Understanding SQL JOIN with Examples

Understanding SQL JOIN with Examples

In SQL, the JOIN clause is used to combine data from two or more tables in a relational database. It lets you retrieve information that spans multiple tables efficiently using a common field.

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Types of SQL JOIN

• INNER JOIN ÔÇö returns only matching rows from both tables.
• LEFT OUTER JOIN ÔÇö all rows from the left table + matches from the right.
• RIGHT OUTER JOIN ÔÇö all rows from the right table + matches from the left.
• FULL OUTER JOIN ÔÇö all rows from both tables.
• CROSS JOIN ÔÇö every combination of rows (Cartesian product).

SQL INNER JOIN Example

Using an Employee table and a Salary_Payment table, we combine matching rows:

SELECT e.ID, e.Employee_NAME, e.Employee_AGE, sp.AMOUNT
FROM Employee e
JOIN Salary_Payment sp ON e.ID = sp.Employee_ID;

The INNER JOIN returns only employees who have a matching payment record.

LEFT JOIN Example

A LEFT JOIN returns ALL employees, even those without payments (NULL for missing payment data):

SELECT e.Employee_NAME, sp.AMOUNT
FROM Employee e
LEFT JOIN Salary_Payment sp ON e.ID = sp.Employee_ID;

RIGHT JOIN Example

SELECT e.Employee_NAME, sp.AMOUNT
FROM Employee e
RIGHT JOIN Salary_Payment sp ON e.ID = sp.Employee_ID;

Why Use SQL JOIN?

• Query data stored across multiple tables.
• Merge rows into one comprehensive result set.
• Build reports that combine related information.

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Key Takeaways

SQL JOINs combine related data from multiple tables. INNER JOIN returns matches only, while LEFT and RIGHT JOINs keep unmatched rows from one side. Choosing the right join ensures accurate results. For more SQL guides, stay tuned to .

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