Understanding Teradata’s SAMPLE Function: Randomly Select Rows and More

What is the Teradata Sample Function?

The SAMPLE function returns a set of randomly selected rows. Here are the characteristics:

• Ask for a sample with an absolute number of rows
• Ask for a sample that contains a certain percentage of the table rows 
• Ask for more than one sample at the same time
• Include the column SAMPLEID to determine from which sample a row originates
• Ask for samples with or without duplicates

The Syntax of the Sample Function

SAMPLE [WITH REPLACEMENT] [RANDOMIZED ALLOCATION][WHEN <condition> THEN] {<number of row> | <percentage>} […,<number-of-rows> | <percentage>][ELSE {<number of rows> | <percentage } END]

Here are some examples of how to use the SAMPLE function:

100 Random Rows

SELECT * FROM CustomerSAMPLE 100;

10% of Table Rows

SELECT * FROM CustomerSAMPLE .10;

2 Samples with 10% of Table Rows (no Duplicates)

SELECT * FROM CustomerSAMPLE .1,.1;

2 Samples with 10 Rows and SAMPLEID

SELECT    t01.* ,   SAMPLEIDFROM Customer t01SAMPLE 10,10;

2 Samples with 10% of Table Rows (with Duplicates)

SELECT * FROM CustomerSAMPLE WITH REPLACEMENT .1,.1;

 

2 Samples with Conditional Logic Applied 

SELECT * FROM CustomerSAMPLE WHEN CustomerId > 1000 THEN .1,.1 ELSE .3,.3 END;