16.20 - Example - Teradata Database

Teradata Vantageā„¢ - SQL Functions, Expressions, and Predicates

Advanced SQL Engine
Teradata Database
March 2019
Programming Reference

The following example illustrates a window function with a nested aggregate. The query is processed as follows:

  1. We use the SUM(balance) aggregate function to calculate the sum of all the balances for a given account in a given quarter.
  2. We check to see if a balance in a given quarter (for a given account) is greater than the balance of the previous quarter.
  3. If the balance increased, we track a cumulative count value. As long as the RESET WHEN condition evaluates to false, the balance is increasing over successive quarters, and we continue to increase the count.
  4. We use the ROW_NUMBER() ordered analytical function to calculate the count value. When we reach a quarter whose balance is less than or equal to that of the previous quarter, the RESET WHEN condition evaluates to true, and we start a new partition and ROW_NUMBER() restarts the count from 1. We specify ROWS BETWEEN 1 PRECEDING AND 1 PRECEDING to access the previous value.
  5. Finally, we subtract 1 to ensure that the count values start with 0.

The balance_increase column shows the number of successive quarters where the balance was increasing. In this example, we only have one quarter (1->2) where the balance has increased.

   SELECT account_key, quarter, sum(balance),
   ROW_NUMBER() over
        (PARTITION BY account_key
         ORDER BY quarter
         RESET WHEN sum(balance) /* current row balance */ <=
         SUM(sum(balance)) over (PARTITION BY account_key ORDER BY quarter
         ROWS BETWEEN 1 PRECEDING AND 1 PRECEDING)/* prev row */
         ) - 1 /* to get the count started at 0 */ as balance_increase
   FROM accounts
   GROUP BY account_key, quarter;

The possible results of the preceding SELECT appear in the table below:

account_key    quarter    balance    balance_increase
-----------    -------    -------    ----------------
          1          1        253                   0
          1          2        258                   1
          1          3        192                   0
          1          4        150                   0