EXISTS

指定一個(gè)子查詢(xún)，檢測行的存在。

語(yǔ)法

EXISTSsubquery

參數

subquery

是一個(gè)受限的 SELECT 語(yǔ)句 (不允許有 COMPUTE 子句和 INTO 關(guān)鍵字)。有關(guān)更多信息，請參見(jiàn)SELECT中有關(guān)子查詢(xún)的討論。

結果類(lèi)型

Boolean

結果值

如果子查詢(xún)包含行，則返回 TRUE。

示例

A. 在子查詢(xún)中使用 NULL 仍然返回結果集

這個(gè)例子在子查詢(xún)中指定 NULL，并返回結果集，通過(guò)使用 EXISTS 仍取值為 TRUE。

USE Northwind
GO
SELECT CategoryName
FROM Categories
WHERE EXISTS (SELECT NULL)
ORDER BY CategoryName ASC
GO

B. 比較使用 EXISTS 和 IN 的查詢(xún)

這個(gè)例子比較了兩個(gè)語(yǔ)義類(lèi)似的查詢(xún)。第一個(gè)查詢(xún)使用 EXISTS 而第二個(gè)查詢(xún)使用 IN。注意兩個(gè)查詢(xún)返回相同的信息。

USE pubs
GO
SELECT DISTINCT pub_name
FROM publishers
WHERE EXISTS
    (SELECT *
    FROM titles
    WHERE pub_id = publishers.pub_id
    AND type = \'business\')
GO

-- Or, using the IN clause:

USE pubs
GO
SELECT distinct pub_name
FROM publishers
WHERE pub_id IN
    (SELECT pub_id
    FROM titles
    WHERE type = \'business\')
GO

下面是任一查詢(xún)的結果集：

pub_name                                 
----------------------------------------
Algodata Infosystems                     
New Moon Books                           

(2 row(s) affected)

C.比較使用 EXISTS 和 = ANY 的查詢(xún)

本示例顯示查找與出版商住在同一城市中的作者的兩種查詢(xún)方法：第一種方法使用 = ANY，第二種方法使用 EXISTS。注意這兩種方法返回相同的信息。

USE pubs
GO
SELECT au_lname, au_fname
FROM authors
WHERE exists
    (SELECT *
    FROM publishers
    WHERE authors.city = publishers.city)
GO

-- Or, using = ANY

USE pubs
GO
SELECT au_lname, au_fname
FROM authors
WHERE city = ANY
    (SELECT city
    FROM publishers)
GO

下面是任一查詢(xún)的結果集：

au_lname                                  au_fname             
---------------------------------------- --------------------
Carson                                    Cheryl               
Bennet                                    Abraham              

(2 row(s) affected)

D.比較使用 EXISTS 和 IN 的查詢(xún)

本示例所示查詢(xún)查找由位于以字母 B 開(kāi)頭的城市中的任一出版商出版的書(shū)名：

USE pubs
GO
SELECT title
FROM titles
WHERE EXISTS
    (SELECT *
    FROM publishers
    WHERE pub_id = titles.pub_id
    AND city LIKE \'B%\')
GO

-- Or, using IN:

USE pubs
GO
SELECT title
FROM titles
WHERE pub_id IN
    (SELECT pub_id
    FROM publishers
    WHERE city LIKE \'B%\')
GO

下面是任一查詢(xún)的結果集：

title                                                                            
------------------------------------------------------------------------
The Busy Executive\'s Database Guide                                              
Cooking with Computers: Surreptitious Balance Sheets                             
You Can Combat Computer Stress!                                                  
Straight Talk About Computers                                                    
But Is It User Friendly?                                                         
Secrets of Silicon Valley                                                        
Net Etiquette                                                                    
Is Anger the Enemy?                                                              
Life Without Fear                                                                
Prolonged Data Deprivation: Four Case Studies                                    
Emotional Security: A New Algorithm                                              

(11 row(s) affected)

E. 使用 NOT EXISTS

NOT EXISTS 的作用與 EXISTS 正相反。如果子查詢(xún)沒(méi)有返回行，則滿(mǎn)足 NOT EXISTS 中的 WHERE 子句。本示例查找不出版商業(yè)書(shū)籍的出版商的名稱(chēng)：

USE pubs
GO
SELECT pub_name
FROM publishers
WHERE NOT EXISTS
    (SELECT *
    FROM titles
    WHERE pub_id = publishers.pub_id
    AND type = \'business\')
ORDER BY pub_name
GO

下面是結果集：

pub_name                                 
----------------------------------------
Binnet & Hardley                         
Five Lakes Publishing                    
GGG&G                                    
Lucerne Publishing                       
Ramona Publishers                        
Scootney Books                           

(6 row(s) affected)