sql - 具有空值的SQL连接

当我没有在myComputer表中设置ProcessorID时，查询不起作用
如果在ProcessorID中分配了值，如何进行查询以显示ProcessorName；如果没有，如何进行查询以显示NULL？

CREATE TABLE myProcessor
(
 ID INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
 Name VARCHAR(255) NOT NULL
) ENGINE=InnoDB;

INSERT INTO myProcessor (Name) VALUES ("xeon");

CREATE TABLE myComputer
(
 ID INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
 Name VARCHAR(255) NOT NULL,
 ProcessorID INT DEFAULT NULL,
 FOREIGN KEY (ProcessorID) REFERENCES myProcessor(ID) ON DELETE CASCADE
) ENGINE=InnoDB;

INSERT INTO myComputer (Name) VALUES ("Newton");

SELECT p.Name as ProcessorName, c.Name as ComputerName
FROM myComputer c, myProcessor p
WHERE c.ProcessorID = p.ID
AND c.Name = "Newton";

如果未设置processorID，则上述select查询当前返回null。

最佳答案

如果计算机未分配处理器，则当前查询不返回行，如您提供的示例所示。
您可能希望使用left outer join，而不是使用隐式inner join：

SELECT     p.Name as ProcessorName, c.Name as ComputerName
FROM       myComputer c
LEFT JOIN  myProcessor p ON (c.ProcessorID = p.ID)
WHERE      c.Name = "Newton";

返回：

+---------------+--------------+
| ProcessorName | ComputerName |
+---------------+--------------+
| NULL          | Newton       |
+---------------+--------------+
1 row in set (0.00 sec)

关于sql - 具有空值的SQL连接，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/3562889/