Question

如果我查询不存在的输出，那么我将不会返回任何内容。我正在寻找在该场景中返回的默认值（0）

select sum(case when a2.status='SUCCESS' THEN A2.a else 0 end) as success,
sum(case when a2.status='FAILED' THEN A2.a else 0 end) as failed,
sum(case when a2.status='ERROR' THEN A2.a else 0 end) as error
from
(select a.stauts,count(1) a 
from table1 a,table2 b
where a.id=b.id
a.date=sysdate
group by a.status)a2;

注意：sysdate没有记录。我要求默认值“0”应该返回状态。

Answer 1

此查询应始终返回一行，即使没有匹配项：

select sum(case when a.status = 'SUCCESS' then 1 else 0 end) as success,
       sum(case when a.status = 'FAILED' then 1 else 0 end) as failed,
       sum(case when a.status = 'ERROR' then 1 else 0 end) as error
from table1 a join
        table2 b
        on a.id = b.id
where a.date = trunc(sysdate);

请注意，我更改了where逻辑。 sysdate（尽管它的名字）有一个时间组件。如果date有时间组件，您可能需要：

where a.date >= trunc(sysdate) and a.date < trunc(sysdate + 1)

编辑：

如果过滤条件没有匹配任何行，那么您将使用：

获得0

select count(case when a.status = 'SUCCESS' then 1 end) as success,
       count(case when a.status = 'FAILED' then 1 end) as failed,
       count(case when a.status = 'ERROR' then 1 end) as error
from table1 a join
        table2 b
        on a.id = b.id
where a.date = trunc(sysdate);

Answer 2

您可以生成缺失值：

WITH cte AS (
     select a.status,count(1) a 
     from table1 a                  --JOIN syntax
     join table2 b
       on a.id=b.id
     WHERE a.date=sysdate           -- are you sure you want precision with time?
     group by a.status
), placeholder AS (
     SELECT *
     FROM cte
     UNION ALL
     SELECT *
     FROM (SELECT 'SUCCESS' AS status, 0 AS a FROM dual UNION ALL
           SELECT 'ERROR',   0 FROM dual UNION ALL
           SELECT 'FAILED',  0 FROM dual)  p
     WHERE NOT EXISTS (SELECT * FROM cte WHERE cte.status = p.status)
)
SELECT 
  sum(case when status='SUCCESS' THEN a else 0 end) as success,
  sum(case when status='FAILED' THEN a else 0 end) as failed,
  sum(case when status='ERROR' THEN a else 0 end) as error
FROM placeholder;

Answer 3

我想到的唯一建议是在子查询中使用左连接并将整个WHERE逻辑移动到ON子句：

SELECT
    SUM(CASE WHEN a2.status = 'SUCCESS' THEN A2.a ELSE 0 END) AS success,
    SUM(CASE WHEN a2.status = 'FAILED'  THEN A2.a ELSE 0 END) AS failed,
    SUM(CASE WHEN a2.status = 'ERROR'   THEN A2.a ELSE 0 END) AS error
FROM
(
    SELECT a.status, COUNT(1) a 
    FROM table1 a
    LEFT JOIN table2 b
        ON a.id = b.id AND
           a.date = SYSDATE
    GROUP BY a.status
) a2;

您当前的查询使用的是古老的连接语法，这使得很难看到实际发生的情况。特别是，在您希望保留的加入期间，您很难看到是否可能丢弃信息。

Answer 4

如果您使用COUNT()，则与NVL()的情况不同，您不需要COALESCE()或NULL来处理SUM()。当参数为COUNT()或没有匹配行时，NULL将始终返回 value = 0 的行。GROUP BY也不需要。{ / p>

SELECT COUNT(CASE WHEN a.status = 'SUCCESS' THEN 1 END) AS success,
       COUNT(CASE WHEN a.status = 'FAILED'  THEN 1 END) AS failed,
       COUNT(CASE WHEN a.status = 'ERROR'   THEN 1 END) AS error
FROM table1 a
JOIN table2 b ON a.id = b.id
WHERE a.date = TRUNC(SYSDATE);

如果您只是想清楚，请测试这些查询并注意结果。

select SUM(1)    FROM DUAL WHERE 1=0; --NULL
select SUM(NULL) FROM DUAL WHERE 1=0; --NULL
select SUM(NULL) FROM DUAL WHERE 1=1; --NULL
select COUNT(1)  FROM DUAL WHERE 1=0;  -- 0
select COUNT(NULL) FROM DUAL WHERE 1=0; -- 0 
select COUNT(NULL) FROM DUAL WHERE 1=1; -- 0

Demo

Answer 5

Aggregation without GROUP BY always returns a row, so your existing query will return NULLs.

To change a NULL to zero simply apply COALESCE:

select
   coalesce(sum(case when a2.status='SUCCESS' THEN A2.a end), 0) as success,
   coalesce(sum(case when a2.status='FAILED'  THEN A2.a end), 0) as failed,
   coalesce(sum(case when a2.status='ERROR'   THEN A2.a end), 0) as error
from
 (
   select a.status,count(1) a 
   from table1 a join table2 b
     on a.id=b.id
   where a.date=sysdate
   group by a.status
 ) a2;

Answer 6

如果我想确保即使对于找不到任何要返回的行的查询也总会有结果，我会在双表上进行左连接（对于oracle）：

select q.*  FROM DUAL d LEFT JOIN ( your_query )q on 1=1

通过这种方式，无论如何，你总会得到一排！

如何在sql中返回零而不是使用case时选择没有行

6 个答案: