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One type consists of mathematical functions, to enable more calculations you can do
The second are aggregating functions, where you combine multiple values into one, which is necessary to reduce the results of some functions of the third type
Relation functions, which fetch values from other related issues, so you can get values from subtasks for example
Info 

Jira automatically rounds values to three decimal places. Be careful when you need exact results. 
Mathematical Function  Description  Example  

 Rounds the passed value to the specified number of significant decimal digits. 
 
 Rounds the passed value to the nearest lower integer. 
 
 Rounds the passed value to the nearest higher integer. 
 
 Gives the absolute value, i. e. the positive value. 
 
 Normalises negative values to 1, zero to 0, and positive values to 1. 
 
 Calculates the remainder of the dividend when divided by the divisor. 
 
Aggregating Function  Description  Example  
 Sum of the passed values 
 
 Average of the passed values 
 
 Smallest of the passed values 
 
 Largest of the passed values 
 
Relation Function  Description  Example  
 Retrieves the values of the passed field from all subtasks. Must be aggregated before using outside of a function. 
 
 Retrieves the values of the passed field from the parent of a subtask. 
 
 Retrieves the values of the passed field from all issues in an epic. Must be aggregated before using outside of a function. 
 
 Retrieves the values of the passed field from the epic of an issue. 
 
 Retrieves the values of the passed field from all linked issues. Must be aggregated before using outside of a function.


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