如何在sql中使用case返回零而不是未选择行

如果我查询一个不存在的输出，那么我将不会得到任何返回。我正在寻找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。我要求状态的默认值“0”应为返回值

您可能会生成缺少的值：

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;

---

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过时的联接语法，这使得很难看到实际发生的情况。特别是，它使您很难看到您是否会在加入过程中丢弃希望保留的信息。

此查询应始终返回一行，即使没有匹配的信息：

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);

注意，我更改了

where

逻辑

sysdate

（尽管名称不同）有一个时间组件。如果

date

有时间成分，您可能需要：

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

如果使用

COUNT（）

，则不需要

NVL（）

或

COALESCE（）

来处理

NULL

s，这与

SUM（）

的情况不同<当参数为

NULL

或没有匹配行时，code>COUNT（）将始终返回值为0的行。

groupby

也不是必需的

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);

如果您只是想弄清楚，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

---

无分组依据的聚合始终返回一行，因此现有查询将返回空值

要将空值更改为零，只需应用合并：

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;

---

如果我想确保即使对于找不到任何要返回的行的查询，也始终有一个结果，我会在双表上执行左联接（对于oracle）：

---

这样，不管发生什么事，你都会再次吵架

不应该是count（case-when）而不是sum（）？您还应该在sum上添加NVL或COALESCE，以处理查询返回no的情况rows@AlexZen：具有聚合且没有分组依据的查询将始终只返回一行。该行将包含整个结果集的统计信息，即使它是空集。@Andomar这是真的，但如果结果集为空，它将返回一行，其中的值为null。因此，您将NVL或COALESCE放在一起，以实现OP要求的（默认值“0”）选择NVL（sum（当a.status='SUCCESS'然后1 else 0 end）作为成功，

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;

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