Question

我必须在sql中编写一个使用exists命令的create view语句。我尝试在网上查找，但我遇到了一些困难。我尽力编写创建视图文件，但它现在无法正常工作。我知道我需要在我的声明中使用关键字EXIST。我想要创建的声明是

Write a query that shows returns the name and city of the university that has no people in database
that are associated with it.

我到目前为止编写的代码是

CREATE VIEW exist AS
SELECT a.university_name, a.city
FROM lab5.university as a
INNER JOIN lab5.person as b
ON a.uid = b.uid
WHERE b.uid  NOT EXIST

我正在使用的表是

         Table "table.university"
     Column      |         Type          |                        Modifiers     
-----------------+-----------------------+--------------------------------------
 uid             | integer               | not null default nextval('university_uid_seq'::regclass)
 university_name | character varying(50) | 
 city            | character varying(50) |

和

            Table "table.person"
 Column |         Type          |                      Modifiers                       
--------+-----------------------+-----------------------------------------------
 pid    | integer               | not null default nextval('person_pid_seq'::reg class)
 uid    | integer               | 
 fname  | character varying(25) | not null
 lname  | character varying(25) | not null

Answer 1

您好使用以下代码并告诉我反馈

创建视图checkuni AS 选择a.university_name，a.city FROM 大学作为一个什么不存在（选择p.uid FROM person为p 在哪里p.uid = a.uid）

Answer 2

EXISTS是一个谓词，因此您的查询应该类似于：

SELECT a.university_name, a.city
FROM lab5.university as a
WHERE NOT EXISTS (
    SELECT 1 
    FROM lab5.person as b
    WHERE a.uid = b.uid
);

您还可以使用外部联接和IS NULL谓词来表达相同的内容：

SELECT a.university_name, a.city
FROM lab5.university as a
LEFT JOIN lab5.person as b
    ON a.uid = b.uid
WHERE b.uid IS NULL

在某些情况下，您可能需要使用SELECT DISTINCT，但我想在您的情况下不一定需要。

使用EXIST命令创建sql视图语句

2 个答案: