SQL INNER JOIN

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In SQL, an INNER JOIN is one of the most commonly used types of join. It allows you to combine rows from two or more tables based on a related column. When you use an INNER JOIN, only the rows that have matching values in both tables are included in the result set.

SELECT columns
FROM table1
INNER JOIN table2
ON table1.column = table2.column;

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1. What is SQL INNER JOIN?

An INNER JOIN is used to select records from two or more tables that have matching values in the specified columns. It returns the intersection of both tables, meaning that only rows with a match in both tables will appear in the result set.

The INNER JOIN operates on the concept of relational databases, where tables are linked by primary and foreign key relationships.

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2. Syntax of SQL INNER JOIN

The syntax for an INNER JOIN is straightforward:

SELECT columns
FROM table1
INNER JOIN table2
ON table1.column = table2.column;

• SELECT: Specifies the columns to retrieve from the joined tables.
• FROM table1: Specifies the first table (left table).
• INNER JOIN table2: Specifies the second table (right table) to join.
• ON table1.column = table2.column: Defines the condition for the join (usually a primary/foreign key relationship between the tables).

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3. How INNER JOIN Works

• The INNER JOIN only returns rows where there is a match in both tables based on the specified condition.
• If there is no match between the rows of the two tables, the row is excluded from the result.
• The result of an INNER JOIN is a set of rows that have matching values in both tables.

Example Scenario:

Consider two tables:

1. employees: Stores information about employees.
2. departments: Stores information about departments.

-- employees table
+-------------+-----------------+---------------+
| employee_id | employee_name   | department_id |
+-------------+-----------------+---------------+
| 1           | Alice           | 1             |
| 2           | Bob             | 2             |
| 3           | Charlie         | 1             |
| 4           | David           | NULL          |
+-------------+-----------------+---------------+

-- departments table
+---------------+--------------------+
| department_id | department_name    |
+---------------+--------------------+
| 1             | HR                 |
| 2             | Finance            |
+---------------+--------------------+

INNER JOIN Example:

SELECT employees.employee_id, employees.employee_name, departments.department_name
FROM employees
INNER JOIN departments
ON employees.department_id = departments.department_id;

Explanation:

• This query selects the employee_id, employee_name, and department_name from the employees and departments tables.
• The query uses INNER JOIN to match rows where employees.department_id equals departments.department_id.

Result:

employee_id	employee_name	department_name
1	Alice	HR
2	Bob	Finance
3	Charlie	HR

Explanation of Result:

• Only employees who are assigned to a department (those with a non-NULL department_id) appear in the result.
• The row for David, who has a NULL department, is excluded because there's no matching department in the departments table.

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4. Using Multiple INNER JOINs

You can combine multiple INNER JOINs in a single query to retrieve data from more than two tables. When joining multiple tables, you need to specify the join condition for each one.

Example with Multiple INNER JOINs:

Let's say you also have a projects table that stores information about the projects each employee works on:

-- projects table
+------------+-------------+----------------+
| project_id | project_name| employee_id    |
+------------+-------------+----------------+
| 1          | Project X   | 1              |
| 2          | Project Y   | 2              |
| 3          | Project Z   | 3              |
+------------+-------------+----------------+

Now, to get the employee_name, department_name, and project_name for each employee, you can use multiple INNER JOINs:

SELECT employees.employee_name, departments.department_name, projects.project_name
FROM employees
INNER JOIN departments ON employees.department_id = departments.department_id
INNER JOIN projects ON employees.employee_id = projects.employee_id;

Explanation:

• The query joins three tables: employees, departments, and projects.
• It first joins employees and departments based on department_id, and then joins employees and projects based on employee_id.

Result:

employee_name	department_name	project_name
Alice	HR	Project X
Bob	Finance	Project Y
Charlie	HR	Project Z

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5. INNER JOIN vs. LEFT JOIN

One common question that comes up is the difference between INNER JOIN and LEFT JOIN. Here's a brief comparison:

• INNER JOIN: Returns only the rows that have matching values in both tables.
• LEFT JOIN (or LEFT OUTER JOIN): Returns all rows from the left table, even if there is no match in the right table. If there’s no match, NULL values are returned for columns from the right table.

Example of LEFT JOIN:

SELECT employees.employee_name, departments.department_name
FROM employees
LEFT JOIN departments ON employees.department_id = departments.department_id;

This query would return all employees, even those without a department, with NULL for the department_name if there's no match.

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6. Performance Considerations for INNER JOIN

• Indexes: To improve the performance of INNER JOINs, ensure that the columns used in the ON condition (such as employee_id, department_id, etc.) are indexed.
• Join Conditions: Always make sure that the columns you’re joining on have a logical relationship, and avoid unnecessary joins to improve query efficiency.
• Query Optimization: For large datasets, consider limiting the number of records in the tables you join using conditions like WHERE to reduce the amount of data processed.

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