2.できる

1.JOIN(INNERJOIN)-内部連結

は、両方とも値を持つ行(NOT NULL)

を返す.

「INNER」はオプションの

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;

💡 JOINはテーブルをたくさん作ることができます

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;

💡 JOINテーブルGROUP

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-同一テープ

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-外部接続

の他端にデータ(NULL)があるか否かにかかわらず、選択方向において出力-行数

を決定する.

「OUTER」はオプションの

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-クロスジョイント

無条件にすべての組合せ(A*B)

を返す

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