Question

我做了以下事情 -

    create table test (col char(10)); 
    insert into test values ('hello');
    select * from test where col = 'hello'

我被建议上面不应该返回任何结果，因为'col'是10个字符，它将用空格填充，因此与'hello'相比不会返回结果。但我得到了结果。有人可以解释一下吗？我正在使用11gR2

Answer 1

查看literals上的Oracle文档：

文本文字具有CHAR和VARCHAR2数据类型的属性：


在表达式和条件中，Oracle通过使用空白填充的比较语义来比较文本文字，就好像它们具有数据类型CHAR一样。

以及blank-padded comparison semantics州的文档：

使用空白填充语义，如果两个值具有不同的长度，则Oracle首先将空白添加到较短值的末尾，因此它们的长度相等。然后，Oracle将逐个字符的值与不同的第一个字符进行比较。在第一个不同位置具有较大字符的值被认为更大。如果两个值没有不同的字符，那么它们被认为是相等的。此规则意味着如果两个值仅在尾随空白的数量上不同，则它们是相等的。仅当比较中的两个值都是数据类型 CHAR ， NCHAR ，文本文字或值的表达式时，Oracle才使用空白填充比较语义由 USER 函数返回。

由于比较的左侧是CHAR(10)而右侧是文本文字，因此使用空白填充的比较语义并且'hello ' = 'hello'为真。

您可以在简单示例中看到这一点：

SELECT * FROM DUAL WHERE 'hello    ' = 'hello';

<强>更新：

[TL; DR]至少从Oracle 7（1992年发布）以来，所有版本的Oracle都出现过此行为。我停止搜索二十多年前发布的文档，但我希望你会发现这是大多数（所有？）版本中的行为。

以下是各种版本的文档：

Oracle 12c Text Literals＆amp; blank-padded semantics
Oracle 11g Text Literals＆amp; blank-padded semantics
Oracle 10gR2 Text Literals＆amp; blank-padded semantics
Oracle 9 Text Literals＆amp; blank-padded semantics
Oracle 8 Text Literals＆amp; blank-padded semantics
Oracle 7 Text Literals

Answer 2

Oracle在此示例中使用“空白填充”比较语义，因为其中一个表达式是数据类型CHAR而另一个表达式是字符串文字。

有关Oracle何时使用空白填充比较和非填充比较语义的规则。它在某处的Oracle文档中有所涉及。

给出了建议（Oracle不会返回结果，因为CHAR列右边用空格填充）是错误的。

Answer 3

你已经得到了正确的答案。现在，您可能想知道“我如何强制进行比较，比较字符串'hello'被解释为VARCHAR2，以便查询不返回任何行？”

答案是，你必须强制'hello'被视为VARCHAR2。像这样：

npm start

> angular2-quickstart@1.0.0 start /home/ubuntu/workspace/bootopsnjs
> tsc && concurrently "npm run tsc:w" "npm run lite" 

