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# INNER JOIN in SQL

An `INNER JOIN` in SQL is used to combine rows from two or more tables based on a related column between them. It returns only the rows where there is a match in both tables. If there is no match, the rows are excluded from the result set. This is the most commonly used type of join in SQL.

## Key Points

1. **Matching Rows**: `INNER JOIN` returns only the rows where there is a match in both tables.
2. **Common Column**: The tables being joined must have at least one common column, usually a primary key and foreign key.
3. **Exclusion of Non-Matching Rows**: Rows that do not have a match in both tables are excluded from the result set.
4. **Alias Support**: You can use table aliases to simplify queries.

## Syntax

```sql theme={"system"}
SELECT column1, column2, ...
FROM table1
INNER JOIN table2 ON table1.common_column = table2.common_column;
```

* `column1, column2, ...`: The columns you want to retrieve.
* `table1, table2`: The tables you want to join.
* `common_column`: The column that relates the two tables.

## Examples

Suppose you have two tables: `Employees` and `Departments`.

Table: Employees

| EmployeeID | Name    | City       | DepartmentID |
| ---------- | ------- | ---------- | ------------ |
| 1          | Anand   | Chennai    | 101          |
| 2          | Bala    | Coimbatore | 102          |
| 3          | Kavitha | Karaikal   | 103          |
| 4          | Raj     | Madurai    | 104          |
| 5          | Kumar   | Trichy     | 105          |

Table: Departments

| DepartmentID | DepartmentName |
| ------------ | -------------- |
| 101          | HR             |
| 102          | Finance        |
| 103          | IT             |
| 104          | Marketing      |
| 105          | Sales          |

To retrieve employee names along with their department names:

```sql theme={"system"}
SELECT Employees.Name, Departments.DepartmentName
FROM Employees
INNER JOIN Departments ON Employees.DepartmentID = Departments.DepartmentID;
```

Result

| Name    | DepartmentName |
| ------- | -------------- |
| Anand   | HR             |
| Bala    | Finance        |
| Kavitha | IT             |
| Raj     | Marketing      |
| Kumar   | Sales          |

### Using INNER JOIN with WHERE Clause

You can combine `INNER JOIN` with the `WHERE` clause to filter the results further. For example, retrieve employees who work in the IT department.

```sql theme={"system"}
SELECT Employees.Name, Departments.DepartmentName
FROM Employees
INNER JOIN Departments ON Employees.DepartmentID = Departments.DepartmentID
WHERE Departments.DepartmentName = 'IT';
```

Result

| Name    | DepartmentName |
| ------- | -------------- |
| Kavitha | IT             |

### Using INNER JOIN with Multiple Tables

You can use `INNER JOIN` to join more than two tables. Suppose you have an additional table named `Projects`.

Table: Projects

| ProjectID | ProjectName | DepartmentID |
| --------- | ----------- | ------------ |
| 201       | Project A   | 101          |
| 202       | Project B   | 102          |
| 203       | Project C   | 103          |

To retrieve employee names, their department names, and the projects they are working on:

```sql theme={"system"}
SELECT Employees.Name, Departments.DepartmentName, Projects.ProjectName
FROM Employees
INNER JOIN Departments ON Employees.DepartmentID = Departments.DepartmentID
INNER JOIN Projects ON Departments.DepartmentID = Projects.DepartmentID;
```

Result

| Name    | DepartmentName | ProjectName |
| ------- | -------------- | ----------- |
| Anand   | HR             | Project A   |
| Bala    | Finance        | Project B   |
| Kavitha | IT             | Project C   |

## Practical Use Case

Suppose you have a table named `Students` and another table named `Courses`.

Table: Students

| StudentID | Name    | City       | CourseID |
| --------- | ------- | ---------- | -------- |
| 1         | Ram     | Chennai    | 101      |
| 2         | Karthik | Coimbatore | 102      |
| 3         | David   | Bangalore  | 103      |

Table: Courses

| CourseID | CourseName  |
| -------- | ----------- |
| 101      | Mathematics |
| 102      | Science     |
| 103      | English     |

To retrieve student names along with the courses they are enrolled in:

```sql theme={"system"}
SELECT Students.Name, Courses.CourseName
FROM Students
INNER JOIN Courses ON Students.CourseID = Courses.CourseID;
```

Result

| Name    | CourseName  |
| ------- | ----------- |
| Ram     | Mathematics |
| Karthik | Science     |
| David   | English     |

## Key Takeaways

1. `INNER JOIN` returns only the rows where there is a match in both tables.
2. It is used to combine rows from two or more tables based on a related column.
3. Non-matching rows are excluded from the result set.
4. You can use `INNER JOIN` with the `WHERE` clause to filter results further.
5. `INNER JOIN` can be used to join multiple tables in a single query.