Question

我需要编写一个返回满足特定条件的所有值的总和的查询，但如果没有找到行，则查询需要返回0，而不是null。例如：

tab    
+---------------+-----+
| descr         | num |
+---------------+-----+
| hello there   | 5   |
| hi there      | 10  |
| hello         | 10  |
| hi there!     | 15  |
+---------------+-----+

此查询：

SELECT sum(num) AS val FROM tab WHERE descr LIKE "%hello%";

应该而且确实会返回15。但是：

SELECT sum(num) AS val FROM tab WHERE descr LIKE "%greetings%";

应该返回0，但会返回null。

有人可以解释这是否可行？

Answer 1

怎么样：

SELECT COALESCE(sum(num), 0) AS val FROM tab WHERE descr LIKE "%greetings%";

COALESCE函数基本上表示“返回第一个参数，除非它返回null，在这种情况下返回第二个参数” - 在这些情况下它非常方便。

Answer 2

检查MySQL documentation for IFNULL。

SELECT SUM(IFNULL(num, 0)) as val FROM tab WHERE descr LIKE "%greetings%";

当然，这假设您的num字段可以为空并且没有默认值。另一种可能的解决方案是为num字段设置默认值0，这可以解决您遇到的问题。

Answer 3

为了正确地执行此操作，您可能希望区分在您要求求和的数据中存在实际NULL结果的情况，以及根本没有值要求求和的情况。

假设我们有以下内容：

mysql> select * from t;
+----------+------+
| descr    | num  |
+----------+------+
| hiya     |    5 |
| hi there |   10 |
| yo       | NULL |
+----------+------+

我们希望空的总和为零，但是NULL的总和为NULL。一种（相当折磨）的方式是：

mysql> SELECT IF(has_null, NULL, total) AS sum FROM (
    ->    SELECT COALESCE(MAX(num IS NULL), 0) AS has_null, COALESCE(SUM(num), 0) AS total
    ->    FROM t WHERE num < 'ciao')
    -> AS u;
+------+
| sum  |
+------+
|    0 |
+------+
1 row in set, 1 warning (0.00 sec)

mysql> SELECT IF(has_null, NULL, total) AS sum FROM (
    ->    SELECT COALESCE(MAX(num IS NULL), 0) AS has_null, COALESCE(SUM(num), 0) AS total
    ->    FROM t)
    -> AS u;
+------+
| sum  |
+------+
| NULL |
+------+
1 row in set (0.00 sec)

mysql> SELECT IF(has_null, NULL, total) AS sum FROM (
    ->    SELECT COALESCE(MAX(num IS NULL), 0) AS has_null, COALESCE(SUM(num), 0) AS total
    ->    FROM t WHERE descr < 'namaste')
    -> AS u;
+------+
| sum  |
+------+
|   15 |
+------+
1 row in set (0.00 sec)

mysql> SELECT IF(has_null, NULL, total) AS sum FROM (
    ->    SELECT COALESCE(MAX(num IS NULL), 0) AS has_null, COALESCE(SUM(num), 0) AS total
    ->    FROM t WHERE descr > 'namaste')
    -> AS u;
+------+
| sum  |
+------+
| NULL |
+------+
1 row in set (0.00 sec)

也许我没有想到更好的方式。

不幸的是，SQL标准defines SUM to be null when no elements are summed和MySQL别无选择，只能遵循该标准。

Answer 4

这有效：

SELECT IF(SUM(num) IS NULL, 0, SUM(num)) AS val FROM tab WHERE descr LIKE "%whatever%";

IF（）接受三个参数：（1）语句，（2）语句为真时应用的值，（3）语句为假时应用的值。

如果没有记录，如何选择sum -or- 0？

4 个答案: