How to perform Join between multiple tables in LINQ lambda

How to perform Join between multiple tables in LINQ lambda

Performing joins between multiple tables in LINQ using lambda expressions involves utilizing the Join() method along with the appropriate key selectors. Here's an example that demonstrates how to perform a join between three tables:

var result = table1
    .Join(table2, t1 => t1.CommonKey, t2 => t2.CommonKey, (t1, t2) => new { t1, t2 })
    .Join(table3, t => t.t2.AnotherKey, t3 => t3.AnotherKey, (t, t3) => new { t.t1, t.t2, t3 })
    .Select(t => new
    {
        t.t1.Column1,
        t.t2.Column2,
        t.t3.Column3
        // Add more columns as needed
    });

In this example, we have three tables (table1, table2, table3) that need to be joined based on common keys (CommonKey and AnotherKey).

The Join() method is used twice to perform the joins. The first Join() call joins table1 and table2, and the second Join() call joins the result of the first join (t) with table3.

The key selectors (t1 => t1.CommonKey, t2 => t2.CommonKey, t => t.t2.AnotherKey, t3 => t3.AnotherKey) specify which properties are used for joining the tables.

The anonymous objects (new { t1, t2 }, new { t.t1, t.t2, t3 }) are created to hold the joined results.

Finally, the Select() method is used to select the desired columns from the joined tables and create a new anonymous type or a custom class that represents the combined result.

You can customize the columns included in the final result by modifying the Select() clause and including the desired properties from the joined tables.

Note: It's important to ensure that the types of the common keys match between the tables to perform successful joins.

Examples

  1. "LINQ lambda join multiple tables example"

    • Code:
      var result = table1
    .Join(table2, t1 => t1.Key, t2 => t2.Key, (t1, t2) => new { t1, t2 })
    .Join(table3, combined => combined.t1.Foreignkey, t3 => t3.Foreignkey, (combined, t3) => new { combined.t1, combined.t2, t3 })
    .Select(final => new { final.t1, final.t2, final.t3 });
    • Description: Provides a basic example of performing a join between three tables using LINQ lambda expressions, combining results into a final projection.

  2. "LINQ lambda join with multiple conditions"

    • Code:
      var result = table1
    .Join(table2, t1 => new { t1.Key, t1.Category }, t2 => new { t2.Key, t2.Category }, (t1, t2) => new { t1, t2 })
    .Join(table3, combined => combined.t1.Foreignkey, t3 => t3.Foreignkey, (combined, t3) => new { combined.t1, combined.t2, t3 })
    .Select(final => new { final.t1, final.t2, final.t3 });
    • Description: Extends the join example to include multiple conditions for the join, using composite keys for more complex matching.

  3. "LINQ lambda join with left outer join"

    • Code:
      var result = table1
    .GroupJoin(table2, t1 => t1.Key, t2 => t2.Key, (t1, t2Group) => new { t1, t2Group })
    .SelectMany(combined => combined.t2Group.DefaultIfEmpty(), (combined, t2) => new { combined.t1, t2 })
    .Join(table3, combined => combined.t1.t1.Foreignkey, t3 => t3.Foreignkey, (combined, t3) => new { combined.t1.t1, combined.t1.t2, t3 })
    .Select(final => new { final.t1, final.t2, final.t3 });
    • Description: Demonstrates a left outer join in LINQ lambda expressions, combining results from multiple tables with the option for null values.

  4. "LINQ lambda join with anonymous types"

    • Code:
      var result = table1
    .Join(table2, t1 => new { t1.Key, t1.Category }, t2 => new { t2.Key, t2.Category }, (t1, t2) => new { t1, t2 })
    .Join(table3, combined => new { combined.t1.Foreignkey, combined.t1.t2.OtherKey }, t3 => new { t3.Foreignkey, t3.OtherKey }, (combined, t3) => new { combined.t1, combined.t2, t3 })
    .Select(final => new { final.t1, final.t2, final.t3 });
    • Description: Utilizes anonymous types for the join conditions, showcasing a flexible approach when dealing with complex matching criteria.

