如果不存在记录，如何选择 sum -or- 0?

How to select sum -or- 0 if no records exist?(如果不存在记录，如何选择 sum -or- 0?)

问题描述

我需要编写一个查询，返回满足某个条件的所有值的总和，但是如果没有找到任何行，则该查询需要返回 0，而不是 null.例如:

I need to write a query that returns the sum of all values that meet a certain criteria, but the query needs to return 0 if no rows are found, rather than null. For example:

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

这个查询:

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

应该并且确实返回 15.然而:

should, and does, return 15. However:

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

应该返回0，但确实返回null.

should return 0, but does return null.

有人可以解释一下吗?

推荐答案

怎么样:

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

COALESCE 函数基本上说返回第一个参数，除非它是 null 在这种情况下返回第二个参数" - 在这些情况下非常方便.

The COALESCE function basically says "return the first parameter, unless it's null in which case return the second parameter" - It's quite handy in these scenarios.

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