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Oracle® Fusion Middleware EPL Language Reference for Oracle Event Processing
11g Release 1 (11.1.1.7)

Part Number E14304-07
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4 EPL Reference: Functions

This chapter provides a reference to functions in Event Processing Language (EPL). Note that EPL is deprecated; new development should use Oracle Continuous Query Language (Oracle CQL).

This chapter includes the following sections:

4.1 Single-row Functions

Single-row functions return a single value for every single result row generated by your statement. These functions can appear anywhere where expressions are allowed.

EPL allows static Java library methods as single-row functions, and also features built-in single-row functions.

EPL auto-imports the following Java library packages:

Thus Java static library methods can be used in all expressions as shown in below example:

SELECT symbol, Math.round(volume/1000)
  FROM StockTickEvent RETAIN 30 SECONDS

Other arbitrary Java classes may also be used, however their names must be fully qualified or configured to be imported. For more information, see Section 4.3, "User-Defined functions."

The table below outlines the built-in single-row functions available.

Table 4-1 Built-In Single-Row Functions

Single-row Function Result See
MAX(expression, expression [, expression [,…])

Returns the highest numeric value among the two or more comma-separated expressions.

Section 4.1.1, "The MIN and MAX Functions"

MIN(expression, expression [, expression [,…])

Returns the lowest numeric value among the two or more comma-separated expressions.

Section 4.1.1, "The MIN and MAX Functions"

COALESCE(expression, expression [, expression [,…])

Returns the first non-null value in the list, or null if there are no non-null values.

Section 4.1.2, "The COALESCE Function"

CASE value
  WHEN compare_value THEN result
  [WHEN compare_value THEN result …]
  [ELSE result]
END

Returns result where the first value equals compare_value.

Section 4.1.3, "The CASE Control Flow Function"

CASE value
  WHEN condition THEN result
  [WHEN condition THEN result …]
  [ELSE result]
END

Returns the result for the first condition that is true.

Section 4.1.3, "The CASE Control Flow Function"

PREV(expression, event_property)

Returns a property value of a previous event, relative to the event order within a data window.

Section 4.1.4, "The PREV Function"

PRIOR(integer, event_property)

Returns a property value of a prior event, relative to the natural order of arrival of events

Section 4.1.5, "The PRIOR Function"

SELECT 
    CASE 
      WHEN INSTANCEOF(item, com.mycompany.Service) THEN serviceName?
      WHEN INSTANCEOF(item, com.mycompany.Product) THEN productName? 
     END
  FROM OrderEvent

Returns a boolean value indicating whether the type of value returned by the expression is one of the given types

Section 4.1.6, "The INSTANCEOF Function"

SELECT CAST(item.price?, double) FROM OrderEvent

Casts the return type of an expression to a designated type.

Section 4.1.7, "The CAST Function"

SELECT EXISTS(item.serviceName?) FROM OrderEvent

Returns a boolean value indicating whether the dynamic property, provided as a parameter to the function, exists on the event.

Section 4.1.8, "The EXISTS Function"


You may also create user-defined single-row functions. See Section 4.3, "User-Defined functions".

4.1.1 The MIN and MAX Functions

The MIN and MAX functions take two or more expression parameters. The MIN function returns the lowest numeric value among these comma-separated expressions, while the MAX function returns the highest numeric value. The return type is the compatible aggregated type of all return values.

The next example shows the MAX function that has a Double return type and returns the value 1.1.

SELECT MAX(1, 1.1, 2 * 0.5) 
  FROM ...

The MIN function returns the lowest value. The statement below uses the function to determine the smaller of two timestamp values.

SELECT symbol, MIN(ticks.timestamp, news.timestamp) AS minT
  FROM StockTickEvent AS ticks, NewsEvent AS news RETAIN 30 SECONDS
  WHERE ticks.symbol = news.symbol

The MIN and MAX functions are also available as aggregate functions. See Section 4.2, "Aggregate functions" for a description of this usage.

4.1.2 The COALESCE Function

The result of the COALESCE function is the first expression in a list of expressions that returns a non-null value. The return type is the compatible aggregated type of all return values.

This example returns a String type result with a value of foo.

SELECT COALESCE(NULL, 'foo') 
  FROM …

4.1.3 The CASE Control Flow Function

The CASE control flow function has two versions. The first version takes a value and a list of compare values to compare against, and returns the result where the first value equals the compare value. The second version takes a list of conditions and returns the result for the first condition that is true.

