oracle sql查询中返回的限制字符答案

【问题标题】：Limit characters returned in oracle sql queryoracle sql查询中返回的限制字符
【发布时间】：2017-03-24 12:22:16
【问题描述】：

我需要将此字段限制为 4000 个字符。如何将其限制在列表 agg 上？谢谢。

LISTAGG(ORDER_IMPRESSION.IMPRESSION, ',') WITHIN GROUP (ORDER BY ORDER_IMPRESSION.LINE)

【问题讨论】：

Data Warehouse Insider 博客在此处有一个相关条目，blogs.oracle.com/datawarehousing/entry/…。

标签： sql oracle

【解决方案1】：

只需要一点努力。像这样的：

select listagg((case when running_len < 4000 then oi.impression end), ',') within group (order by oi.line)
from (select oi.*,
             sum(length(oi.impression) + 1) over (partition by ?? order by oi.line) as running_len
      from order_impression oi
     ) oi
group by ??;

这会计算运行长度，并且只聚合不超过长度的值。 ?? 是您用于聚合的任何内容。这确实假设line 是唯一的，因此order by 是稳定的。

这将不包括超过长度的impression - 之后什么都没有。它不会切断印象。这种逻辑是可能的，但它确实使查询复杂化。

【讨论】：

【解决方案2】：

您可以计算字符串长度的总和（使用SUM(...) OVER (...) 解析函数），然后使用它来将字符串截断为 4000 个字符：

SELECT LISTAGG(
         CASE
           WHEN prev_len >= 4000 THEN NULL
           WHEN prev_len + len <= 4000 THEN value
           ELSE SUBSTR( value, 1, 4000 - prev_len )
         END
       ) WITHIN GROUP ( ORDER BY line ) AS value
FROM   (
  SELECT impression,
         line,
         LENGTH( impression ) AS len,
         COALESCE(
           SUM( 1 + LENGTH( impression ) )
             OVER ( ORDER BY line ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING ),
           0
         ) AS prev_len
  FROM   order_impression
);

【讨论】：

【解决方案3】：

您可以编写自定义聚合函数将VARCHAR2s 聚合成CLOB：

CREATE OR REPLACE TYPE CLOBAggregation AS OBJECT(
  value CLOB,

  STATIC FUNCTION ODCIAggregateInitialize(
    ctx         IN OUT CLOBAggregation
  ) RETURN NUMBER,

  MEMBER FUNCTION ODCIAggregateIterate(
    self        IN OUT CLOBAggregation,
    value       IN     VARCHAR2
  ) RETURN NUMBER,

  MEMBER FUNCTION ODCIAggregateTerminate(
    self        IN OUT CLOBAggregation,
    returnValue    OUT CLOB,
    flags       IN     NUMBER
  ) RETURN NUMBER,

  MEMBER FUNCTION ODCIAggregateMerge(
    self        IN OUT CLOBAggregation,
    ctx         IN OUT CLOBAggregation
  ) RETURN NUMBER
);
/

CREATE OR REPLACE TYPE BODY CLOBAggregation
IS
  STATIC FUNCTION ODCIAggregateInitialize(
    ctx         IN OUT CLOBAggregation
  ) RETURN NUMBER
  IS
  BEGIN
    ctx := CLOBAggregation( NULL );
    RETURN ODCIConst.SUCCESS;
  END;

  MEMBER FUNCTION ODCIAggregateIterate(
    self        IN OUT CLOBAggregation,
    value       IN     VARCHAR2
  ) RETURN NUMBER
  IS
  BEGIN
    IF value IS NULL THEN
      NULL;
    ELSIF self.value IS NULL THEN
      self.value := value;
    ELSE
      self.value := self.value || ',' || value;
    END IF;
    RETURN ODCIConst.SUCCESS;
  END;

  MEMBER FUNCTION ODCIAggregateTerminate(
    self        IN OUT CLOBAggregation,
    returnValue    OUT CLOB,
    flags       IN     NUMBER
  ) RETURN NUMBER
  IS
  BEGIN
    returnValue := self.value;
    RETURN ODCIConst.SUCCESS;
  END;

  MEMBER FUNCTION ODCIAggregateMerge(
    self        IN OUT CLOBAggregation,
    ctx         IN OUT CLOBAggregation
  ) RETURN NUMBER
  IS
  BEGIN
    IF self.value IS NULL THEN
      self.value := ctx.value;
    ELSIF ctx.value IS NULL THEN
      NULL;
    ELSE
      self.value := self.value || ',' || ctx.value;
    END IF;
    RETURN ODCIConst.SUCCESS;
  END;
END;
/

CREATE FUNCTION CLOBAgg( value VARCHAR2 )
RETURN CLOB
PARALLEL_ENABLE AGGREGATE USING CLOBAggregation;
/

那么你可以这样做：

SELECT DBMS_LOB.SUBSTR( CLOBAGG( IMPRESSION ), 4000 )
FROM   (
  SELECT   IMPRESSION
  FROM     ORDER_IMPRESSION
  ORDER BY line
)

【讨论】：

【解决方案4】：

如果您是 12.2 的早期采用者，则可以使用 ON OVERFLOW TRUNCATE 子句...

ON OVERFLOW TRUNCATE

【讨论】：