我注意到Oracle 11.2数据库中的v $ reserved_words视图有两行，关键字为null（值为null，而不是null）。
所以这个查询：
select * from v$reserved_words where keyword is null;
返回两行，相同但除了一行保留= Y而另一行保留= N.知道这是什么用途或者它是如何使用的？
答案 0 :(得分：1)
您需要了解如何解释视图v$reserved _words。要确定是否以任何方式保留特定关键字，请检查已保留， RES_TYPE ， RES_ATTR 和 RES_SEMI 列。
SQL> column keyword format A10;
SQL> select * from v$reserved_words where keyword is null;
KEYWORD LENGTH R R R R D CON_ID
---------- ---------- - - - - - ----------
0 Y N N N N 0
0 N N N N N 0
这两行没有＆＃39; Y＆＃39;对于所有列。是的，其中一行保留为＆＃39; Y＆＃39;但长度为0.此外，没有任何属性是＆＃39; Y＆＃39;。
来自文档，
RESERVED VARCHAR2(1) A value of Y means that the keyword cannot be used as an identifier. A value of N means that it is not reserved.
RES_TYPE VARCHAR2(1) A value of Y means that the keyword cannot be used as a type name. A value of N means that it is not reserved.
RES_ATTR VARCHAR2(1) A value of Y means that the keyword cannot be used as an attribute name. A value of N means that it is not reserved.
RES_SEMI VARCHAR2(1) A value of Y means that the keyword is not allowed as an identifier in certain situations, such as in DML. A value of N means that it is not reserved.
DUPLICATE VARCHAR2(1) A value of Y means that the keyword is a duplicate of another keyword. A value of N means that it is not a duplicate.
这澄清了原因。
而不是v $ reserved_words，您还可以通过以下方式检查SQL * Plus中的SQL和PL / SQL保留字：
SQL*Plus: Release 12.1.0.1.0 Production on Fri Sep 5 13:05:15 2014
Copyright (c) 1982, 2013, Oracle. All rights reserved.
Last Successful login time: Thu Sep 04 2014 15:01:52 +05:30
Connected to:
Oracle Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production
With the Partitioning, OLAP, Advanced Analytics and Real Application Testing options
SQL> help reserve
RESERVED WORDS (PL/SQL)
-----------------------
PL/SQL Reserved Words have special meaning in PL/SQL, and may not be used
for identifier names (unless enclosed in "quotes").
An asterisk (*) indicates words are also SQL Reserved Words.
ALL* DESC* JAVA PACKAGE SUBTYPE
ALTER* DISTINCT* LEVEL* PARTITION SUCCESSFUL*
AND* DO LIKE* PCTFREE* SUM
ANY* DROP* LIMITED PLS_INTEGER SYNONYM*
ARRAY ELSE* LOCK* POSITIVE SYSDATE*
AS* ELSIF LONG* POSITIVEN TABLE*
ASC* END LOOP PRAGMA THEN*
AT EXCEPTION MAX PRIOR* TIME
AUTHID EXCLUSIVE* MIN PRIVATE TIMESTAMP
AVG EXECUTE MINUS* PROCEDURE TIMEZONE_ABBR
BEGIN EXISTS* MINUTE PUBLIC* TIMEZONE_HOUR
BETWEEN* EXIT MLSLABEL* RAISE TIMEZONE_MINUTE
BINARY_INTEGER EXTENDS MOD RANGE TIMEZONE_REGION
BODY EXTRACT MODE* RAW* TO*
BOOLEAN FALSE MONTH REAL TRIGGER*
BULK FETCH NATURAL RECORD TRUE
BY* FLOAT* NATURALN REF TYPE
CHAR* FOR* NEW RELEASE UI
CHAR_BASE FORALL NEXTVAL RETURN UNION*
CHECK* FROM* NOCOPY REVERSE UNIQUE*
CLOSE FUNCTION NOT* ROLLBACK UPDATE*
CLUSTER* GOTO NOWAIT* ROW* USE
COALESCE GROUP* NULL* ROWID* USER*
COLLECT HAVING* NULLIF ROWNUM* VALIDATE*
COMMENT* HEAP NUMBER* ROWTYPE VALUES*
COMMIT HOUR NUMBER_BASE SAVEPOINT VARCHAR*
COMPRESS* IF OCIROWID SECOND VARCHAR2*
CONNECT* IMMEDIATE* OF* SELECT* VARIANCE
CONSTANT IN* ON* SEPERATE VIEW*
CREATE* INDEX* OPAQUE SET* WHEN
CURRENT* INDICATOR OPEN SHARE* WHENEVER*
CURRVAL INSERT* OPERATOR SMALLINT* WHERE*
CURSOR INTEGER* OPTION* SPACE WHILE
DATE* INTERFACE OR* SQL WITH*
DAY INTERSECT* ORDER* SQLCODE WORK
DECIMAL* INTERVAL ORGANIZATION SQLERRM WRITE
DECLARE INTO* OTHERS START* YEAR
DEFAULT* IS* OUT STDDEV ZONE
DELETE* ISOLATION
RESERVED WORDS (SQL)
--------------------
SQL Reserved Words have special meaning in SQL, and may not be used for
identifier names unless enclosed in "quotes".
