# Skill 11 - LINQ Querying

## Core Concepts

LINQ lets code describe what sequence result is wanted: filter, project, sort, group, join, aggregate, and transform.

## LINQ Parts

- Extension methods such as `Where`, `Select`, `OrderBy`, `GroupBy`, and `Join` provide the operations.
- Providers execute expressions against in-memory objects, EF Core, XML, or other sources.
- Lambda expressions express predicates and projections.
- Query comprehension syntax is optional and maps to method calls.

## Rules

- Prefer method syntax for simple pipelines.
- Consider query syntax for multi-join/group queries when readability improves.
- Understand deferred execution: queries may not run until enumerated.
- Materialize with `ToList`, `ToArray`, or similar only when needed.
- Avoid multiple enumeration of expensive queries.
- Place filters early.
- For EF Core, ensure the query can translate to SQL.
- Do not call custom .NET methods inside EF queries unless they can be translated.

## Common Operations

- `Where` filters.
- `Select` projects.
- `OrderBy` / `ThenBy` sorts.
- `GroupBy` groups.
- `Join`, `GroupJoin`, `LeftJoin`, and `RightJoin` combine sequences where available.
- `Any` checks existence.
- `All` checks universal conditions.
- `Count`, `Sum`, `Average`, `Min`, `Max` aggregate.

## Performance Rules

- Avoid materializing before filtering.
- Use `Any()` instead of `Count() > 0` for existence.
- Use `TryGetNonEnumeratedCount` when count may be available cheaply.
- Be careful with closures in loops.