Sampling Methods
Random Sampling
Teradata Database supports extracting a random sample from a database table using the SAMPLE clause and specifying one of the following:
This sampling method assumes that rows are sampled without replacement and that they are not reconsidered when another sample of the population is taken. This method results in mutually exclusive samples when you request multiple samples. In addition, the random sampling method assumes proportional allocation of rows across the AMPs in the system.
Random Stratified Sampling
In addition to random sampling option, Teradata Database supports stratified sampling.
Random Stratified Sampling, also called proportional or quota random sampling, involves dividing the population into homogeneous subgroups and taking a random sample in each subgroup. Stratified sampling represents both the overall population and key subgroups of the population. The fraction specification for stratified sampling refers to the fraction of the total number of rows in the stratum.
The following apply to stratified sampling.
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You can specify… |
You cannot specify… |
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stratified sampling in derived tables, views, and macros |
stratified sampling with set operations or subqueries |
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either a fraction or an integer as the sample size for every stratum |
fraction and integer combinations |
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up to 16 mutually exclusive samples for each stratum |