LINQ Multi-field connection detailed explanation

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LINQ (Language Integrated Query) provides a powerful and expressive way to query data. One of its key features is the ability to use the join clause to connect data from multiple data sources. While traditional connections usually involve connections of a single field, LINQ can also implement multi-field connections.

Suppose the following scenario: You need to perform a LINQ query that joins two tables entity and entity2, where field1 in entity The ] and field2 fields need to match the field1 and fields in entity2.

To do this, you can use the following syntax:

var result = from x in entity
             join y in entity2
             on new { x.field1, x.field2 } equals new { y.field1, y.field2 }

Anonymous type

{ x.field1, x.field2 } creates a key combination that combines field1 and from the entity table The value of field2. This key combination is then compared with the key combination { y.field1, y.field2 } from the entity2 table.

It should be noted that this method assumes an equal value connection, where the values ​​in the join field must be the same. If you need non-equivalent joins, such as date range queries, you can add additional conditions in the

where clause.

For example, to connect

field1 and of entity and entity2, and ensure the entity in entity The date field is within the specified range, and you can use:

var result = from x in entity
             join y in entity2
             on new { x.field1, x.field2 } equals new { y.field1, y.field2 }
             where x.date >= startDate && x.date 

This syntax allows for flexible and powerful data manipulation, allowing you to connect data on multiple fields and apply additional constraints as needed.