The median aggregate returns the median from a set of input values. Available only in the Enterprise Edition.

Synopsis

AQL% SELECT median(attribute) FROM array;
AFL% aggregate(array,median(attribute),dimension_1[,dimension_2]...

Inputs

• array:  Specifies an array in the SciDb database.
• median(attribute):  Specifies which attribute to use when performing the median aggregation
• dimension_n:  Specifies which dimension to use when performing the median aggregation.

Library

The median aggregate resides in the Linear Algebra library. Run the following query to load this library:

AFL% load_library('linear_algebra'); 

Summary

The median aggregate returns the median from a set of input values. The median is the middle number (in a sorted list of numbers). The median value for a set of n values is defined as:

Example

To find the median of each column of a 3×3 matrix, do the following:

1. Load the appropriate library:

   AFL% load_library('linear_algebra');

   
   The output is:

   Query was executed successfully

2. Create an array m3x3:

   AFL% CREATE ARRAY m3x3<val:double>[i=0:2; j=0:2];

   
   The output is:

   Query was executed successfully

3. Store values 0–8 in the m3x3 array:
   

   AFL% store(build(m3x3,i*3+j),m3x3); 

   
   The output is:

   {i,j} val
   {0,0} 0
   {0,1} 1
   {0,2} 2
   {1,0} 3
   {1,1} 4
   {1,2} 5
   {2,0} 6
   {2,1} 7
   {2,2} 8

4. Find the median of every column of m3x3 using the attribute val and the dimension j:
   

   AFL% aggregate(m3x3,median(val),j);  

   
   The output is:

   {j} val_median
   {0} 3
   {1} 4
   {2} 5 

   
   

5. Remove the m3x3 array:
   

   AFL% remove(m3x3);

   
   The output is:

   Query was executed successfully