2-2. JOIN - 여러 테이블 조립하기

1. JOIN(INNER JOIN) - 내부 조인

• 양쪽 모두에 값이 있는 행(NOT NULL) 반환
• 'INNER '는 선택사항

SELECT * FROM Categories C
JOIN Products P 
  ON C.CategoryID = P.CategoryID; 

SELECT C.CategoryID, C.CategoryName, P.ProductName
FROM Categories C
JOIN Products P 
  ON C.CategoryID = P.CategoryID; 

-- ambiguous 주의!

SELECT
  CONCAT(
    P.ProductName, ' by ', S.SupplierName
  ) AS Product,
  S.Phone, P.Price
FROM Products P
JOIN Suppliers S
  ON P.SupplierID = S.SupplierID
WHERE Price > 50
ORDER BY ProductName;

💡 여러 테이블을 JOIN할 수 있습니다.

SELECT 
  C.CategoryID, C.CategoryName, 
  P.ProductName, 
  O.OrderDate,
  D.Quantity
FROM Categories C
JOIN Products P 
  ON C.CategoryID = P.CategoryID
JOIN OrderDetails D
  ON P.ProductID = D.ProductID
JOIN Orders O
  ON O.OrderID = D.OrderID;

💡 JOIN한 테이블 GROUP하기

SELECT 
  C.CategoryName,
  MIN(O.OrderDate) AS FirstOrder,
  MAX(O.OrderDate) AS LastOrder,
  SUM(D.Quantity) AS TotalQuantity
FROM Categories C
JOIN Products P 
  ON C.CategoryID = P.CategoryID
JOIN OrderDetails D
  ON P.ProductID = D.ProductID
JOIN Orders O
  ON O.OrderID = D.OrderID
GROUP BY C.CategoryID;

SELECT 
  C.CategoryName, P.ProductName,
  MIN(O.OrderDate) AS FirstOrder,
  MAX(O.OrderDate) AS LastOrder,
  SUM(D.Quantity) AS TotalQuantity
FROM Categories C
JOIN Products P 
  ON C.CategoryID = P.CategoryID
JOIN OrderDetails D
  ON P.ProductID = D.ProductID
JOIN Orders O
  ON O.OrderID = D.OrderID
GROUP BY C.CategoryID, P.ProductID;

💡 SELF JOIN - 같은 테이블끼리

SELECT
  E1.EmployeeID, CONCAT_WS(' ', E1.FirstName, E1.LastName) AS Employee,
  E2.EmployeeID, CONCAT_WS(' ', E2.FirstName, E2.LastName) AS NextEmployee
FROM Employees E1 JOIN Employees E2
ON E1.EmployeeID + 1 = E2.EmployeeID;

-- 1번의 전, 마지막 번호의 다음은?

2. LEFT/RIGHT OUTER JOIN - 외부 조인

• 반대쪽에 데이터가 있든 없든(NULL), 선택된 방향에 있으면 출력 - 행 수 결정
• 'OUTER '는 선택사항

SELECT
  E1.EmployeeID, CONCAT_WS(' ', E1.FirstName, E1.LastName) AS Employee,
  E2.EmployeeID, CONCAT_WS(' ', E2.FirstName, E2.LastName) AS NextEmployee
FROM Employees E1
LEFT JOIN Employees E2
ON E1.EmployeeID + 1 = E2.EmployeeID
ORDER BY E1.EmployeeID;

-- LEFT를 RIGHT로 바꿔서도 실행해 볼 것

SELECT
  C.CustomerName, S.SupplierName,
  C.City, C.Country
FROM Customers C
LEFT JOIN Suppliers S
ON C.City = S.City AND C.Country = S.Country;

-- LEFT를 RIGHT로 바꿔서도 실행해 볼 것

SELECT
  IFNULL(C.CustomerName, '-- NO CUSTOMER --'),
  IFNULL(S.SupplierName, '-- NO SUPPLIER --'),
  IFNULL(C.City, S.City),
  IFNULL(C.Country, S.Country)
FROM Customers C
LEFT JOIN Suppliers S
ON C.City = S.City AND C.Country = S.Country;

-- LEFT를 RIGHT로 바꿔서도 실행해 볼 것

3. CROSS JOIN - 교차 조인

• 조건 없이 모든 조합 반환(A * B)

SELECT
  E1.LastName, E2.FirstName
FROM Employees E1
CROSS JOIN Employees E2
ORDER BY E1.EmployeeID;