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，如何生成一个显示ProcessorName的查询，否则当我没有？时显示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left outer join，而不是使用的隐式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)

Answer 2

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

SQL加入Null值

2 个答案: