在 Oracle SQL select 查询中使用列引用以避免重新计算
Use column references in Oracle SQL select query to avoid re-calculation
我想创建一个汇总的数据库备份报告,其中包括来自多个数据库表的数据,这些表彼此没有关系。例如,我想包含 v$database 视图中的数据库名称:
SQL> Select name from v$database;
以及数据库的大小:
SQL> SELECT sum(bytes) FROM v$datafile;
考虑到这两个视图之间没有公共列。
除此之外,我还想包括以字节、KB、MB、……等为单位的数据库大小。
我正在使用以下查询:
SQL> SELECT
"Database", "DB_SIZE_IN_Bytes", "DB_SIZE_IN_KBs", "DB_SIZE_IN_MBs",
"DB_SIZE_IN_GBs", "DB_SIZE_IN_TBs"
FROM (
SELECT
(SELECT name FROM v$database ) "Database",
(SELECT sum(bytes) FROM v$datafile ) "DB_SIZE_IN_Bytes",
(SELECT sum(bytes)/1024 FROM v$datafile ) "DB_SIZE_IN_KBs",
(SELECT sum(bytes)/1024/1024 FROM v$datafile ) "DB_SIZE_IN_MBs",
(SELECT sum(bytes)/1024/1024/1024 FROM v$datafile ) "DB_SIZE_IN_GBs",
(SELECT sum(bytes)/1024/1024/1024/1024 FROM v$datafile ) "DB_SIZE_IN_TBs"
FROM dual
);
通过这种方式,我避免了 v$database 和 v$datafile 之间的连接条件。
这只是一个例子,但是,在我的项目中我有超过 10 个表。他们中的大多数没有共同的专栏。
问题是在将字节转换为 kbs、mbs 等时,我想使用列引用,例如:
(SELECT DB_SIZE_IN_Bytes/1024 FROM dual ) "DB_SIZE_IN_KBs",
(SELECT DB_SIZE_IN_KBs /1024 FROM dual ) "DB_SIZE_IN_MBs",
(SELECT DB_SIZE_IN_MBs /1024 FROM dual ) "DB_SIZE_IN_GBs",
(SELECT DB_SIZE_IN_GBs /1024 FROM dual ) "DB_SIZE_IN_TBs"
而不是一次又一次的计算。有什么方法可以使用上面示例中提到的列引用而不是重新计算吗?
在 from
子句中使用子查询计算求和结果一次,然后使用:
SELECT (SELECT name FROM v$database ) as "Database",
DB_SIZE_IN_Bytes,
DB_SIZE_IN_Bytes / 1024 as DB_SIZE_IN_KBs,
DB_SIZE_IN_Bytes / (1024 * 1024) as DB_SIZE_IN_MBs,
DB_SIZE_IN_Bytes / (1024 * 1024 * 1024) as DB_SIZE_IN_GBs,
DB_SIZE_IN_Bytes / (1024 * 1024 * 1024 * 1024) as DB_SIZE_IN_TBs
FROM (SELECT sum(bytes) as DB_SIZE_IN_Bytes FROM v$datafile ) x;
你也可以使用横向连接来做到这一点,但对于这个特定的例子来说,这绝对是矫枉过正:
SELECT (SELECT name FROM v$database ) as "Database",
bb.*, bk.*, bm.*, bg.*, bt.*
FROM (SELECT sum(bytes) as DB_SIZE_IN_Bytes FROM v$datafile
) bb CROSS JOIN LATERAL
(SELECT bb.DB_SIZE_IN_Bytes / 1024 as DB_SIZE_IN_Kb
FROM dual
) bk CROSS JOIN LATERAL
(SELECT bk.DB_SIZE_IN_Kb / 1024 as DB_SIZE_IN_Mb
FROM dual
) bm CROSS JOIN LATERAL
(SELECT bm.DB_SIZE_IN_Mb / 1024 as DB_SIZE_IN_Gb
FROM dual
) bg CROSS JOIN LATERAL
