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Practice Problems

How does left join work?

Introduction to LEFT JOIN

LEFT JOIN is a type of join that returns all records from the left table (clients), and the matched records from the right table (purchases). If there is no match, NULL values are returned for columns from the right table.

Sample Data

Clients Table

client_id | full_name ------------------------ 1 | David 2 | Emma 3 | Frank

Purchases Table

purchase_id | user_id | item ------------------------------ 1 | 1 | Smartphone 2 | 1 | Charger 3 | 2 | Headphones

SQL Query Example

The query will look like this. Let's break it down in more detail:

sql
SELECT clients.full_name AS client_name, purchases.item_name AS item FROM clients LEFT JOIN purchases ON clients.client_id = purchases.client_id;

Result of the Query

As a result of executing the query, we will get the following result:

sql
SELECT user_fullname, item_name FROM users LEFT JOIN orders ON users.user_id = orders.order_user_id ORDER BY user_fullname;

Conclusion

In summary, the LEFT JOIN operation allows you to retrieve all records from the left table while including matched records from the right table, making it a powerful tool for data retrieval in SQL.

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