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±ÈÈçÔÚNorthwindÊý¾Ý¿âÖÐ  
  ÓÐÒ»¸ö²éѯΪ  
  SELECT   c.CustomerId,   CompanyName  
  from   Customers   c  
  WHERE   EXISTS(  
              SELECT   OrderID   from   Orders   o  
              WHERE   o.CustomerID   =   cu.CustomerID)  
   
  ÕâÀïÃæµÄEXISTSÊÇÈçºÎÔË×÷ÄØ£¿×Ó²éѯ·µ»ØµÄÊÇOrderId×ֶΣ¬¿ÉÊÇÍâÃæµÄ²éѯҪÕÒµÄÊÇCustomerIDºÍCompanyName×ֶΣ¬ÕâÁ½¸ö×ֶο϶¨²»ÔÚOrderIDÀïÃæ°¡£¬ÕâÊÇÈçºÎÆ¥ÅäµÄÄØ£¿
 
EXISTSÓÃÓÚ¼ì²é×Ó²éѯÊÇ·ñÖÁÉٻ᷵»ØÒ»ÐÐÊý¾Ý£¬¸Ã×Ó²éѯʵ¼ÊÉϲ¢²»·µ»ØÈκÎÊý¾Ý£¬¶øÊÇ·µ»ØÖµTrue»òFalse
 
EXISTS
Ö¸¶¨Ò»¸ö×Ó²éѯ£¬¼ì²âÐеĴæÔÚ¡£
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EXISTS subquery
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subquery
ÊÇÒ»¸öÊÜÏÞµÄ SELECT Óï¾ä (²»ÔÊÐíÓÐ COMPUTE ×Ó¾äºÍ INTO ¹Ø¼ü×Ö)¡£Óйظü¶àÐÅÏ¢£¬Çë²Î¼û SELECT ÖÐÓйØ×Ó²éѯµÄÌÖÂÛ¡£
½á¹ûÀàÐÍ
Boolean
½á¹ûÖµ
Èç¹û×Ó²éѯ°üº¬ÐУ¬Ôò·µ»Ø TRUE¡£
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A. ÔÚ×Ó²éѯÖÐʹÓà NULL ÈÔÈ»·µ»Ø½á¹û¼¯
Õâ¸öÀý×ÓÔÚ×Ó²éѯÖÐÖ¸¶¨ NULL£¬²¢·µ»Ø½á¹û¼¯£¬Í¨¹ýʹÓà EXISTS ÈÔȡֵΪ TRUE¡£
USE Northwind
GO
SELECT CategoryName
from Categories
WHERE EXISTS (SELECT NULL)
ORDER BY CategoryName ASC
GO
B. ±È½ÏʹÓà EXISTS ºÍ IN µÄ²éѯ
Õâ¸öÀý×ӱȽÏÁËÁ½¸öÓïÒåÀàËÆµÄ²éѯ¡£µÚÒ»¸ö²éѯʹÓà EXISTS ¶øµÚ¶þ¸ö²éѯʹÓà IN¡£×¢ÒâÁ½¸ö²éѯ·µ»ØÏàͬµÄÐÅÏ¢¡£
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
ÏÂÃæÊÇÈÎÒ»²éѯµÄ½á¹û¼¯£º
pub_name                      &nbs


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