The 
Optionally, you can insert an integer as a parameter.
derive type:single value: RAND() as:'random' 
Output: For each row, generate a random number between 0 and 1 in the new random
function.
There are no arguments for this function.
In the following example, the random
column is generated by the RAND
function:
derive type:single value:RAND() as:'random' 
source  random 

A 

B 

C 

D 

The RAND
function is typically used to introduce randomness of some kind in your data. In the following example, it is used to perform sampling within your wider dataset.
Tip: Keep in mind that for larger datasets the application displays only a sample of them. This method of randomization is applied to the full dataset during job execution. 
Source:
You want to extract a random sample of 20% of your set of orders for further study:
OrderId  Qty  ProdId 

1001  30  Widgets 
1002  10  Big Widgets 
1003  5  Big Widgets 
1004  40  Widgets 
1005  80  Tiny Widgets 
1006  20  Widgets 
1007  100  Tiny Widgets 
Transform:
You can use the following transform to generate a random integer from one to 10:
derive type:single value:round(RAND() * 10) as:'random' 
You can now use the following transform to keep only the rows that contain random values that are in the top 20%, where the value is 9
or 10
:
keep row:(random > 8) 
Results:
NOTE: Since the results are randomized, your results might vary. 
OrderId  Qty  ProdId  random 

1005  80  Tiny Widgets  9 
1007  100  Tiny Widgets  10 