Скачать или смотреть How to Query Data with Filters from Multiple Tables in Entity Framework using LINQ

Learn how to effectively use LINQ in Entity Framework to query and filter data from multiple tables. This guide gives you practical examples and clear explanations to enhance your C# skills.
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How to Query Data with Filters from Multiple Tables in Entity Framework using LINQ

When working with databases in C# , it's common to find yourself needing to retrieve data from multiple tables simultaneously. This task can be achieved smoothly using Entity Framework (EF) and LINQ. This guide will guide you through the process of querying data from multiple tables by demonstrating how to filter results based on specific criteria.

Understanding the Problem

In your scenario, you have two classes: User and Character. A user can have multiple characters, which creates a relationship between these two tables.

Here's a brief summary of the classes:

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You want to write a query similar to this SQL statement:

[[See Video to Reveal this Text or Code Snippet]]

Now, let’s walk through how to implement this query in LINQ.

Performing Joins and Filtering with LINQ

To retrieve data from both the Users and Characters tables, you'll need to utilize the join keyword and apply filtering criteria.

Step 1: Setup Your DbContext

First, ensure your database context (DbContext) is set up to represent the database:

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Step 2: Writing the LINQ Query

Next, create a method to query and filter users based on your criteria.

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Explanation

Joining Tables: This code uses the LINQ join keyword to connect the Users and Characters tables based on the UserId field.

Filtering Criteria: The where clause applies the filtering conditions to find users whose name, email, or character name contains the specified string.

Execution: .ToList() executes the query and returns the results as a list.

Step 3: Handling LEFT JOIN

If you need to perform a LEFT JOIN instead of an INNER JOIN, you can modify the query accordingly:

[[See Video to Reveal this Text or Code Snippet]]

Additional Explanation

Using into: This groups the results into characterGroup.

LEFT JOIN Implementation: DefaultIfEmpty() ensures all users are included, even if they don’t have associated characters. We also have to check if cg is not null before applying further conditions.

Using Include with LINQ

An alternative, more straightforward syntax utilizes .Include() for eager loading of related data:

[[See Video to Reveal this Text or Code Snippet]]

Explanation

Eager Loading: The .Include(u => u.Characters) method loads the related characters of each user.

Filtering with Any: This checks for any characters that match the filtering criteria.

Conclusion

With Entity Framework and LINQ, combining data from multiple tables can be done easily and effectively. In this post, we've demonstrated how to:

Perform inner and left joins

Apply filtering criteria to retrieve specific data

Use eager loading for efficiency

Feel free to customize the filtering conditions to fit your particular needs! Happy coding!