即使没有返回行也显示一个值

Question

We have a situation in oracle SQL.

select x from A where x=10;

so if 10 exists in the column, value will be displayed 10 . but if it doesn't exists I want to display null . I tried NVL but it is not returning anything as data is not displaying.

Could you please help me to resolve this.

Answer 1

If you want to return multiple rows from the original SELECT then you can use UNION ALL to add a row when the value does not exist:

SELECT x
FROM   A
WHERE  x = 10
UNION ALL
SELECT NULL
FROM   DUAL
WHERE  NOT EXISTS (
  SELECT x
  FROM   A
  WHERE  x = 10
)

If your table is:

CREATE TABLE a ( x ) AS
SELECT 10 FROM DUAL UNION ALL
SELECT 10 FROM DUAL UNION ALL
SELECT 10 FROM DUAL;

Then the query will output:

\n|  X | \n|  -: | \n|  10 | \n|  10 | \n|  10 | \n

Then if you DELETE FROM a; and repeat the query, it will output:

\n|  X | \n|  ---: | \n|  null |\n

db<>fiddle here

Answer 2

Try this:

select max(x) 
from A
where x = 10; --this will return null if no data

And then you can do a NVL:

select nvl(max(x), 0) 
from A
where x = 10; --this will return 0 if no data

Answer 3

Where clause filter out the data what you are looking for, if it doesn't exists then query will return no record.

So, you can remove where clause & do aggregation :

select max(case when a.x = 10 then 10 end)
from a;