sql query to show zeros - simple, elegant method - mysql

Question

I have read answers to similar questions, and solved my problem based on these answers. However the (my) solution is rather complex, it includes two-step views, an additional table with left join. I wonder if there is a simpler, more elegant solution to the problem below as an illustration. The table Test has column B that needs to be summarized based on a criteria that column Marker has values 4 or over. The table TestResult is the desired output of the query. As I say, I do have an awkward, complex solution, but I'm curious if there is a simple, elegant method?
mysql> select * FROM Test;
+----+------+------+--------+
| ID | A | B | Marker |
+----+------+------+--------+
| 1 | 11 | 2 | 4 |
| 2 | 11 | 1 | 5 |
| 3 | 14 | 4 | 4 |
| 4 | 11 | 2 | 1 |
| 5 | 12 | 2 | 2 |
| 6 | 13 | 2 | 3 |
| 7 | 14 | 2 | 2 |
+----+------+------+--------+
7 rows in set (0.00 sec)
mysql> select * FROM TestResult;
+----+------+
| A | SumB |
+----+------+
| 11 | 3 |
| 12 | 0 |
| 13 | 0 |
| 14 | 4 |
+----+------+
4 rows in set (0.00 sec)

Answer 1

Could be achieved by using group by and case:
select A, sum(case when marker >= 4 then B else 0 end)
from Test
group by A;
Check out this SQL Fiddle

Answer 2

Edit: I originally overlooked the "or over" part of the question; leading me to think a different (more difficult to reach) answer was desired. I've corrected the condition, but left the solution for the more difficult answer since someone has apparently found it helpful.
Using the LEFT JOIN assures you get all A values from Test, the subselect provides you an indicator of the A values of rows satisfying the condition.
SELECT A, IF(t1.A IS NULL, 0, SUM(B)) AS sumB
FROM Test AS t0
LEFT JOIN (SELECT DISTINCT A FROM Test WHERE Marker >= 4) AS t1
ON t0.A = t1.A
GROUP BY A;
Alternatively, this is the most succinct; but might be less reliable or even rejected through some types of connections.
SELECT A, IF(SUM(IF(Marker>=4,1,0))>0, SUM(B),0) AS sumB
FROM Test
GROUP BY A;

Answer 3

I'd do it like this. Start with this:
SELECT t.A
FROM Test t
GROUP BY t.A
That's easy enough.
For the second column, we want a SUM of an expression...
SELECT t.A
, SUM(expr) AS SumB
FROM Test t
GROUP BY t.A
The trick is to use an expression that does a conditional tests that determine whether a value of column B should be returned or not. For example, in MySQL specific syntax, something like:
IF(t.Marker >= 4, t.B, 0)
Or, a more ANSI standards compliant equivalent:
CASE WHEN t.Marker >= 4 THEN t.B ELSE 0 END
Putting that all together...
SELECT t.A
, SUM(IF(t.Marker >= 4, t.B, 0)) AS SumB
FROM Test t
GROUP BY t.A
EDIT
What if there are rows like this in Test
+----+------+------+--------+
| ID | A | B | Marker |
+----+------+------+--------+
| 1 | 11 | 2 | 4 |
| 2 | 11 | 1 | 5 |
| 17 | 11 | 4 | 1 |
I'm questioning whether the row with a Marker value of 1 (which is not greater than or equal to 4), should the value of B be included in the SUM?
The query above, you'd get a SumB value of 3. Just the values from first two rows will be included in the SUM.
If you need to return a value of 7 for SumB (including that third row), then you'll need a different query pattern.