Analytic functions perform computations on a window of rows defined by the over_clause. In contrast to GROUP BY, they compute the result for each row of the result set.
The over_clause is used to break up the data in a query into partitions and to determine the order in which the partitions are processed. The over_clause may contain a partition_clause, an order_clause, and a window_frame_clause.
The partition_clause defines expressions that Exasol uses to partition the result set. To compute the result for a row, the function operates only on the rows in the partition containing the current row. Without the partition_clause, Exasol treats the entire result set as a single partition. If the query does not specify an order_clause nor a window_frame_clause, the default window_frame_clause is ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING.
The order_clause specifies the ordering of the data within each partition. If the analytic function contains an order_clause but not a window_frame_clause, the default window_frame_clause is RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW. If the analytic function does not contain an order_clause, Exasol does not sort the data within each partition. Without sorted data, the use of some analytic functions can cause non-deterministic results.
Window Frame Clause
The window_frame_clause defines a window of rows that Exasol uses as input for computing the analytic function. The window can be specified for each row in terms of window_frame_units relative to the current row and within the partition to which the current row belongs. Exasol supports following window_frame_units:
- Number of rows
For example, ROWS BETWEEN 5 PRECEDING AND 5 FOLLOWING, ROWS BETWEEN column_A PRECEDING AND column_B FOLLOWING
- Range of order values relative to the order value in the current row
For example, RANGE BETWEEN 5 PRECEDING AND 5 FOLLOWING, RANGE BETWEEN column_A PRECEDING AND column_B FOLLOWING
- Number of groups of peer rows. Rows with the same order value are called peer rows
For example, GROUPS BETWEEN 5 PRECEDING AND 5 FOLLOWING, GROUPS BETWEEN column_A PRECEDING AND column_B FOLLOWING
Additionally, it is possible to use UNBOUNDED PRECEDING, CURRENT ROW, and UNBOUNDED FOLLOWING clauses to specify the window boundaries.
Exasol also supports a short form for window frames that use CURRENT ROW. For example, RANGE UNBOUNDED PRECEDING is equivalent to RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW.
For ROWS or GROUPS, the computed expr in expr PRECEDING or expr FOLLOWING has to be a numeric value greater or equal 0 with scale 0. For RANGE the computed expr in expr PRECEDING or expr FOLLOWING has to be a numeric (or interval) value greater or equal 0.
The following example shows the different results by using window_frame_units:
part, ord, arg,
LISTAGG(arg, ',') OVER (PARTITION BY part ORDER BY ord ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING) LISTAGG_ROWS,
LISTAGG(arg, ',') OVER (PARTITION BY part ORDER BY ord RANGE BETWEEN 1 PRECEDING AND 1 FOLLOWING) LISTAGG_RANGE,
LISTAGG(arg, ',') OVER (PARTITION BY part ORDER BY ord GROUPS BETWEEN 1 PRECEDING AND 1 FOLLOWING) LISTAGG_GROUPS
FROM ex_table ORDER BY part, ord;
Window Frame Exclusion Clause
The window_frame_clause may also contain the window_frame_exclusion clause, which makes it possible to exclude certain rows from the window before the computation of the analytic function.
|EXCLUDE CURRENT ROW||Excludes the current row|
|EXCLUDE GROUP||Excludes the group of peer rows (rows with same order value) of the current row|
|EXCLUDE TIES||Excludes the group of peer rows, but keeps the current row in the window|
|EXCLUDE NO OTHERS||Does not exclude any row from the window|
If the query does not contain a window_frame_exclusion clause, the default behavior is EXCLUDE NO OTHERS.
Named Window Clause
The window clause allows the definition of named window specifications (i.e., the content of over_clauses) for analytic functions. It allows the association of identifiers (i.e., a window_name) with a specification. By referencing the identifier in the over_clause it is possible to use the same specification several times in a single statement. This shortens the SQL query and increases its readability. Additionally, it is possible to combine named window specifications with each other.
The following example shows the different results by using a named window clause:
id, department, hire_date, starting_salary,
AVG(starting_salary) OVER w2 AVG,
MIN(starting_salary) OVER w2 MIN_STARTING_SALARY,
MAX(starting_salary) OVER (w1 ORDER BY hire_date)
WINDOW w1 as (PARTITION BY department), w2 as (w1 ORDER BY hire_date)
ORDER BY department, hire_date;
- Analytic functions are always evaluated after WHERE, GROUP BY, and HAVING clause, but before the global ORDER BY clause.
- The results within each partition are computed independently from the other partitions in the result set.
- In the window_frame_clause, the lower boundary of the window must lie before, or can be the same as the upper boundary according to the sorting order of the rows. Exasol returns an error if this condition is violated.
- If the analytic function uses DISTINCT, the function must not contain an order_clause and must not contain a window_frame_clause.
- The use of a non-default window_frame_exclusion clause is only possible in combination with an order_clause.
- The use of window frame unit RANGE in combination with expr PRECEDING or expr FOLLOWING requires an order_clause with a single order expression.
- Exasol allows the use of analytic functions only in the select list or in the global ORDER BY clause.
- It is possible to replace the contents of the over_clause partially or completely by referring to a named_window identifier. To use such a window it is necessary to define the identifier and the associate window specification parts (for example, a partition_clause) in the Named Window Clause.
- Exasol does not support nested analytic functions.
- The position of the window_clause is after HAVING in a SELECT statement.
- It is not possible to override a previously defined window_name identifier.
- The scope of the window_clause is only the current SELECT statement. It does not include subqueries.
- It is possible to combine several named window specifications. However, the combination has to contain only one partition_clause, one order_clause, and one window_frame_clause.
The input table share_closing_prices in the example lists closing prices of shares of different companies over 10 days. The following query generates a moving average of the closing price for each share for the previous 4 days and the current day.
COMPANY, TRADING_DATE, CLOSING_PRICE,
AVG(CLOSING_PRICE) over (PARTITION BY COMPANY ORDER BY TRADING_DATE ROWS BETWEEN 4 PRECEDING AND CURRENT ROW) MOVING_AVG
The analytic function used in this example is AVG. The query partitions the table by COMPANY and orders the data by TRADING_DATE. The result for each row is the average of the CLOSING_PRICE for previous 4 days and the current day.
Supported Analytic Functions
Exasol supports the following analytic functions.