Note that in a relational database, you have three-valued logic, because there is always a possibility that a piece of data is unknown. ...