node_modules/@angular/core/src/application_ref.d.ts(39,88): error TS2304: Cannot find name 'Promise'.
node_modules/@angular/core/src/application_ref.d.ts(99,42): error TS2304: Cannot find name 'Promise'.
node_modules/@angular/core/src/application_ref.d.ts(174,33): error TS2304: Cannot find name 'Promise'.
node_modules/@angular/core/src/change_detection/differs/default_keyvalue_differ.d.ts(24,15): error TS2304: Cannot find name 'Map'.
node_modules/@angular/core/src/change_detection/differs/default_keyvalue_differ.d.ts(26,16): error TS2304: Cannot find name 'Map'.
node_modules/@angular/core/src/di/reflective_provider.d.ts(105,123): error TS2304: Cannot find name 'Map'.
node_modules/@angular/core/src/di/reflective_provider.d.ts(105,165): error TS2304: Cannot find name 'Map'.
node_modules/@angular/core/src/facade/async.d.ts(27,33): error TS2304: Cannot find name 'Promise'.
node_modules/@angular/core/src/facade/async.d.ts(28,45): error TS2304: Cannot find name 'Promise'.
node_modules/@angular/core/src/facade/collection.d.ts(1,25): error TS2304: Cannot find name 'MapConstructor'.
node_modules/@angular/core/src/facade/collection.d.ts(2,25): error TS2304: Cannot find name 'SetConstructor'.
node_modules/@angular/core/src/facade/collection.d.ts(4,27): error TS2304: Cannot find name 'Map'.
node_modules/@angular/core/src/facade/collection.d.ts(4,39): error TS2304: Cannot find name 'Map'.
node_modules/@angular/core/src/facade/collection.d.ts(7,9): error TS2304: Cannot find name 'Map'.
node_modules/@angular/core/src/facade/collection.d.ts(8,30): error TS2304: Cannot find name 'Map'.
node_modules/@angular/core/src/facade/collection.d.ts(11,43): error TS2304: Cannot find name 'Map'.
node_modules/@angular/core/src/facade/collection.d.ts(12,27): error TS2304: Cannot find name 'Map'.
node_modules/@angular/core/src/facade/collection.d.ts(14,23): error TS2304: Cannot find name 'Map'.
node_modules/@angular/core/src/facade/collection.d.ts(15,25): error TS2304: Cannot find name 'Map'.
node_modules/@angular/core/src/facade/collection.d.ts(100,41): error TS2304: Cannot find name 'Set'.
node_modules/@angular/core/src/facade/collection.d.ts(101,22): error TS2304: Cannot find name 'Set'.
node_modules/@angular/core/src/facade/collection.d.ts(102,25): error TS2304: Cannot find name 'Set'.
node_modules/@angular/core/src/facade/lang.d.ts(4,17): error TS2304: Cannot find name 'Map'.
node_modules/@angular/core/src/facade/lang.d.ts(5,17): error TS2304: Cannot find name 'Set'.
node_modules/@angular/core/src/facade/lang.d.ts(70,59): error TS2304: Cannot find name 'Map'.
node_modules/@angular/core/src/facade/promise.d.ts(2,14): error TS2304: Cannot find name 'Promise'.
node_modules/@angular/core/src/facade/promise.d.ts(8,32): error TS2304: Cannot find name 'Promise'.
node_modules/@angular/core/src/facade/promise.d.ts(9,38): error TS2304: Cannot find name 'Promise'.
node_modules/@angular/core/src/facade/promise.d.ts(10,35): error TS2304: Cannot find name 'Promise'.
node_modules/@angular/core/src/facade/promise.d.ts(10,93): error TS2304: Cannot find name 'Promise'.
node_modules/@angular/core/src/facade/promise.d.ts(11,34): error TS2304: Cannot find name 'Promise'.
node_modules/@angular/core/src/facade/promise.d.ts(11,50): error TS2304: Cannot find name 'Promise'.
node_modules/@angular/core/src/facade/promise.d.ts(12,32): error TS2304: Cannot find name 'Promise'.
node_modules/@angular/core/src/facade/promise.d.ts(12,149): error TS2304: Cannot find name 'Promise'.
node_modules/@angular/core/src/facade/promise.d.ts(13,43): error TS2304: Cannot find name 'Promise'.
node_modules/@angular/core/src/linker/component_resolver.d.ts(8,53): error TS2304: Cannot find name 'Promise'.
node_modules/@angular/core/src/linker/component_resolver.d.ts(12,44): error TS2304: Cannot find name 'Promise'.
node_modules/@angular/core/src/linker/dynamic_component_loader.d.ts(62,148): error TS2304: Cannot find name 'Promise'.
node_modules/@angular/core/src/linker/dynamic_component_loader.d.ts(103,144): error TS2304: Cannot find name 'Promise'.
node_modules/@angular/core/src/linker/dynamic_component_loader.d.ts(108,139): error TS2304: Cannot find name 'Promise'.
node_modules/@angular/core/src/linker/dynamic_component_loader.d.ts(109,135): error TS2304: Cannot find name 'Promise'.
node_modules/@angular/core/src/reflection/reflector.d.ts(28,22): error TS2304: Cannot find name 'Map'.
node_modules/@angular/core/src/reflection/reflector.d.ts(30,15): error TS2304: Cannot find name 'Map'.
node_modules/@angular/core/src/reflection/reflector.d.ts(32,15): error TS2304: Cannot find name 'Map'.
node_modules/@angular/core/src/reflection/reflector.d.ts(34,15): error TS2304: Cannot find name 'Map'.
node_modules/@angular/core/src/reflection/reflector.d.ts(36,16): error TS2304: Cannot find name 'Set'.
node_modules/@angular/core/src/testability/testability.d.ts(40,20): error TS2304: Cannot find name 'Map'.
node_modules/rxjs/Observable.d.ts(10,66): error TS2304: Cannot find name 'Promise'.
node_modules/rxjs/Observable.d.ts(66,60): error TS2304: Cannot find name 'Promise'.
node_modules/rxjs/Observable.d.ts(66,70): error TS2304: Cannot find name 'Promise'.

npm ERR! Linux 3.13.0-71-generic
npm ERR! argv "/usr/bin/node" "/usr/bin/npm" "start"
npm ERR! node v0.10.45
npm ERR! npm  v2.15.1
npm ERR! code ELIFECYCLE
npm ERR! angular2-quickstart@1.0.0 start: `tsc && concurrently "npm run tsc:w" "npm run lite" `
npm ERR! Exit status 2
npm ERR! 
npm ERR! Failed at the angular2-quickstart@1.0.0 start script 'tsc && concurrently "npm run tsc:w" "npm run lite" '.
npm ERR! This is most likely a problem with the angular2-quickstart package,
npm ERR! not with npm itself.
npm ERR! Tell the author that this fails on your system:
npm ERR!     tsc && concurrently "npm run tsc:w" "npm run lite" 
npm ERR! You can get information on how to open an issue for this project with:
npm ERR!     npm bugs angular2-quickstart
npm ERR! Or if that isn't available, you can get their info via:
npm ERR! 
npm ERR!     npm owner ls angular2-quickstart
npm ERR! There is likely additional logging output above.

npm ERR! Please include the following file with any support request:
npm ERR!     /home/ubuntu/workspace/bootopsnjs/npm-debug.log
ubuntu@ubuntu:~/workspace/bootopsnjs$

（请注意... where col = cast ('hello' as VARCHAR2(10))函数的语法要求您指定长度。

即使您指定长度为10，因为强制转换为VARCHAR2，结果实际上只有5个字符 - 没有VARCHAR2值的空白填充。

在where子句中忽略空白字符

3 个答案: