How to Perform Multiple Table Joins Using LINQ Lambda Expressions?

Use LINQ Lambda expressions to perform multi-table joins

LINQ allows joining multiple tables using lambda expressions. Let's say we have three classes:

<code>Product { Id, ProdName, ProdQty }
Category { Id, CatName }
ProductCategory { ProdId, CatId }</code>

Concatenation using Lambda expressions

These tables can be joined using the following code:

<code>var query = product.Join(
    productcategory,
    p => p.Id,
    pc => pc.ProdID,
    (p, pc) => new { product = p, productcategory = pc }
).Join(
    category,
    ppc => ppc.productcategory.CatId,
    c => c.Id,
    (ppc, c) => new { productproductcategory = ppc, category = c }
);</code>

This will generate objects of the following classes:

<code>QueryClass { productproductcategory, category }</code>

Populate new object

To populate a new object with the connection result properties, you can use the following Select method:

<code>var categorizedProducts = query.Select(m => new {
    ProdId = ???, 
    CatId = ???, 
    // 其他赋值
});</code>

Get attribute value

The specific property to be assigned can be obtained from the connected object as follows:

<code>ProdId = m.ppc.p.Id, // 或 m.ppc.pc.ProdId
CatId = m.c.CatId
// 其他赋值</code>

Query syntax alternatives

Alternatively, you can use query syntax, which provides a more concise approach:

<code>var categorizedProducts = from p in product
                       join pc in productcategory on p.Id equals pc.ProdId
                       join c in category on pc.CatId equals c.Id
                       select new {
                           ProdId = p.Id, // 或 pc.ProdId
                           CatId = c.CatId
                           // 其他赋值
                       };</code>

The results of this query are the same as the lambda expression method. If readability is more important, it is recommended to use query syntax.

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