The return type of a CASE expression is the compatible aggregated type of all return values.

The example below shows the first version of a CASE statement. It has a String return type and returns the value one.

SELECT CASE 1 WHEN 1 THEN 'one' WHEN 2 THEN 'two' ELSE 'more' END 
  FROM …

The second version of the CASE function takes a list of conditions. The next example has a Boolean return type and returns the Boolean value true.

SELECT CASE WHEN 1>0 THEN true ELSE false END 
  FROM …

4.1.4 The PREV Function

The PREV function returns the property value of a previous event. The first parameter denotes the ith previous event in the order established by the data window. The second parameter is a property name for which the function returns the value for the previous event.

This example selects the value of the price property of the second previous event from the current Trade event.

SELECT PREV(2, price) 
  FROM Trade RETAIN 10 EVENTS

Because the PREV function takes the order established by the data window into account, the function works well with sorted windows. In the following example the statement selects the symbol of the three Trade events that had the largest, second-largest and third-largest volume.

SELECT PREV(0, symbol), PREV(1, symbol), PREV(2, symbol)
  FROM Trade RETAIN 10 EVENTS WITH LARGEST volume

The ith previous event parameter can also be an expression returning an Integer type value. The next statement joins the Trade data window with a RankSelectionEvent event that provides a rank property used to look up a certain position in the sorted Trade data window:

SELECT PREV(rank, symbol) 
  FROM Trade, RankSelectionEvent RETAIN 10 EVENTS WITH LARGEST volume

The PREV function returns a NULL value if the data window does not currently hold the ith previous event. The example below illustrates this using a time batch window. Here the PREV function returns a null value for any events in which the previous event is not in the same batch of events. The PRIOR function as discussed below can be used if a null value is not the desired result.

SELECT PREV(1, symbol) 
  FROM Trade RETAIN BATCH OF 1 MINUTE

4.1.4.1 Previous Event Per Group

The combination of the PREV function and the PARTITION BY clause returns the property value for a previous event in the given group.

For example, assume we want to obtain the price of the previous event of the same symbol as the current event.

The statement that follows solves this problem. It partitions the window on the symbol property over a time window of one minute. As a result, when the engine encounters a new symbol value that it hasn't seen before, it creates a new window specifically to hold events for that symbol. Consequently, the PREV function returns the previous event within the respective time window for that event's symbol value.

SELECT PREV(1, price) AS prevPrice 
  FROM Trade RETAIN 1 MIN PARTITION BY symbol

4.1.4.2 Restrictions

The following restrictions apply to the PREV functions and its results:

  • The function always returns a null value for remove stream (old data) events.

  • The function may only be used on streams that are constrained by a RETAIN clause.

4.1.5 The PRIOR Function

The PRIOR function returns the property value of a prior event. The first parameter is an integer value that denotes the ith prior event in the natural order of arrival. The second parameter is a property name for which the function returns the value for the prior event.

This example selects the value of the price property of the second prior event to the current Trade event.

SELECT PRIOR(2, price) 
  FROM Trade RETAIN ALL

The PRIOR function can be used on any event stream or view and does not require a stream to be constrained by a RETAIN clause as with the PREV function. The function operates based on the order of arrival of events in the event stream that provides the events.

The next statement uses a length batch window to compute an average volume for every 3 Trade events, posting results every 3 events. The SELECT clause employs the PRIOR function to select the current average and the average before the current average:

SELECT AVG(volume) AS avgVolume, AVG(PRIOR(3, volume))
  FROM Trade RETAIN BATCH OF 3 EVENTS

4.1.5.1 Comparison to the PREV Function

The PRIOR function is similar to the PREV function. The key differences between the two functions are as follows:

  • The PREV function returns previous events in the order provided by the window, while the PRIOR function returns prior events in the order of arrival in the stream.

  • The PREV function requires a RETAIN clause while the PRIOR function does not.

  • The PREV function returns the previous event taking into account any grouping. The PRIOR function returns prior events regardless of any grouping.

  • The PREV function returns a null value for remove stream events, i.e. for events leaving a data window. The PRIOR function does not have this restriction.

4.1.6 The INSTANCEOF Function

The INSTANCEOF function returns a boolean value indicating whether the type of value returned by the expression is one of the given types. The first parameter to the INSTANCEOF function is an expression to evaluate. The second and subsequent parameters are Java type names.

