What is the difference between IEnumerable and IQueryable in C#? Why to use IQueryable for sql queries?

IQueryable inherited from IEnumerable interface only, so anything you can do with a IEnumerable, you can also do with an IQueryable also.

IEnumerable bring all result from database and then filter it at code side, which is a network load and performance issue.

IQueryable filter the result at database only and then get only filtered result, therefore less network load and better performance.

IQueryable is under SYSTEM.LINQ namespace. IEnuberable is under System.Collections namespace. 

IEnumerable and IQueryable are both interfaces used for querying collections of data. However, they have different purposes and behaviors.

IEnumerable:

• The IEnumerable interface is used for querying in-memory collections, such as arrays, lists, or other enumerable objects.
• It provides a simple way to iterate over a collection and perform operations like filtering, sorting, and projection using LINQ (Language-Integrated Query) operators.
• The LINQ operators execute the query in memory on the entire collection and are suitable for small or in-memory datasets.
• IEnumerable is part of the System.Collections namespace.

IQueryable:

• The IQueryable interface is used for querying data sources that support querying operations, such as databases or remote data services.
• It represents a query that can be executed against a specific data source (e.g., SQL Server) and supports deferred execution.
• Query operations written using IQueryable are translated into a query language (e.g., SQL) and executed on the data source itself.
• IQueryable extends IEnumerable and is part of the System.Linq namespace.

Why use IQueryable for SQL queries?

• When dealing with large datasets or querying against a database, IQueryable is more suitable as it allows the query to be translated and executed at the data source itself (e.g., SQL Server).
• IQueryable enables the use of query providers that generate optimized SQL queries based on LINQ expressions, leading to more efficient execution.
• By using IQueryable, you can leverage the capabilities of the underlying data source, such as indexes, query optimization, and efficient data retrieval.
• IQueryable supports additional query operations that are specific to the data source, allowing you to write more powerful and complex queries.

Here's an example illustrating the usage of IEnumerable and IQueryable:

// Using IEnumerable
IEnumerable<string> names = GetNamesFromMemory();
var result1 = names.Where(n => n.StartsWith("A")).OrderBy(n => n).Take(5);

// Using IQueryable
IQueryable<string> namesQuery = GetNamesFromDatabase().AsQueryable();
var result2 = namesQuery.Where(n => n.StartsWith("A")).OrderBy(n => n).Take(5);

In the above example, result1 and result2 are both LINQ queries written using Where, OrderBy, and Take operators. However, result1 operates on an in-memory collection (IEnumerable), while result2 operates on a queryable data source (IQueryable), which can be a database.

By using IQueryable for SQL queries, you can take advantage of deferred execution, query translation, and efficient execution against the underlying data source. This can lead to improved performance and scalability when dealing with large datasets or complex queries.