为什么ON子句用于全外连接？

Question

根据我的理解，FULL OUTER JOIN是LEFT OUTER JOIN和RIGHT OUTER JOIN的组合。在这种情况下，它只是连接两个表与所有条目。请告诉我为什么我们为ON提供“FULL OUTER JOIN”条款？或者解释如何应用ON子句或者它在查询中有什么区别？

例如查询：

SELECT Customers.CustomerName, Orders.OrderID
FROM Customers
FULL OUTER JOIN Orders
ON Customers.CustomerID=Orders.CustomerID

Answer 1

此图表将帮助您了解连接。

Answer 2

ON 是加入条件，将用于除交叉加入之外的所有联接类型

全外连接类似于：

SELECT employee.LastName, employee.DepartmentID,
       department.DepartmentName, department.DepartmentID
FROM employee
INNER JOIN department ON employee.DepartmentID = department.DepartmentID

UNION ALL

SELECT employee.LastName, employee.DepartmentID,
       CAST(NULL AS VARCHAR(20)), CAST(NULL AS INTEGER)
FROM employee
WHERE NOT EXISTS (
    SELECT * FROM department
             WHERE employee.DepartmentID = department.DepartmentID)

UNION ALL

SELECT CAST(NULL AS VARCHAR(20)), CAST(NULL AS INTEGER),
       department.DepartmentName, department.DepartmentID
FROM department
WHERE NOT EXISTS (
    SELECT * FROM employee
             WHERE employee.DepartmentID = department.DepartmentID)

测试数据：

CREATE TABLE department
(
 DepartmentID INT,
 DepartmentName VARCHAR(20)
);

CREATE TABLE employee
(
 LastName VARCHAR(20),
 DepartmentID INT
);

INSERT INTO department VALUES(31, 'Sales');
INSERT INTO department VALUES(33, 'Engineering');
INSERT INTO department VALUES(34, 'Clerical');
INSERT INTO department VALUES(35, 'Marketing');

INSERT INTO employee VALUES('Rafferty', 31);
INSERT INTO employee VALUES('Jones', 33);
INSERT INTO employee VALUES('Heisenberg', 33);
INSERT INTO employee VALUES('Robinson', 34);
INSERT INTO employee VALUES('Smith', 34);
INSERT INTO employee VALUES('John', NULL);

Reference

Answer 3

cross join在两个表之间生成笛卡尔积，返回所有行的所有可能组合。它有 no on clause ，因为您只是将所有内容加入到所有内容中。

full outer join是左外连接和右外连接的组合。它返回两个表中与查询的where子句匹配的所有行，如果这些行不能满足on条件，则为未填充的字段设置空值。

来自@Donnie answer