使用COALESCE 来避免这种结果.

Use COALESCE to avoid that outcome.

SELECT COALESCE(SUM(column),0)
FROM   table
WHERE  ...

要查看它的实际效果，请查看此 sql fiddle:http://www.sqlfiddle.com/#!2/d1542/3/0

To see it in action, please see this sql fiddle: http://www.sqlfiddle.com/#!2/d1542/3/0

更多信息:

给定三个表(一个包含所有数字，一个包含所有空值，一个包含混合):

Given three tables (one with all numbers, one with all nulls, and one with a mixture):

SQL 小提琴

MySQL 5.5.32 架构设置:

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);

查询 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

结果:

| 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 |