Sql 即使没有匹配的记录，也获取零

我在EMP_DETAILS表中有数据

EMPLOYEE_NUMBER      ROLE
NSA5421              CONTRACTOR
NSA390               CONTRACTOR
E8923                EMPLOYEE
E2390                EMPLOYEE

我想有零显示，即使没有记录，我尝试了以下，但没有得到理想的结果

SELECT  CASE WHEN cnt IS NULL THEN 0 ELSE cnt END cnt,
        CASE WHEN role IS NULL THEN '0' ELSE role END role
FROM
(SELECT   COUNT (*) cnt, role
     FROM   emp_details
    WHERE   employee_number = 'E3400'
GROUP BY   role)

请尝试：

select 
  NVL(y."ROLE", '0') "ROLE",  
  case when y."ROLE" is null then 0 else  COUNT(*) end cnt 
from(
      SELECT 'E3400' employee_number from dual
  )x left join emp_details y on x.employee_number=y.employee_number
group by "ROLE"

请尝试：

select 
  NVL(y."ROLE", '0') "ROLE",  
  case when y."ROLE" is null then 0 else  COUNT(*) end cnt 
from(
      SELECT 'E3400' employee_number from dual
  )x left join emp_details y on x.employee_number=y.employee_number
group by "ROLE"

---

您的内部查询只返回空值，而不返回任何内容

SELECT   COUNT (*) cnt, role
     FROM   emp_details
    WHERE   employee_number = 'E3400'
GROUP BY   role

---

Use EXISTS子句

您的内部查询只返回空值，而不返回任何内容

SELECT   COUNT (*) cnt, role
     FROM   emp_details
    WHERE   employee_number = 'E3400'
GROUP BY   role

SELECT  CASE WHEN cnt IS NULL THEN 0 ELSE cnt END cnt,
        CASE WHEN role IS NULL THEN '0' ELSE role END role
FROM
(SELECT   COUNT (*) cnt, role
     FROM   emp_details
    WHERE   employee_number = 'E3400'
GROUP BY   ROLE
UNION
SELECT NULL, NULL  FROM dual
WHERE NOT EXISTS
     ( SELECT   COUNT (*) cnt, role
     FROM   emp_details
    WHERE   employee_number = 'E3400'
GROUP BY   ROLE
     )
);

用另一种方式使用EXISTS子句

SELECT  CASE WHEN cnt IS NULL THEN 0 ELSE cnt END cnt,
        CASE WHEN role IS NULL THEN '0' ELSE role END role
FROM
(SELECT   COUNT (*) cnt, role
     FROM   emp_details
    WHERE   employee_number = 'E3400'
GROUP BY   ROLE
UNION
SELECT NULL, NULL  FROM dual
WHERE NOT EXISTS
     ( SELECT   COUNT (*) cnt, role
     FROM   emp_details
    WHERE   employee_number = 'E3400'
GROUP BY   ROLE
     )
);

SELECT a.role,
       count(b.role)
  FROM emp_details a left outer join emp_details b
    on a.employee_number = b.employee_number
   and a.employee_number = 'E3400'
 group by a.role;

输出：

|      ROLE  | COUNT(B.ROLE)  |
-------------|---------- -----|
| CONTRACTOR |              0 |
|   EMPLOYEE |              0 |

.

还有另一种方法

SELECT a.role,
       count(b.role)
  FROM emp_details a left outer join emp_details b
    on a.employee_number = b.employee_number
   and a.employee_number = 'E3400'
 group by a.role;

输出：

|      ROLE  | COUNT(B.ROLE)  |
-------------|---------- -----|
| CONTRACTOR |              0 |
|   EMPLOYEE |              0 |

---

.

根据您的数据，员工编号为“E3400”，则没有返回记录=）根据您的数据，员工编号为“E3400”，则没有返回记录=）显示除“E3400”以外的员工编号的2条记录。@techdo，已更新，请查看。显示除“E3400”以外的员工编号的2条记录。@techdo，已更新，请复习。