What is a result of the query below for table with 10 rows when there are 2 clients with the headcount column having a NULL value, there are 7 customers with 1 head and 1 customer with 2 heads?
SELECT SUM(headcount) FROM clients
/*
ClientId
HeadCount
1
1
2
NULL
3
1
4
2
5
1
6
NULL
7
1
8
1
9
1
10
1
*/
Answer
Explanation
The result is 9 because if you use SUM(column), only rows with NOT NULL value in the column are summed.
Example
DECLARE @clients TABLE (headcount INT, name NVARCHAR(50))
INSERT INTO @clients (headcount, name) VALUES (1, '1 Peter Johnson')
INSERT INTO @clients (headcount, name) VALUES (1, '2 Peter Johnson')
INSERT INTO @clients (headcount, name) VALUES (1, '3 Peter Johnson')
INSERT INTO @clients (headcount, name) VALUES (1, '4 Peter Johnson')
INSERT INTO @clients (headcount, name) VALUES (1, '5 Peter Johnson')
INSERT INTO @clients (headcount, name) VALUES (2, '6 Peter Johnson')
INSERT INTO @clients (headcount, name) VALUES (1, '7 Peter Johnson')
INSERT INTO @clients (headcount, name) VALUES (1, '8 Peter Johnson')
INSERT INTO @clients (headcount, name) VALUES (NULL, NULL)
INSERT INTO @clients (headcount, name) VALUES (NULL, NULL)
SELECT SUM(headcount) FROM @clients
-- Returns 9, because rows with NULL values are ignored
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