The INSTANCEOF function determines the return type of the expression at runtime by evaluating the expression, and compares the type of object returned by the expression to the defined types. If the type of object returned by the expression matches any of the given types, the function returns true. If the expression returned null or a type that does not match any of the given types, the function returns false.

The INSTANCEOF function is often used in conjunction with dynamic (unchecked) properties. Dynamic properties are properties whose type is not known at compile type.

The following example uses the INSTANCEOF function to select different properties based on the type:

SELECT 
    CASE 
      WHEN INSTANCEOF(item, com.mycompany.Service) THEN serviceName?
      WHEN INSTANCEOF(item, com.mycompany.Product) THEN productName? 
     END
  FROM OrderEvent

The INSTANCEOF function returns false if the expression tested by INSTANCEOF returned null.

Valid parameters for the type parameter are:

  • Any of the Java built-in types: int, long, byte, short, char, double, float, string, where string is a short notation for java.lang.String. The type name is not case-sensitive. For example, the following function tests if the dynamic price property is either of type float or type double:

    INSTANCEOF(price?, double, float)
    
  • The fully-qualified class name of the class to test, for example:

    INSTANCEOF(product, org.myproducer.Product)
    

Valid parameters for the type parameter list are:

The INSTANCEOF function considers an event class's superclasses as well as all the directly or indirectly-implemented interfaces by superclasses.

4.1.7 The CAST Function

The CAST function casts the return type of an expression to a designated type. The function accepts two parameters: the first parameter is the property name or expression that returns the value to be casted and the second parameter is the type to cast to.

Valid parameters for the second (type) parameter are:

  • Any of the Java built-in types: int, long, byte, short, char, double, float, string, where string is a short notation for java.lang.String. The type name is not case-sensitive. For example:

    cast(price, double)
    
  • The fully-qualified class name of the class to cast to, for example:

    cast(product, org.myproducer.Product)
    

The CAST function is often used to provide a type for dynamic (unchecked) properties. Dynamic properties are properties whose type is not known at compile type. These properties are always of type java.lang.Object.

The following example shows how to use the CAST function to cast the price dynamic property of an item in the OrderEvent to a double value.

SELECT CAST(item.price?, double) 
  FROM OrderEvent

The CAST function returns a null value if the expression result cannot be casted to the desired type, or if the expression result itself is null.

The CAST function adheres to the following type conversion rules:

  • For all numeric types, the CAST function utilizes java.lang.Number to convert numeric types, if required.

  • For casts to string or java.lang.String, the CAST function calls toString on the expression result.

  • For casts to other objects, including application objects, the CAST function considers a Java class's superclasses as well as all directly or indirectly-implemented interfaces by superclasses .

4.1.8 The EXISTS Function

The EXISTS function returns a boolean value indicating whether the dynamic property, provided as a parameter to the function, exists on the event. The EXISTS function accepts a single dynamic property name as its only parameter.

Use the EXISTS function with dynamic (unchecked) properties. Dynamic properties are properties whose type is not known at compile type. Dynamic properties return a null value if the dynamic property does not exist on an event, or if the dynamic property exists but the value of the dynamic property is null.

The following example of using the EXISTS function returns true if the item property contains an object that has a serviceName property. It returns false if the item property is null, or if the item property does not contain an object that has a property named serviceName:

SELECT EXISTS(item.serviceName?) 
  FROM OrderEvent

4.2 Aggregate functions

The aggregate functions are SUM, AVG, COUNT, MAX, MIN, MEDIAN, STDDEV, AVEDEV. You can use aggregate functions to calculate and summarize data from event properties. For example, to find out the total price for all stock tick events in the last 30 seconds:

SELECT SUM(price) 
  FROM StockTickEvent RETAIN 30 SECONDS

Here is the syntax for aggregate functions:

aggregate_function( [ALL | DISTINCT] expression)

You can apply aggregate functions to all events in an event stream window or other view, or to one or more groups of events. From each set of events to which an aggregate function is applied, EPL generates a single value.

The expression is usually an event property name. However it can also be a constant, function, or any combination of event property names, constants, and functions connected by arithmetic operators.

For example, to find out the average price for all stock tick events in the last 30 seconds if the price was doubled:

SELECT AVG(price * 2) 
  FROM StockTickEvent RETAIN 30 SECONDS

You can use the optional keyword DISTINCT with all aggregate functions to eliminate duplicate values before the aggregate function is applied. The optional keyword ALL which performs the operation on all events is the default.