An asterisk (*) indicates words are also ANSI Reserved Words.
Oracle prefixes implicitly generated schema object and subobject names
with "SYS_". To avoid name resolution conflict, Oracle discourages you
from prefixing your schema object and subobject names with "SYS_".
ACCESS DEFAULT* INTEGER* ONLINE START
ADD* DELETE* INTERSECT* OPTION* SUCCESSFUL
ALL* DESC* INTO* OR* SYNONYM
ALTER* DISTINCT* IS* ORDER* SYSDATE
AND* DROP* LEVEL* PCTFREE TABLE*
ANY* ELSE* LIKE* PRIOR* THEN*
AS* EXCLUSIVE LOCK PRIVILEGES* TO*
ASC* EXISTS LONG PUBLIC* TRIGGER
AUDIT FILE MAXEXTENTS RAW UID
BETWEEN* FLOAT* MINUS RENAME UNION*
BY* FOR* MLSLABEL RESOURCE UNIQUE*
CHAR* FROM* MODE REVOKE* UPDATE*
CHECK* GRANT* MODIFY ROW USER*
CLUSTER GROUP* NOAUDIT ROWID VALIDATE
COLUMN HAVING* NOCOMPRESS ROWNUM VALUES*
COMMENT IDENTIFIED NOT* ROWS* VARCHAR*
COMPRESS IMMEDIATE* NOWAIT SELECT* VARCHAR2
CONNECT* IN* NULL* SESSION* VIEW*
CREATE* INCREMENT NUMBER SET* WHENEVER*
CURRENT* INDEX OF* SHARE WHERE
DATE* INITIAL OFFLINE SIZE* WITH*
DECIMAL* INSERT* ON* SMALLINT*
SQL>
添加更多信息以澄清这两行是不同的TYPE。
My Oracle Support 中的支持说明https://support.oracle.com/epmos/faces/SearchDocDisplay?_adf.ctrl-state=csshtn48w_9&_afrLoop=2343138470228212#990809表示最好查看视图中的TYPE： X $ KWDDEF
SQL> select indx, keyword, length, type from X$KWDDEF where keyword is NULL;
INDX KEYWORD LENGTH TYPE
---------- ---------- ---------- ----------
2087 0 2
2088 0 1
现在，如何从X $ KWDDEF形成V $ RESERVED_WORDS视图？这是基础查询：
SELECT inst_id, keyword, LENGTH,
DECODE (MOD (TRUNC (TYPE / 2), 2), 0, 'N', 1, 'Y', '?') reserved,
DECODE (MOD (TRUNC (TYPE / 4), 2), 0, 'N', 1, 'Y', '?') res_type,
DECODE (MOD (TRUNC (TYPE / 8), 2), 0, 'N', 1, 'Y', '?') res_attr,
DECODE (MOD (TRUNC (TYPE / 16), 2), 0, 'N', 1, 'Y', '?') res_semi,
DECODE (MOD (TRUNC (TYPE / 32), 2), 0, 'N', 1, 'Y', '?') duplicate
FROM x$kwddef;
那么TYPE实际上是什么？ TYPE列用作分组单词的存储桶。
SQL> select type, count(*)
2 from x$kwddef
3 group by type
4 order by 1
5 /
TYPE COUNT(*)
---------- ----------
1 1939
2 96
9 2
16 28
33 20
34 4
6 rows selected.
因此，基于TYPE（如Oracle建议的那样），两行不一样。它们属于不同的类型。
关键字＆＃39; NULL＆＃39;不应该误解这些行中的NULL VALUE。 NULL关键字完全不同，LENGTH = 4。
SQL> select indx, keyword, length, type from X$KWDDEF where keyword = 'NULL';
INDX KEYWORD LENGTH TYPE
---------- ---------- ---------- ----------
338 NULL 4 2
因为＆＃39; X $ KWDDEF＆＃39;有一个条目为＆＃39; NULL＆＃39;关键字为TYPE 2，可以安全地忽略这两行。我猜X $ KWDDEF意味着内核字词定义，只是一个猜测！