Použijte COALESCE
abyste se tomuto výsledku vyhnuli.
SELECT COALESCE(SUM(column),0)
FROM table
WHERE ...
Chcete-li to vidět v akci, podívejte se na tyto housle sql:http://www .sqlfiddle.com/#!2/d1542/3/0
Další informace:
Jsou dány tři tabulky (jedna se všemi čísly, jedna se všemi nulami a jedna se směsí):
Nastavení schématu MySQL 5.5.32 :
CREATE TABLE foo
(
id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
val INT
);
INSERT INTO foo (val) VALUES
(null),(1),(null),(2),(null),(3),(null),(4),(null),(5),(null),(6),(null);
CREATE TABLE bar
(
id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
val INT
);
INSERT INTO bar (val) VALUES
(1),(2),(3),(4),(5),(6);
CREATE TABLE baz
(
id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
val INT
);
INSERT INTO baz (val) VALUES
(null),(null),(null),(null),(null),(null);
Dotaz 1 :
SELECT 'foo' as table_name,
'mixed null/non-null' as description,
21 as expected_sum,
COALESCE(SUM(val), 0) as actual_sum
FROM foo
UNION ALL
SELECT 'bar' as table_name,
'all non-null' as description,
21 as expected_sum,
COALESCE(SUM(val), 0) as actual_sum
FROM bar
UNION ALL
SELECT 'baz' as table_name,
'all null' as description,
0 as expected_sum,
COALESCE(SUM(val), 0) as actual_sum
FROM baz
Výsledky :
| TABLE_NAME | DESCRIPTION | EXPECTED_SUM | ACTUAL_SUM |
|------------|---------------------|--------------|------------|
| foo | mixed null/non-null | 21 | 21 |
| bar | all non-null | 21 | 21 |
| baz | all null | 0 | 0 |