如何显示空值的行？

Question

我想弄清楚如何显示带有“JobTitleID”列的空值的行。

SQL文件：

DROP TABLE Employee; 
DROP TABLE JobTitle; 


-- create our table 
CREATE TABLE JobTitle 
(
JobTitleID Number(38) PRIMARY KEY, 
JobTitle VARCHAR2(25) 
) 
; 


CREATE TABLE Employee 
(
EmployeeID Number(38) PRIMARY KEY, 
EmployeeFName VARCHAR2(25), 
EmployeeLName VARCHAR2(25), 
ClientName VARCHAR2(25), 
HoursWorked Number(5,2), 
ChargeRate Number(5,2), 
JobTitleID Number(38), 
CONSTRAINT fkJobTitle FOREIGN KEY (JobTitleID) REFERENCES JobTitle(JobTitleID) 
) 
; 



-- populate tables 

INSERT ALL 
INTO JobTitle (JobTitleID, JobTitle) VALUES (1,'Project Manager') 
INTO JobTitle (JobTitleID, JobTitle) VALUES (2,'Programmer') 
INTO JobTitle (JobTitleID, JobTitle) VALUES (3,'Network Specialist') 
INTO JobTitle (JobTitleID, JobTitle) VALUES (4,'Technical Support') 
INTO JobTitle (JobTitleID, JobTitle) VALUES (5,'Cable Installer') 
INTO JobTitle (JobTitleID, JobTitle) VALUES (6,'DBA') 
INTO JobTitle (JobTitleID, JobTitle) VALUES (7,'Telecom Engineer') 
SELECT * FROM dual; 

INSERT ALL 
INTO Employee (EmployeeID, EmployeeFName,EmployeeLName,ClientName,HoursWorked,ChargeRate,JobTitleID) VALUES (1,'Bob','Smith','Acme Corp.',125.5,72.25,1) 
INTO Employee (EmployeeID, EmployeeFName,EmployeeLName,ClientName,HoursWorked,ChargeRate,JobTitleID) VALUES(2,'Bob','Smith','Astro Electric',32,72.25,1) 
INTO Employee (EmployeeID, EmployeeFName,EmployeeLName,ClientName,HoursWorked,ChargeRate,JobTitleID) VALUES(3,'Jane','Doe','Acme Corp.',160,25,2) 
INTO Employee (EmployeeID, EmployeeFName,EmployeeLName,ClientName,HoursWorked,ChargeRate,JobTitleID) VALUES(4,'Jane','Doe','Astro Electric.',220,25,2) 
INTO Employee (EmployeeID, EmployeeFName,EmployeeLName,ClientName,HoursWorked,ChargeRate,JobTitleID) VALUES (5,'Jane','Doe','Durham Tiles',12,25,2) 
INTO Employee (EmployeeID, EmployeeFName,EmployeeLName,ClientName,HoursWorked,ChargeRate,JobTitleID) VALUES (6,'Henry','Jones','Lighting Unlimited',245,15,4) 
INTO Employee (EmployeeID, EmployeeFName,EmployeeLName,ClientName,HoursWorked,ChargeRate,JobTitleID) VALUES(7,'Jane','Fonda','Whale Mart',275,72.25,1) 
INTO Employee (EmployeeID, EmployeeFName,EmployeeLName,ClientName,HoursWorked,ChargeRate,JobTitleID) VALUES(8,'Bill','Murray','Whale Mart',145,20,5) 
INTO Employee (EmployeeID, EmployeeFName,EmployeeLName,ClientName,HoursWorked,ChargeRate,JobTitleID) VALUES(9,'Bill','Murray','ABC Logistics',45,20,5) 
INTO Employee (EmployeeID, EmployeeFName,EmployeeLName,ClientName,HoursWorked,ChargeRate,JobTitleID) VALUES (10,'John','Jameson','Whale Mart',160,20,5) 
INTO Employee (EmployeeID, EmployeeFName,EmployeeLName,ClientName,HoursWorked,ChargeRate,JobTitleID) VALUES (11,'John','Jameson','ABC Logistics',130,20,5) 
INTO Employee (EmployeeID, EmployeeFName,EmployeeLName,ClientName,HoursWorked,ChargeRate,JobTitleID) VALUES(12,'John','Jameson','ABM Systems',8,20,5) 
INTO Employee (EmployeeID, EmployeeFName,EmployeeLName,ClientName,HoursWorked,ChargeRate,JobTitleID) VALUES(13,'Homer','Simpson','Flitter',345,25,2) 
INTO Employee (EmployeeID, EmployeeFName,EmployeeLName,ClientName,HoursWorked,ChargeRate,JobTitleID) VALUES(14,'John','Carpenter','Speedy Messengers',25,15,4) 
INTO Employee (EmployeeID, EmployeeFName,EmployeeLName,ClientName,HoursWorked,ChargeRate,JobTitleID) VALUES (15,'John','Carpenter','Flitter',123,15,4) 
INTO Employee (EmployeeID, EmployeeFName,EmployeeLName,ClientName,HoursWorked,ChargeRate,JobTitleID) VALUES (16,'John','Carpenter','ABM Systems',67,15,4) 
INTO Employee (EmployeeID, EmployeeFName,EmployeeLName,ClientName,HoursWorked,ChargeRate,JobTitleID) VALUES(17,'John','Carpenter','ABC Logistics',23,15,4) 
INTO Employee (EmployeeID, EmployeeFName,EmployeeLName,ClientName,HoursWorked,ChargeRate,JobTitleID) VALUES(18,'Mary','Jane','ABM Systems',43,72.25,1) 
INTO Employee (EmployeeID, EmployeeFName,EmployeeLName,ClientName,HoursWorked,ChargeRate,JobTitleID) VALUES(19,'Mary','Jane','Flitter',156,72.25,1) 
INTO Employee (EmployeeID, EmployeeFName,EmployeeLName,ClientName,HoursWorked,ChargeRate,JobTitleID) VALUES (20,'Frank','Herbert','Acme Corp.',55,45,3) 
INTO Employee (EmployeeID, EmployeeFName,EmployeeLName,ClientName,HoursWorked,ChargeRate,JobTitleID) VALUES (21,'Kathy','Smith','',0,45,3) 
SELECT * FROM dual; 



