SQL : Subqueries, Joins and Unions

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In this lesson of the SQL tutorial, you will learn...

1. To write queries with subqueries.
2. To select columns from multiple tables with joins.
3. To select records from multiple tables with unions.

Subqueries

Subqueries are queries embedded in queries. They are used to retrieve data from one table based on data in another table. They generally are used when tables have some kind of relationship. For example, in the Northwind database, the Orders table has a CustomerID field, which references a customer in the Customers table. Retrieving the CustomerID for a specific order is pretty straightforward.  more...