SQL SELECT from Multiple Tables

When working with relational databases, retrieving data from multiple tables is a common requirement. The SELECT statement allows us to fetch specific fields from different tables by using JOIN queries to establish relationships between them.

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Example Query for Selecting from Multiple Tables

Let’s consider an example where we have two supplier-related tables and an orders table. Using the INNER JOIN clause, we can retrieve relevant data from both tables:

SELECT orders.order_id, suppliers.company_name   
FROM suppliers  
INNER JOIN orders  
ON suppliers.supplier_id = orders.supplier_id  
ORDER BY order_id;  

Consideration of Three Tables Example

Suppose we have three tables: two customer tables named client1 and client2, and a sales table that stores product sales information.

client1 Table:

client_id	first_name
101	Alice
102	Bob

client2 Table:

client_id	last_name
101	Johnson
102	Smith

sales Table:

sale_id	client_id	product_name
501	101	Laptop
502	102	Smartphone
503	C101	Printer
504	C102	Desk

SQL Query to Select from Multiple Tables

The following SQL query uses LEFT JOIN to retrieve data from the sales table while including information from both client1 and client2 tables:

SELECT s.sale_id, s.client_id, s.product_name, c1.first_name, c2.last_name  
FROM sales AS s  
LEFT JOIN client1 AS c1  
ON s.client_id = c1.client_id  
LEFT JOIN client2 AS c2  
ON s.client_id = c2.client_id;  

Expected Output:

sale_id	client_id	product_name	first_name	last_name
501	101	Laptop	Alice	Johnson
502	102	Smartphone	Bob	Smith
503	C101	Printer	NULL	NULL
504	C102	Desk	NULL	NULL

Explanation:

• The LEFT JOIN ensures that all records from the sales table are returned, even if there is no matching record in client1 or client2.
• If a corresponding match is found in client1 or client2, it retrieves the respective values; otherwise, it returns NULL.

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Conclusion

Fetching data from multiple tables using SQL joins is an essential technique that helps in efficiently combining and analyzing data from different sources. With proper table relationships and join conditions, you can get meaningful insights with ease.

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