How to Use INNER JOIN in T-SQL: Combining Related Table Data

Identify Related Tables Before writing your JOIN, identify the tables you want to combine and the related columns that connect them.

These are typically primary and foreign key relationships. Look for columns with matching or related values that logically link your tables together.

Determine the Join Condition Select the specific columns you'll use to match rows between tables. This is typically done using an equality comparison (=) between a column in the first table and a corresponding column in the second table. The join condition defines how rows are paired up.

Write the Basic INNER JOIN Syntax Use the following basic structure:

SELECT columns

FROM FirstTable INNER JOIN SecondTable

ON FirstTable.Column = SecondTable.Column

This syntax tells SQL Server to return only the rows that have matching values in both tables.

Select Desired Columns Choose which columns you want to retrieve from both tables. You can select specific columns or use * to select all columns. Remember to use table aliases if you're selecting columns with the same name from multiple tables.

Refine and Test Your Query After writing your initial JOIN, run the query and verify the results.

Check that:

- The number of returned rows makes sense

- The data matches your expectations

- You've included any additional filtering (WHERE clauses) if needed

- Performance meets your requirements, especially for larger datasets

Pro Tip: INNER JOIN returns only the rows with matching values in both tables. If a row in one table has no match in the other, it will be excluded from the result set.

Example:

SELECT Employees.Name, Departments.DepartmentName FROM Employees INNER JOIN Departments ON Employees.DepartmentID = Departments.DepartmentID