# Querying and Saving Data — .NET Core: Build, Test and Ship Web APIs

Source: https://www.geekswithgeeks.com/en/dotnet-core/ef-querying-saving

> Read with LINQ, avoid the N+1 problem and write changes with SaveChangesAsync.

## LINQ becomes SQL

EF Core translates `Where` and `Select` into SQL, so filtering happens in the database. Use `AsNoTracking()` for read-only queries to skip change tracking, and always use the `Async` methods in web apps.

## CRUD endpoints

Read cheap books without tracking, and add a book then save once. Related data is loaded explicitly with `Include`.

```csharp
app.MapGet("/books", async (ShopDb db) =>
    await db.Books.AsNoTracking()
        .Where(b => b.Price < 500)
        .OrderBy(b => b.Title)
        .ToListAsync());

app.MapPost("/books", async (Book b, ShopDb db) =>
{
    db.Books.Add(b);
    await db.SaveChangesAsync();
    return Results.Created($"/books/{b.Id}", b);
});
```

## The N+1 trap

Looping over orders and touching each order's items can fire one query per order. Load related rows in a single query with `Include`, or project to a DTO with `Select`. Log SQL in development to notice this early.

**Quiz:** What does AsNoTracking() do?

- [ ] Deletes the rows
- [ ] Encrypts the query
- [ ] Disables caching of the table
- [x] Skips change tracking for a faster read-only query

*Answer:* Skips change tracking for a faster read-only query. Returned entities are not tracked, which saves memory and time when you will not update them.