Business analysis glossary
JOIN
What it is
A JOIN combines rows from two tables based on a matching column, usually a foreign key matching a primary key. Common types are INNER JOIN (only matches) and LEFT JOIN (all rows from the left table plus matches). The join condition says which columns must match, for example orders.customer_id = customers.customer_id.
Why it matters
Data is split across tables, so most useful questions need a join. Choosing the wrong type silently drops rows or duplicates them and produces misleading numbers. Counting rows before and after a join is a simple habit that catches many accidental duplicates, especially when one table has several matching rows for each key.
Example
To list every customer of Cedar Credit Union and their loan count, including customers with no loans, a LEFT JOIN from customers to loans keeps all customers. An INNER JOIN would hide those with zero loans. Comparing the count of customers with and without the join confirms that no customer was lost, and the analyst notes the difference in the report.