How can you handle an IN sub-query with LINQ to SQL?

Any for EXISTS, because LINQ is not EXISTent without Any!

// SQL EXISTS in Lisp-like language, you got this!
var result = dbContext.ParentTable
                      .Where(p => dbContext.ChildTable.Any(c => c.ParentId == p.Id));
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let your complex subqueries shine

// LINQ with let keyword: because, why not?
var query = from p in dbContext.ParentTable
            let subQuery = from c in dbContext.ChildTable
                           where p.Id == c.ParentId
                           select c
            where subQuery.Any()
            select p;


Shutdown complexity with let and Contains

// Thought SQL was fun? Wait till you meet LINQ!
var query = from p in dbContext.ParentTable
            let ids = subQuery.Select(c => c.Id)
            where ids.Contains(p.Id)
            select p;
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From SQL complexity to LINQ simplicity

Translate complex SQL to LINQ by deconstructing the query into smaller parts and using relevant LINQ methods.

Fine-tuning LINQ sub-queries

Crafting SQL sub-queries and turning them into LINQ can be a bit tricky. Here's a quick remedy to address some of the main pain points:

• Deferred execution: LINQ queries execute when enumerated. Watch for context changes along the line.
• Performance tune-ups: Sub-queries can be sluggish if mismanaged. Use .Join() over nested .Where() if possible.
• Avoiding re-evaluation: Cache sub-query outcomes in a variable to avoid repeat execution.
• Data types alignment: Ensure .Contains() collection item type matches the field type it is compared against.

Advanced LINQ sub-query patterns

Condition handling for the Pros

var complexQuery = dbContext.Table
                            .Where(row => ids.Contains(row.Id) &&
                                          row.Date > startDate &&
                                          row.Date < endDate); 
                                          // SQL, Riddikulus! (Harry Potter fans, anyone?)


Map and fetch like a Cartographer

var subQueryDictionary = dbContext.ChildTable
                                  .Where(c => c.SomeCondition)
                                  .ToDictionary(c => c.Key, c => c.Value);

var mainQuery = dbContext.ParentTable
                         .Where(p => subQueryDictionary.ContainsKey(p.Id) &&
                                     subQueryDictionary[p.Id] == someValue);

Nested queries: An inception moment!

var nestedQuery = dbContext.GrandParentTable
                           .Select(g => new {
                               GrandParent = g,
                               Parents = g.ParentTable
                                          .Where(p => ids.Contains(p.Id))
                                          .Select(p => new {
                                              Parent = p,
                                              // It's inception, darling! We're going one layer deeper!
                                              Children = p.ChildTable
                                                          .Where(c => c.SomeCondition)
                                          })
                           });