The MIN and MAX aggregate functions are also available as single row functions. See Section 4.1.1, "The MIN and MAX Functions" for a description of this usage.

The syntax of the aggregation functions and the results they produce are shown in table below.

Table 4-2 Aggregate Functions

Aggregate Function Result
SUM([ALL|DISTINCT] expression)

Totals the (distinct) values in the expression, returning a value of long, double, float or integer type depending on the expression.

AVG([ALL|DISTINCT] expression)

Average of the (distinct) values in the expression, returning a value of double type.

COUNT([ALL|DISTINCT] expression)

Number of the (distinct) non-null values in the expression, returning a value of long type.

COUNT(*)

Number of events, returning a value of long type.

MAX([ALL|DISTINCT] expression)

Highest (distinct) value in the expression, returning a value of the same type as the expression itself returns.

MIN([ALL|DISTINCT] expression)

Lowest (distinct) value in the expression, returning a value of the same type as the expression itself returns.

MEDIAN([ALL|DISTINCT] expression)

Median (distinct) value in the expression, returning a value of double type.

STDDEV([ALL|DISTINCT] expression)

Standard deviation of the (distinct) values in the expression, returning a value of double type.

Oracle Event Processing uses a common measure called sample standard deviation when internally implementing STDDEV, rather than the precise mathematical definition. The definition used by Oracle Event Processing for STDDEV is as follows:

stddev(x) = (1/(N-1) * SUM{i=1 ... n}(xi - xavg) ) ^ (1/2)

AVEDEV([ALL|DISTINCT] expression)

Mean deviation of the (distinct) values in the expression, returning a value of double type.

Oracle Event Processing uses the following definition for AVEDEV:

avedev(x) = 1/n * SUM{i=1 . . . n} (x_i ? xavg)

TREND(expression)

Number of consecutive up ticks (as positive number), down ticks (as negative number), or no change (as zero) for expression.


You can use aggregation functions in a SELECT clause and in a HAVING clause. You cannot use aggregate functions in a WHERE clause, but you can use the WHERE clause to restrict the events to which the aggregate is applied. The next query computes the average and sum of the price of stock tick events for the symbol ACME only, for the last 10 stock tick events regardless of their symbol.

SELECT 'ACME stats' AS title, AVG(price) AS avgPrice, SUM(price) AS sumPrice
  FROM StockTickEvent RETAIN 10 EVENTS
  WHERE symbol='ACME'

In the preceding example the length window of 10 elements is not affected by the WHERE clause, in other words, all events enter and leave the length window regardless of their symbol. If we only care about the last 10 ACME events, we need to add a MATCHING clause as shown below.

SELECT 'ACME stats' AS title, AVG(price) AS avgPrice, SUM(price) AS sumPrice
  FROM (SELECT * FROM StockTickEvent WHERE symbol='ACME') 
  RETAIN 10 EVENT

You can use aggregate functions with any type of event property or expression, with the following restriction:

EPL ignores any null values returned by the event property or expression on which the aggregate function is operating, except for the COUNT(*) function, which counts null values as well. All aggregate functions return null if the data set contains no events, or if all events in the data set contain only null values for the aggregated expression.

You may also create an aggregate user-defined function. For more information, see Section 4.3, "User-Defined functions".

4.3 User-Defined functions

A user-defined function can be invoked anywhere as an expression itself or within an expression. The function must simply be a public static method that the class loader can resolve at statement creation time. The engine resolves the function reference at statement creation time and verifies parameter types.

Caution:

Only idempotent user-defined functions in EPL queries are allowed. That is, multiple invocations of the user-defined function does not change the result.

The example below assumes a class MyClass that exposes a public static method myFunction accepting two parameters, and returning a numeric type such as double.

SELECT 3 * MyClass.myFunction(price, volume) as myValue
  FROM StockTick RETAIN 30 SECONDS

User-defined functions also take array parameters as this example shows. Section 3.6, "Array Definition Operator" outlines in more detail the types of arrays produced.

SELECT * 
  FROM RFIDEvent RETAIN 10 MINUTES
  WHERE com.mycompany.rfid.MyChecker.isInZone(zone, {10, 20, 30})

Oracle Event Processing supports both single-row and aggregate user-defined functions.

For more information, see "wlevs:function" in the Oracle Fusion Middleware Developer's Guide for Oracle Event Processing for Eclipse.