COMMIT; 

到目前为止，我有这样的：

COLUMN Employee FORMAT a25; 
SELECT JOBTITLE.JOBTITLE AS "Job Title" 
FROM EMPLOYEE 
FULL JOIN JOBTITLE 
ON EMPLOYEE.JOBTITLEID = JOBTITLE.JOBTITLEID 
WHERE EMPLOYEE.JOBTITLEID = 0 
ORDER BY "Job Title"; /*Ordering it by alphabetical order/* 

现在我知道WHERE EMPLOYEE.JOBTITLEID = 0不会因为表JobTitle不为0的有效JobTitleID的工作。

，我追求的是输出继电器：

Job Title 
----------------------- 
DBA 
Telecom Engineer 

因为没有任何的“员工”的那些行的。

我目前没有显示任何行。

Answer 1

您可以在jobtitle表上使用left join。

SELECT DISTINCT JOBTITLE.JOBTITLE AS "Job Title" 
FROM JOBTITLE 
LEFT JOIN EMPLOYEE 
ON EMPLOYEE.JOBTITLEID = JOBTITLE.JOBTITLEID 
WHERE EMPLOYEE.JOBTITLEID IS NULL 
ORDER BY "Job Title"; 

或使用not exists。

select jobtitle 
from jobtitle j 
where not exists (select 1 from employee where jobtitleid = j.jobtitleid) 
order by 1 

Answer 2

你只需要改变

EMPLOYEE.JOBTITLEID = 0 

到

EMPLOYEE.JOBTITLEID is null 

因为在连接条件，其 没有发现会自动空例如

列在连接表中的值

students 
+-------------+--------------+--------- 
| id   | name   | section| 
+-------------+--------------+--------- 
|   1 | Stud 1  | 1 | 
|   2 | Stud 2  | 3 | 
+-------------+--------------+--------- 

sections 

+-------------+--------------+---- 
| id   | name | teacher | 
+-------------+--------------+---- 
|   1 | A | Teacher 1 | 
|   2 | B | Teacher 2 | 
+-------------+--------------+---- 

Select students.id, students.name, sections.name from students 
join sections on students.section = sections.id 

RESULT 

+-------------+--------------+--------- 
| id   | name   | section| 
+-------------+--------------+--------- 
|   1 | Stud 1 |  | A | 
|   2 | Stud 2 |  | null | 
+-------------+--------------+--------- 

，因为你看到未找到结果部分中的联接为空

Answer 3

你只需要检查表设计，如果你正在寻找没有值的字段是非数字的，并允许NULL，那么你可以使用“IS NULL“在where子句，否则，如果它的数字，你可以使用”='0'“