Difference between "TOP" and "SAMPLE" in TeraData SQL

sql teradata
Steve picture Steve · Jun 16, 2011 · Viewed 16.1k times · Source

What is the difference between "TOP" and "SAMPLE" in TeraData SQL? Are they the same?

Answer

Yuck picture Yuck · Jun 16, 2011

From TOP vs SAMPLE:

TOP 10 means "first 10 rows in sorted order". If you don't have an ORDER BY, then by extension it will be interpreted as asking for "ANY 10 rows" in any order. The optimizer is free to select the cheapest plan it can find and stop processing as soon as it has found enough rows to return.

If this query is the only thing running on your system, TOP may appear to always give you exactly the same answer, but that behavior is NOT guaranteed.

SAMPLE, as you have observed, does extra processing to try to randomize the result set yet maintain the same approximate distribution. At a very simple level, for example, it could pick a random point at which to start scanning the table and a number of rows to skip between rows that are returned.

Related questions

SQL SELECT multi-columns INTO multi-variable

I'm converting SQL from Teradata to SQL Server in Teradata, they have the format SELECT col1, col2 FROM table1 INTO @variable1, @variable2 In SQL Server, I found SET @variable1 = ( SELECT col1 FROM table1 ); That only allows a single column/variable …

Read More
What is ROWS UNBOUNDED PRECEDING used for in Teradata?

I am just starting on Teradata and I have come across an Ordered Analytical Function called "Rows unbounded preceding" in Teradata. I tried several sites to learn about the function but all of them uses a complicated example explaining the …

Read More
Find which rows have different values for a given column in Teradata SQL

I am trying to compare two addresses from the same ID to see whether they match. For example: Id Adress Code Address 1 1 123 Main 1 2 123 Main 2 1 456 Wall 2 2 456 Wall 3 1 789 Right 3 2 100 Left I'm just trying to figure out whether the address for each ID …

Read More