Mathematical notation for a conditional sum I'm looking for the correct way to define a sum of all elements in a set that fullfill a certain condition.
Example:
$$
n(A) = \frac{\sum_{F \in A}{
\begin{cases}
A_F\text{ is }\mathrm{NULL} & w_F\\
\text{else} & 0
\end{cases}}}{\sum_{F \in A}{w_F}}
$$
A is to supposed to be a record with a number of fields $F$. $A_F$ is the value of field F in the record A. $w_F$ is a weight associated with the Field $F$. I want to calculate how much percent of the record is NULL weighted according to $w_F$.
What I want to know is if this below is a valid way to formulate this or if there is a better way.
If someone has some pointers on a more formal / mathematical notation for database/records I would also be very grateful.
Thanks!
 A: For the sake of clarity, I would describe such an operation with the help of a short sentence. Also, it is quite common to put some summation conditions in the index spot of the sum symbol, as in
$$\sum_{p \text{ prime}} 1/p$$
In my opinion, it is best to describe what you have written above as
(Brief description of what kinds of objects $A$, $w_F$, $A_f$, etc are and a quick name for what $n(A)$ describes)
$$n(A) = \sum_{F \in A, A_F = 0} w_F/\sum_{F \in A}w_F$$
A: I know it is not any cool notation, but maybe you can simply define a dummy set, one that satisfies all your conditions, and then, make the sum of wf on each element of this dummy set .
By adding a dummy set B, this part of your formula
$$\sum_{F \in A}{
\begin{cases}
A_F\text{ is }\mathrm{NULL} & w_F\\
\text{else} & 0
\end{cases}}$$
could be transformed into
$$
\sum_{F \in B}{w_F}
$$
Then add at the end of your formula that set B is all the elements f of set A for which Af is null:
$$B=\{f \in A | A_f\text{ is }\mathrm{NULL}\}
$$
Thus, my final answer to your question would look like this:
$$
n(A) = \frac{\sum_{F \in B}{w_F}}{\sum_{F \in A}{w_F}}
$$
where
$$B=\{f \in A | A_f\text{ is }\mathrm{NULL}\}
$$
**Also, I don't know yet if this notation is allowed:
$$
n(A) = \frac{\sum_{F \in \{f \in A | A_f\text{ is }\mathrm{NULL}\}}{w_F}}{\sum_{F \in A}{w_F}}
$$ but it is the integration of the B set definition into one big formula. I personnaly think that defining B on a separated formula is clearer, but I guess sometimes we need to use only one formula. 
A: Although I don't think it is quite mainstream, but you could look into Iverson Notation. In short, you have a bracket and inside you write a condition. The bracket value is 1 if the condition holds, otherwise it's 0. Its most popular usage was in Concrete Mathematics (by Knuth et al).
$$
n(A) = \frac{\sum_{F \in A}{w_F[A_F \neq NULL]}}{\sum_{F \in A}{w_F}}
$$
