Window examples

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Window examples

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The following examples show a sample input data set, the definitions for several windows, and the contents of those windows at various times after 10:00, the time data starts to arrive for this example.

The windows are defined as follows:



 sum(amount) OVER lastHour,

 count(*) OVER lastHour

 sum(amount) OVER lastThree

FROM Trades



 lastThree AS (ROWS 3 PRECEDING),

 lastZeroRows AS (ROWS CURRENT ROW),

 lastZeroSeconds AS (RANGE CURRENT ROW),

 lastTwoSameTicker AS (PARTITION BY ticker ROWS 2 PRECEDING),


First Example: time-based windows versus row-based windows

As shown on the right side of the figure below, the time-based lastHour window contains varying numbers of rows, because window membership is defined by time range.


Examples of windows containing rows

The row-based lastThree window generally contains four rows: the three preceding and the current row. However for the row 10:10 IBM, it only contains two rows, because there is no data before 10:00.

A row-based window can contain several rows whose ROWTIME value is the same, though they arrive at different times (wall-clock times). The order of such a row in the row-based window depends on its arrival time; indeed, the row's arrival time can determine which window includes it.

For example, the middle lastThree window in Figure 1 shows the arrival of a YHOO trade with ROWTIME 11:15 (and the last three trades before it). However, this window excludes the next trade, for IBM, whose ROWTIME is also 11:15 but which must have arrived later than the YHOO trade. This 11:15 IBM trade is included in the 'next' window, as is the 11:15 YHOO trade, its immediate predecessor.

Second Example: zero width windows, row-based and time-based

Figure 2: Examples of zero-width windows shows row-based and time-based windows of zero width. The row-based window lastZeroRows includes just the current row, and therefore always contains precisely one row. Note that ROWS CURRENT ROW is equivalent to ROWS 0 PRECEDING.

The time-based window lastZeroSeconds contains all rows with the same timestamp, of which there may be several. Note that RANGE CURRENT ROW is equivalent to RANGE INTERVAL '0' SECOND PRECEDING.


Third Example: Partitioning applied to row-based and time-based windows

Figure 3 shows windows that are similar to those in Figure 1 but with a PARTITION BY clause. For time-based window lastTwoSameTicker and the row-based window lastHourSameTicker, the window contains rows that meet the window criteria and have the same value of the ticker column. Note: Partitions are evaluated before windows.