Oracle SQL 按子句分组

Question

我想写一个 sql 来获取前 5 个表空间存储指标。下面的查询给出了关于所有 tbspace 的指标。欣赏是否有人对此进行微调以使其只有前 N

SELECT
ts.tablespace_name AS TBNAME,
round((ts.tablespace_size/1024/1024),2) AS SIZE_MB,
round((ts.tablespace_used_size/1024/1024),2) AS USED_MB,
round(((ts.tablespace_size - ts.tablespace_used_size)/1024/1024),2) AS     FREE_MB
FROM
mgmt$db_tablespaces ts,
(SELECT d.target_guid, d.tablespace_name, count(d.file_name) df_count,
sum(decode(d.autoextensible, 'YES', 1, 0)) auto_extend
FROM mgmt$db_datafiles d, mgmt$target t
WHERE t.target_guid = '<id>' AND
(t.target_type='rac_database' OR
(t.target_type='oracle_database' AND t.TYPE_QUALIFIER3 != 'RACINST')) AND
t.target_guid = d.target_guid
GROUP BY d.target_guid, d.tablespace_name) df
WHERE
ts.target_guid = df.target_guid AND
df.tablespace_name = ts.tablespace_name
ORDER BY ts.tablespace_size;`

谢谢

Answer 1

您可以使用ROWNUM。 Oracle 在返回结果后将rownum 应用于结果。 您需要在返回结果后对其进行过滤，因此需要子查询。您还可以使用 RANK() 函数来获取 Top-N 结果。

  SELECT 
       * 
  FROM
      (
         SELECT
            ts.tablespace_name AS TBNAME,
            round((ts.tablespace_size/1024/1024),2) AS SIZE_MB,
            round((ts.tablespace_used_size/1024/1024),2) AS USED_MB,
            round(((ts.tablespace_size - ts.tablespace_used_size)/1024/1024),2) AS     FREE_MB
       FROM
          mgmt$db_tablespaces ts,
          (SELECT d.target_guid, d.tablespace_name, count(d.file_name) df_count,
              sum(decode(d.autoextensible, 'YES', 1, 0)) auto_extend
            FROM mgmt$db_datafiles d, mgmt$target t
            WHERE t.target_guid = '<id>' AND
            (t.target_type='rac_database' OR
            (t.target_type='oracle_database' AND t.TYPE_QUALIFIER3 != 'RACINST')) AND
            t.target_guid = d.target_guid
           GROUP BY d.target_guid, d.tablespace_name) df
       WHERE
        ts.target_guid = df.target_guid AND
        df.tablespace_name = ts.tablespace_name
       ORDER BY ts.tablespace_size
      )
  WHERE ROWNUM <= 5;