SQL：包含 NULLS 的两列的总和？

Question

I want to sum two columns to get the third column. If a row has NULL for both columns, I want the third column to have NULL , instead of a 0 . The reason is because I want to get the average of the third column and having a 0 will change that average.

| Value1 | Value2 | Value3 |
|--------|--------|--------|
| 10     | NULL   | 10     |
| 20     | NULL   | 20     |
| NULL   | NULL   | NULL   |
| 20     | 30     | 50     |
| NULL   | NULL   | NULL   |

I have tried the following approaches, but they all give a 0 for NULL rows. What is the best way to achieve this?

CREATE TABLE #AMOUNTS (
    Value1 INT,
    Value2 INT
)

INSERT INTO #AMOUNTS VALUES
(10, NULL), (20, NULL), (NULL, NULL), (20, 30), (NULL, NULL)

SELECT 
Value1, Value2

,ISNULL(Value1, 0) + ISNULL(Value2, 0) AS Value3
,COALESCE(Value1, 0) + COALESCE(Value2, 0) AS Value3
,CASE WHEN Value1 = NULL AND Value2 = NULL THEN NULL
    ELSE ISNULL(Value1, 0) + ISNULL(Value2, 0)
END AS Value3
FROM #AMOUNTS

| Value1 | Value2 | Value3 | Value3 | Value3 |
|--------|--------|--------|--------|--------|
| 10     | NULL   | 10     | 10     | 10     |
| 20     | NULL   | 20     | 20     | 20     |
| NULL   | NULL   | 0      | 0      | 0      |
| 20     | 30     | 50     | 50     | 50     |
| NULL   | NULL   | 0      | 0      | 0      |

Answer 1

When testing for NULL use operator IS not = :

CASE WHEN Value1 IS NULL AND Value2 IS NULL THEN NULL
ELSE ISNULL(Value1, 0) + ISNULL(Value2, 0)

Answer 2

Use a case expression:

select (case when value1 is null and value2 is null
             then null
             else coalesce(value1, 0) + coalesce(value2, 0)
        end)

That said, apply is a simple way to get the average:

select t.*, v.avg_value
from t cross apply
     (select avg(v.val) as avg_value
      from (values (value1), (value2)) v(val)
     ) v;
         

Answer 3

If you don't want 0 s in the result column you can do it with ISNULL() and NULLIF() :

SELECT *, NULLIF(ISNULL(Value1, 0) + ISNULL(Value2, 0), 0) Value3
FROM Amounts

See the demo .