How to Use Join Query

To write a SQL query using the JOIN keyword and specifying the tables and join type, you need to follow these steps:

🔍 Step 1: Determine the type of join needed

There are several types of joins in SQL including INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL OUTER JOIN. Each join type is useful in different situations depending on what data you want to retrieve.

• INNER JOIN: returns only the matching rows from both tables.

• LEFT JOIN: returns all the rows from the left table and the matching rows from the right table.

• RIGHT JOIN: returns all the rows from the right table and the matching rows from the left table.

• FULL OUTER JOIN: returns all the rows from both tables, including any non-matching rows.

📚 Step 2: Identify the tables to join

Before writing the SQL query, you need to identify which tables you want to join. For example, suppose you have two tables: customer and order. You might want to join these tables to get a list of all customers and their corresponding orders.

🧑‍💻 Step 3: Write the SQL query

Once you have identified the type of join and the tables to join, you can write the SQL query using the JOIN keyword and specifying the tables and join type.

Here is an example of a SQL query that uses an INNER JOIN to join the customer and order tables on the customer_id column:

SELECT *
FROM customer
INNER JOIN order ON customer.customer_id = order.customer_id;

This query will select all columns from both tables where the customer_id column in customer matches the customer_id column in order.

🚀 Step 4: Execute the SQL query

After writing the SQL query, you can execute it using a database management tool like SQLite. The result set will contain all columns from both tables where the join condition is satisfied.

🤯 Real-world Example

Suppose you have a database that contains information about your company's employees, departments, and salaries. You might want to join these tables to get a list of all employees and their corresponding departments and salaries.

Here is an example of a SQL query that uses a LEFT JOIN to join the employee, department, and salary tables:

SELECT *
FROM employee
LEFT JOIN department ON employee.department_id = department.department_id
LEFT JOIN salary ON employee.employee_id = salary.employee_id;

This query will select all columns from the employee table and matching columns from the department and salary tables where the join conditions are satisfied. The LEFT JOIN ensures that all employees are included in the result set, even if they do not have a matching department or salary record.

👉 Understanding how to write SQL queries using JOINs is essential for anyone working with databases. By following the steps outlined above, you can easily join multiple tables and retrieve the data you need.