有个问题请老师帮我解决一下
select 6.66+1.11
结果为7.77 没有问题
select 6.66+1.11+isnull(a,0)
其中a为空值，结果为 7.8
select 6.66+1.11+isnull(a,0.000000001)
结果还是7.8
请问我如何得到7.77结果。
mysql 上查询到的isnull 函数的用法，和你写的不一样。你要不检查一下。它的含义应该是test这个表达式是不是null，如果是null，返回1，否则返回0
http://dev.mysql.com/doc/refman/5.7/en/comparison-operators.html#function_isnull
ISNULL(expr)
If expr is NULL, ISNULL() returns 1, otherwise it returns 0.
mysql> SELECT ISNULL(1+1);
-> 0
mysql> SELECT ISNULL(1/0);
-> 1
ISNULL() can be used instead of = to test whether a value is NULL. (Comparing a value to NULL using = always yields false.)
The ISNULL() function shares some special behaviors with the IS NULL comparison operator. See the description of IS NULL.