  5. "LINQ lambda join with multiple tables and projections"

    • Code:
      var result = table1
    .Join(table2, t1 => t1.Key, t2 => t2.Key, (t1, t2) => new { t1, t2 })
    .Join(table3, combined => combined.t1.Foreignkey, t3 => t3.Foreignkey, (combined, t3) => new { combined.t1, combined.t2, t3 })
    .Select(final => new
    {
        Property1 = final.t1.Column1,
        Property2 = final.t2.Column2,
        Property3 = final.t3.Column3
    });
    • Description: Illustrates a join with multiple tables and custom projections, selecting specific properties from each joined table.

  6. "LINQ lambda join with method syntax"

    • Code:
      var result = table1
    .Join(table2, t1 => t1.Key, t2 => t2.Key, (t1, t2) => new { t1, t2 })
    .Join(table3, combined => combined.t1.Foreignkey, t3 => t3.Foreignkey, (combined, t3) => new { combined.t1, combined.t2, t3 })
    .Select(final => new { final.t1, final.t2, final.t3 });
    • Description: Demonstrates performing a join using method syntax in LINQ lambda expressions for those who prefer this style.

  7. "LINQ lambda join with group by"

    • Code:
      var result = table1
    .Join(table2, t1 => t1.Key, t2 => t2.Key, (t1, t2) => new { t1, t2 })
    .Join(table3, combined => combined.t1.Foreignkey, t3 => t3.Foreignkey, (combined, t3) => new { combined.t1, combined.t2, t3 })
    .GroupBy(final => new { final.t1.Category, final.t2.Type })
    .Select(group => new
    {
        GroupKey = group.Key,
        TotalCount = group.Count(),
        AverageValue = group.Average(item => item.t3.Value)
    });
    • Description: Incorporates a group by clause after performing a join, enabling aggregation and summary calculations on the grouped results.

  8. "LINQ lambda join with composite keys"

    • Code:
      var result = table1
    .Join(table2, t1 => new { t1.Key1, t1.Key2 }, t2 => new { t2.Key1, t2.Key2 }, (t1, t2) => new { t1, t2 })
    .Join(table3, combined => new { combined.t1.Foreignkey, combined.t1.t2.OtherKey }, t3 => new { t3.Foreignkey, t3.OtherKey }, (combined, t3) => new { combined.t1, combined.t2, t3 })
    .Select(final => new { final.t1, final.t2, final.t3 });
    • Description: Handles composite keys during the join process, ensuring accurate matching of multiple key components.

  9. "LINQ lambda join with conditional join"

    • Code:
      var result = table1
    .Join(table2, t1 => t1.Key, t2 => t2.Key, (t1, t2) => new { t1, t2 })
    .GroupJoin(table3, combined => combined.t1.Foreignkey, t3 => t3.Foreignkey, (combined, t3Group) => new { combined.t1, combined.t2, t3Group })
    .SelectMany(combined => combined.t3Group.DefaultIfEmpty(), (combined, t3) => new { combined.t1, combined.t2, t3 })
    .Where(final => final.t3 != null)
    .Select(final => new { final.t1, final.t2, final.t3 });
    • Description: Demonstrates a conditional join, combining tables and filtering results based on a specified condition during the join process.

  10. "LINQ lambda join with multiple OR conditions"

    • Code:
      var result = table1
    .Join(table2, t1 => t1.Key, t2 => t2.Key1, (t1, t2) => new { t1, t2 })
    .Join(table3, combined => combined.t1.Foreignkey, t3 => t3.Foreignkey, (combined, t3) => new { combined.t1, combined.t2, t3 })
    .Where(final => final.t1.Category == "A" || final.t2.Type == "B" || final.t3.Value > 10)
    .Select(final => new { final.t1, final.t2, final.t3 });
    • Description: Incorporates multiple OR conditions within the Where clause during the join, allowing for flexible filtering based on different criteria.

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