How to Perform a Comprehensive Multi-Table Join Using LINQ Lambda?

Use LINQ Lambda to achieve efficient joins of multiple tables

When processing multi-table data in LINQ, join operations are the key to efficiently obtain related data. This article will introduce how to use LINQ Lambda to perform join operations between the three tables of Product, Category and ProductCategory.

The following code snippet shows how to achieve this:

<code class="language-csharp">var categorizedProducts = product
    .Join(productcategory, p => p.Id, pc => pc.ProdId, (p, pc) => new { p, pc })
    .Join(category, ppc => ppc.pc.CatId, c => c.Id, (ppc, c) => new { ppc, c })
    .Select(m => new { 
        ProdId = m.ppc.p.Id, // 或 m.ppc.pc.ProdId
        CatId = m.c.CatId
        // 其他字段映射
    });</code>

This code achieves its goal through a series of join operations:

1. Join the Product table and ProductCategory table based on the condition p.Id == pc.ProdId.
2. Join the result of the first join with the Category table, the condition is ppc.pc.CatId == c.Id.
3. Finally, project the result of the second join into a new anonymous type that contains the fields required by the CategorizedProducts class.

With this approach, you can efficiently join multiple tables and extract the required data into a single object.

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