EPL Reference: Clauses 2-11
symbol and not other windows for different stock symbols. Thus, in this case, only the average price of AAA would be affected.
2.4.7 Using WITH Clause to Keep LargestSmallestUnique Values
The WITH clause allows the largest, smallest, and unique property values to be kept in the window. For example, to keep the two highest priced stocks, the following
statement would be used:
SELECT stockSymbol, price FROM StockTick RETAIN ALL WITH 2 LARGEST price
In the case of time-based windows, the [n] qualifier before the LARGEST or SMALLEST keyword determines how many values are kept. For example, the following statement
would keep the two smallest prices seen over one minute:
SELECT stockSymbol, price FROM StockTick RETAIN 1 MINUTE WITH 2 SMALLEST price
In the absence of this qualifier, the single largest or smallest value is kept. The UNIQUE qualifier causes the window to include only the most recent among
events having the same value for the specified property. For example, the following query would keep only the last stock tick for each unique stock symbol:
SELECT FROM StockTick RETAIN 1 DAY WITH UNIQUE stockSymbol
Prior events of the same property value would be posted as old events by the engine.
2.5 MATCHING
Either a MATCHING or a FROM clause must appear in an EPL statement. The MATCHING clause is an alternate mechanism for determining which events are used by the EPL
statement. It allows for the detection of a series of one or more events occurring that satisfies a specified pattern. Pattern expressions consist of references to streams
separated by logical operators such as AND, OR, and FOLLOWED BY to define the sequence of events that compose the pattern. You may include an optional RETAIN
clause, as specified in
Section 2.4, RETAIN, to define the characteristics of the
window containing the matched events. The MATCHING clause executes prior to the WHERE or HAVING clauses.
The MATCHING clause syntax is as follows: MATCHING pattern_expression [RETAIN retain_clause]
with pattern_expression having the following syntax: [NOT|EVERY] stream_expression
AND | OR | [NOT] FOLLOWED BY stream_expression [WITHIN time_interval]
You can use the NOT operator to detect the absence of an event and the EVERY operator to control how pattern matching continues after a match. The stream_
expression is a stream source name optionally bound to a variable and filtered by a
parenthesized expression: stream_expression: [var_name:=]stream_name [ filter_expression ]
