为什么包含空值的多列的Sql Sum返回不正确的结果？

Question

Table containing the following values:

Column1  Column2
1        NULL
NULL     4
2        NULL
NULL     5
3        6

The following query:

SELECT 
    SUM([Column1] + [Column2] ) 
FROM [myTable]

returns a value of 9 when it should be returning 21 . Why? How does it arrive at the value?

I know the SUM can be corrected by adding ISNULL like so:

SELECT 
    SUM(ISNULL([Column1], 0) + ISNULL([Column2], 0)) 
FROM [myTable]

but I would like to know the logic behind the value 9

Answer 1

use COALESCE to convert null into 0 . ( that's it if you want null values as zero. )

SELECT SUM(COALESCE(column1,0) + COALESCE(column2,0))
FROM table1

• See SQLFIDDLE Demo

Answer 2

What is the sum of null and a number, exactly? Note where the 9 comes from: the only row which has non-null Column1 and Column2 .

One viable solution has of course already been posted. But then, where's the fun in jumping right to the fix?

(copypasta'd at OP's request)

Answer 3

Because it is adding value +NULL before summing

Try sum(column1) + sum(column2)

Answer 4

使用ISNULL函数获得所需的行为：

SELECT SUM(ISNULL(Column1, 0) + ISNULL(Column2, 0)) FROM [myTable]

Answer 5

Its a problem with a Null value.

SELECT SUM(IsNull(Column1, 0) + IsNull(Column2, 0) ) FROM [myTable]

to ensure it is always 0 at minimum.

Thank you

Answer 6

1. number + NULL = NULL
2. SUM( expression ) function is manipulated by calculating the expression by each row separately and return a NOT_NULL value . And then all the results will be summed and returned.

That's reason why your result is 9. ISNULL(expression, replacement_value) can help you in this situation. :)

Answer 7

An alternative explanation (just in case it works better for someone):

NULLs affect + but do not affect SUM() : where a NULL is + -ed, it evaluates to NULL, where it is SUMmed, it is ignored. (But SUM() can return NULL when not a single argument was a value .)

So, there is only one row in your data sample (the last one) that produces a non-NULL result for the + , and that result is 9 , which is what SUM() returns as well.