SQL加入Null值

Question

当我没有在myComputer表中设置ProcessorID时,我的查询不起作用当我没有在ProcessorID中指定值时,我怎么能创建一个显示ProcessorName的查询？

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";

如果尚未设置processorID,则上面的select查询当前返回null.

Answer 1

如果未为计算机分配处理器,则当前查询不返回任何行,如您提供的示例所示.

您可能希望使用左外连接,而不是使用的隐式内连接:

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)