이것이 C#이다(Chapter 15. LINQ) ★★★★

1. LINQ 란? : ★★★

• LINQ 는 Collection(List, 배열 등)을 SQL 처럼 다룰 수 있는 문법을 말한다.
• Collection 만 사용이 가능한데, 이는 IEnumerable<T> 인터페이스를 Collection이 상속하고 있기 때문임.

예)

1) Query

var result =
    from p in products
    where p.Price > 100
    select p.Name;

 

2) Method

var result = products
    .Where(p => p.Price > 100)
    .Select(p => p.Name);

• from(범위) : data 라는 Collection 에서,
• where(조건) : x > 10 인 경우에만,
• select(조회) : x * 2 로 x를 변환하여 > result 에 담는다.

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2. Select(조회 및 변환) : ★★★★

1) Query

var result =
    from p in products
    select p.Name;

 

2) Method

var result = prodcuts.Select(p => p.Name);

> Product 에서 Name 만 추출

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3. Where(조건) : ★★★★

1) Query

var result =
    from p in products
    where p.Price > 100
    select p;

 

2) Method

var result = products.Where(p => p.Price > 100);

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4. OrderBy / OrderByDescending(정렬) : ★★★

1) Query

var result =
    from p in products
    orderby p.Price descending
    select p;

 

2) Method

var result = products
    .OrderByDescending(p => p.Price);

---

5. GroupBy(그룹) : ★★★

1) Query

var result =
    from p in products
    group p by p.Category;

 

2) Method

var result = products
    .GroupBy(p => p.Category);

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6. Join(연결) : ★★★

1) Query

var result =
    from p in products
    join c in categories
    on p.CategoryId equals c.Id
    select new { p.Name, c.CategoryName };

 

2) Method

var result = products.Join(
    categories,
    p => p.CategoryId,
    c => c.Id,
    (p, c) => new { p.Name, c.CategoryName });

---

7. Group + Select : ★★★

1) Query

var result =
    from p in products
    group p by p.Category into g
    select new
    {
        Category = g.Key,
        Count = g.Count()
    };

 

2) Method

var result = products
    .GroupBy(p => p.Category)
    .Select(g => new
    {
        Category = g.Key,
        Count = g.Count()
    });

---

8. SelectMany : ★★★★

1) Query(2중 from)

var result =
    from t in titles
    from word in t.Split(' ')
    select word;

 

2) Method

var result = titles
    .SelectMany(t => t.Split(' '));

---

9. 핵심예제 : ★★★★

1) Query

var result =
    from t in titles
    from word in t.Split(' ')
    group word by word.ToLower() into g
    orderby g.Count() descending
    select new { Word = g.Key, Count = g.Count() };

 

2) Method

var result = titles
    .SelectMany(t => t.Split(' '))
    .GroupBy(w => w.ToLower())
    .OrderByDescending(g => g.Count())
    .Select(g => new { Word = g.Key, Count = g.Count() });

 

3) 정리

from        → 시작
where       → 조건
select      → 결과
orderby     → 정렬
group by    → 그룹
into        → 그룹 후 재사용