Oracle伪列
在oracle10g和11g下，伪列包括如下内容：
l Hierarchical Query Pseudocolumns
分级查询是oracle提供的递归查询语法，在这里不做展开。只有在分级查询下，才可以使用以下伪列：
1.CONNECT_BY_ISCYCLE Pseudocolumn
2.CONNECT_BY_ISLEAF Pseudocolumn
3.LEVEL Pseudocolumn
l Sequence Pseudocolumns
Sequence有以下2个伪列：
1.CURRVAL：返回当前sequence值
2.NEXTVAL：增加sequence并返回下一个值
一般用法：
sequence.CURRVAL
sequence.NEXTVAL
l Version Query Pseudocolumns
Version Query伪列只有在Flashback Version Query时才有效，内容如下：
1.VERSIONS_STARTSCN and VERSIONS_STARTTIME
2.VERSIONS_ENDSCN and VERSIONS_ENDTIME
3.VERSIONS_XID
4.VERSIONS_OPERATION
l COLUMN_VALUE Pseudocolumn
l OBJECT_ID Pseudocolumn
l OBJECT_VALUE Pseudocolumn
l ORA_ROWSCN Pseudocolumn
l ROWID Pseudocolumn
l ROWNUM Pseudocolumn
l XMLDATA Pseudocolumn
在对普通表做查询时，比较常用的伪列有：ORA_ROWSCN、ROWID、ROWNUM。
ORA_ROWSCN
虽然叫ORA_ROWSCN，不过默认情况下，查询出的该值是从数据文件块头获取的，也就是说，查询出的是block的最近事务的scn，而不是精确到row的scn。
在创建表时，可以指定ROWDEPENDENCIES来使ora_rowscn真正记录行一级的scn。
看一下ROWDEPENDENCIES 的定义：Specify ROWDEPENDENCIES if you want to enable row-level dependency tracking. This setting is useful primarily to allow for parallel propagation in replication environments. It increases the size of each row by 6 bytes.
从这里也可以看出一个SCN占用的空间（6 bytes）。SCN的最大值是0xffff.ffffffff，共48位。包括2bytes的高位字节（SCN wrap）和4bytes的低位字节（SCN base）。
SQL>select ora_rowscn,username from t;
ORA_ROWSCN USERNAME
---------- ------------------------------
86516279 SYSTEM
86516279 SYS
86516279 OUTLN
86516279 LINCINQ
86516279 TEST
86516279 LINC
86516279 SPLEXUC
86516279 DIP
86516279 TSMSYS
86516279 WMSYS
86516279 DBSNMP
11 rows selected.
需要查询scn对于的具体时间，可以用函数SCN_TO_TIMESTAMP
SQL>alter session set nls_timestamp_format='yyyy-mm-dd hh24:mi:ss';
Session altered.
SQL>select scn_to_timestamp(ora_rowscn),username from t;
SCN_TO_TIMESTAMP(ORA_ROWSCN) USERNAME
------------------------------ ------------------------------
2011-06-01 13:13:31 SYSTEM
2011-06-01 13:13:31 SYS
2011-06-01 13:13:31 OUTLN
2011-06-01 13:13:31 LINCINQ
2011-06-01 13:13:31 TEST
2011-06-01 13:13:31 LINC
2011-06-01 13:13:31 SPLEXUC
2011-06-01 13:13:31 DIP
2011-06-01 13:13:31 TSMSYS
2011-06-01 13:13:31 WMSYS
2011-06-01 13:13:31 DBSNMP
11 rows selected.
ROWID
Rowid是基于64位编码的18个字符，格式如下：
data_object_id file_id block_number row_number
OOOOOO FFF BBBBBB RRR
SQL>select rowid,username from t;
ROWID USERNAME
------------------ ------------------------------
AAAE0LAAOAABQqMAAA SYSTEM
AAAE0LAAOAABQqMAAB SYS
AAAE0LAAOAABQqMAAC OUTLN
AAAE0LAAOAABQqMAAD LINCINQ
AAAE0LAAOAABQqMAAE TEST
AAAE0LAAOAABQqMAAF LINC
AAAE0LAAOAABQqMAAG SPLEXUC
AAAE0LAAOAABQqMAAH DIP
AAAE0LAAOAABQqMAAI TSMSYS
AAAE0LAAOAABQqMAAJ WMSYS
AAAE0LAAOAABQqMAAK DBSNMP
11 rows selected.
通过dbms_rowid包，我们可以获得rowid对应的记录详细信息。
SQL>select dbms_rowid.rowid_object('&1') data_object_id#,
2 dbms_rowid.rowid_relative_fno('&1') rfile#,
3 dbms_rowid.rowid_block_number('&1') block#,
4 dbms_rowid.rowid_row_number('&1') row# from dual;
Enter value for 1: AAAE0LAAOAABQqMAAA
old 1: select dbms_rowid.rowid_object('&1') data_object_id#,
new 1: select dbms_rowid.rowid_object('AAAE0LAAOAABQqMAAA') data_object_id#,
Enter value for 1: AAAE0LAAOAABQqMAAA
old 2: dbms_rowid.rowid_relative_fno('&1') rfile#,
new 2: dbms_rowid.rowid_relative_fno('AAAE0LAAOAABQqMAAA') rfile#,
Enter value for 1: AAAE0LAAOAABQqMAAA
old 3: dbms_rowid.rowid_block_number('&1') block#,
new 3: dbms_rowid.rowid_block_number('AAAE0LAAOAABQqMAAA') block#,
Enter value for 1: AAAE0LAAOAABQqMAAA
old 4: dbms_rowid.rowid_row_number('&1') row# from dual
new 4: dbms_rowid.rowid_row_number('AAAE0LAAOAABQqMAAA') row# from dual
DATA_OBJECT_ID# RFILE# BLOCK# ROW#
--------------- ---------- ---------- ----------
19723 14 330380 0
ROWNUM
在对表的查询中，每返回一条记录，rownum伪列就返回一个数字，代表查询返回的行的编号。
SQL>select rownum,username from t;
ROWNUM USERNAME
---------- ------------------------------
1 SYSTEM
2 SYS
3 OUTLN
4 LINCINQ
5 TEST
6 LINC
7 SPLEXUC
8 DIP
9 TSMSYS
10 WMSYS
11 DBSNMP
11 rows selected.
从下面的例子可以看出，rownum返回的是查询过程中返回记录的顺序，并不是查询结果的序列号。
SQL>select rownum,username from t order by username;
ROWNUM USERNAME
---------- ------------------------------
11 DBSNMP
8 DIP
6 LINC
4 LINCINQ
3 OUTLN
7 SPLEXUC
2 SYS
1 SYSTEM
5 TEST
9 TSMSYS
10 WMSYS
11 rows selected.
SQL>select rownum,username from (select username from t order by username);
ROWNUM USERNAME
---------- ------------------------------
1 DBSNMP
2 DIP
3 LINC
4 LINCINQ
5 OUTLN
6 SPLEXUC
7 SYS
8 SYSTEM
9 TEST
10 TSMSYS
11 WMSYS
11 rows selected.