之前在论坛中见到一个针对in/exists的讨论。原帖懒得找了,这里介绍一下近期的学习小结:
NOT IN和NOT EIXTS在对同意为null的列查询时会有一定的风险。特别是NOT IN,假设子查询包括了最少一个NULL,会出现非预期的结果。以下做一个演示。
IF OBJECT_ID('ShipmentItems', 'U') IS NOT NULL DROP TABLE dbo.ShipmentItems; GO CREATE TABLE dbo.ShipmentItems ( ShipmentBarcode VARCHAR(30) NOT NULL , Description VARCHAR(100) NULL , Barcode VARCHAR(30) NOT NULL ); GO INSERT INTO dbo.ShipmentItems ( ShipmentBarcode , Barcode , Description ) SELECT '123456' , '1010203' , 'Some cool widget' UNION ALL SELECT '123654' , '1010203' , 'Some cool widget' UNION ALL SELECT '123654' , '1010204' , 'Some cool stuff for some gadget'; GO -- retrieve all the items from shipment 123654 -- that are not shipped in shipment 123456 SELECT Barcode FROM dbo.ShipmentItems WHERE ShipmentBarcode = '123654' AND Barcode NOT IN ( SELECT Barcode FROM dbo.ShipmentItems WHERE ShipmentBarcode = '123456' ); /* Barcode ------------------------------ 1010204 */
能够看出得到了期待结果。以下看看改动表结构,同意列为null的情况:
ALTER TABLE dbo.ShipmentItems ALTER COLUMN Barcode VARCHAR(30) NULL; INSERT INTO dbo.ShipmentItems ( ShipmentBarcode , Barcode , Description ) SELECT '123456' , NULL , 'Users manual for some gadget'; GO SELECT Barcode FROM dbo.ShipmentItems WHERE ShipmentBarcode = '123654' AND Barcode NOT IN ( SELECT Barcode FROM dbo.ShipmentItems WHERE ShipmentBarcode = '123456' ); /* Barcode ------------------------------ */
非常多人会认为这是一个bug,有时候能查出数据。有时候却不能。可是实际上不是bug,当NOT IN子句返回最少一个NULL时,查询会返回空。以下的语句能更好地说明这个想法:
SELECT CASE WHEN 1 NOT IN ( 2, 3 ) THEN 'True' ELSE 'Unknown or False' END , CASE WHEN 1 NOT IN ( 2, 3, NULL ) THEN 'True' ELSE 'Unknown or False' END; /* ---- ---------------- True Unknown or False */
实际上,因为IN的本质是OR操作,所以:
SELECT CASE WHEN 1 IN ( 1, 2, NULL ) THEN 'True' ELSE 'Unknown or False' END ;
中,1 in 1。也就是为TRUE,所以返回true。这个语句的逻辑实际上是:
SELECT CASE WHEN ( 1 = 1 ) OR ( 1 = 2 ) OR ( 1 = NULL ) THEN 'True' ELSE 'Unknown or False' END ;
当使用NOT IN 时,如以下的语句:
SELECT CASE WHEN 1 NOT IN ( 1, 2, NULL ) THEN 'True' ELSE 'Unknown or False' END ;
会转变成:
SELECT CASE WHEN NOT ( ( 1 = 1 ) OR ( 1 = 2 ) OR ( 1 = NULL ) ) THEN 'True' ELSE 'Unknown or False' END ;
依据离散数学的概念,能够转换为:
SELECT CASE WHEN ( ( 1 <> 1 ) AND ( 1 <> 2 ) AND ( 1 <> NULL ) ) THEN 'True' ELSE 'Unknown or False' END ;
谓词有短路特性,即在AND条件中。仅仅要有一个条件为false,整个条件都为false,而1<>1是为false,所以后面的也不须要推断了,直接返回else部分。即使是1<>null,依据集合论的特性,NULL和实际数据的对照总是返回unknown,所以也是为false。假设你非要用NOT IN 。请确保子查询永远不会有NULL返回。或者须要额外处理去除NULL,比方:
SELECT Barcode FROM dbo.ShipmentItems WHERE ShipmentBarcode = '123654' AND Barcode NOT IN ( SELECT Barcode FROM dbo.ShipmentItems WHERE ShipmentBarcode = '123456' AND Barcode IS NOT NULL ) ;
另一种方法就是改写语句,用NOT EXISTS来等价替换:
SELECT i.Barcode FROM dbo.ShipmentItems AS i WHERE i.ShipmentBarcode = '123654' AND NOT EXISTS ( SELECT * FROM dbo.ShipmentItems AS i1 WHERE i1.ShipmentBarcode = '123456' AND i1.Barcode = i.Barcode ); /* Barcode ------------------------------ 1010204 */
另外,基于SARG要求,一般不建议用NOT IN/NOT EXISTS这样的反向扫描,避免影响性能。另一个选择使用IN/EXISTS的要点。就是多列匹配的问题,在T-SQL中,多列同一时候匹配要用EXISTS,而单列匹配能够用EXISTS/IN。可能能够用其它写法来实现IN的多列匹配,可是一般我个人会选择使用EXISTS来匹配多列。
原文出自:CSDN博客:黄钊吉的博客