(SELECT bb.DB_SIZE_IN_Gb / 1024 as DB_SIZE_IN_Tb
FROM dual
) t;
我想创建一个汇总的数据库备份报告,其中包括来自多个数据库表的数据,这些表彼此没有关系。例如,我想包含 v$database 视图中的数据库名称:
SQL> Select name from v$database;
以及数据库的大小:
SQL> SELECT sum(bytes) FROM v$datafile;
考虑到这两个视图之间没有公共列。
除此之外,我还想包括以字节、KB、MB、……等为单位的数据库大小。
我正在使用以下查询:
SQL> SELECT
"Database", "DB_SIZE_IN_Bytes", "DB_SIZE_IN_KBs", "DB_SIZE_IN_MBs",
"DB_SIZE_IN_GBs", "DB_SIZE_IN_TBs"
FROM (
SELECT
(SELECT name FROM v$database ) "Database",
(SELECT sum(bytes) FROM v$datafile ) "DB_SIZE_IN_Bytes",
(SELECT sum(bytes)/1024 FROM v$datafile ) "DB_SIZE_IN_KBs",
(SELECT sum(bytes)/1024/1024 FROM v$datafile ) "DB_SIZE_IN_MBs",
(SELECT sum(bytes)/1024/1024/1024 FROM v$datafile ) "DB_SIZE_IN_GBs",
(SELECT sum(bytes)/1024/1024/1024/1024 FROM v$datafile ) "DB_SIZE_IN_TBs"
FROM dual
);
通过这种方式,我避免了 v$database 和 v$datafile 之间的连接条件。
这只是一个例子,但是,在我的项目中我有超过 10 个表。他们中的大多数没有共同的专栏。
问题是在将字节转换为 kbs、mbs 等时,我想使用列引用,例如:
(SELECT DB_SIZE_IN_Bytes/1024 FROM dual ) "DB_SIZE_IN_KBs",
(SELECT DB_SIZE_IN_KBs /1024 FROM dual ) "DB_SIZE_IN_MBs",
(SELECT DB_SIZE_IN_MBs /1024 FROM dual ) "DB_SIZE_IN_GBs",
(SELECT DB_SIZE_IN_GBs /1024 FROM dual ) "DB_SIZE_IN_TBs"
而不是一次又一次的计算。有什么方法可以使用上面示例中提到的列引用而不是重新计算吗?
在 from
子句中使用子查询计算求和结果一次,然后使用:
SELECT (SELECT name FROM v$database ) as "Database",
DB_SIZE_IN_Bytes,
DB_SIZE_IN_Bytes / 1024 as DB_SIZE_IN_KBs,
DB_SIZE_IN_Bytes / (1024 * 1024) as DB_SIZE_IN_MBs,
DB_SIZE_IN_Bytes / (1024 * 1024 * 1024) as DB_SIZE_IN_GBs,
DB_SIZE_IN_Bytes / (1024 * 1024 * 1024 * 1024) as DB_SIZE_IN_TBs
FROM (SELECT sum(bytes) as DB_SIZE_IN_Bytes FROM v$datafile ) x;
你也可以使用横向连接来做到这一点,但对于这个特定的例子来说,这绝对是矫枉过正:
SELECT (SELECT name FROM v$database ) as "Database",
bb.*, bk.*, bm.*, bg.*, bt.*
FROM (SELECT sum(bytes) as DB_SIZE_IN_Bytes FROM v$datafile
) bb CROSS JOIN LATERAL
(SELECT bb.DB_SIZE_IN_Bytes / 1024 as DB_SIZE_IN_Kb
FROM dual
) bk CROSS JOIN LATERAL
(SELECT bk.DB_SIZE_IN_Kb / 1024 as DB_SIZE_IN_Mb
FROM dual
) bm CROSS JOIN LATERAL
(SELECT bm.DB_SIZE_IN_Mb / 1024 as DB_SIZE_IN_Gb
FROM dual
) bg CROSS JOIN LATERAL
(SELECT bb.DB_SIZE_IN_Gb / 1024 as DB_SIZE_IN_Tb
FROM dual
) t;