如果不存在记录，如何选择 sum -or- 0?
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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