org.h2.api

接口 AggregateFunction

public interface AggregateFunction

A user-defined aggregate function needs to implement this interface. The class must be public and must have a public non-argument constructor.

Please note this interface only has limited support for data types. If you need data types that don't have a corresponding SQL type (for example GEOMETRY), then use the Aggregate interface.

方法概要

限定符和类型	方法和说明
void	add(Object value) This method is called once for each row.
Object	getResult() This method returns the computed aggregate value.
int	getType(int[] inputTypes) This method must return the SQL type of the method, given the SQL type of the input data.
default void	init(Connection conn) This method is called when the aggregate function is used.

方法详细资料

init

default void init(Connection conn)
           throws SQLException

This method is called when the aggregate function is used. A new object is created for each invocation.

参数:

conn - a connection to the database

抛出:

SQLException - on SQL exception

getType

int getType(int[] inputTypes)
     throws SQLException

This method must return the SQL type of the method, given the SQL type of the input data. The method should check here if the number of parameters passed is correct, and if not it should throw an exception.

参数:

inputTypes - the SQL type of the parameters, Types

返回:

the SQL type of the result

抛出:

SQLException - on failure

add

void add(Object value)
  throws SQLException

This method is called once for each row. If the aggregate function is called with multiple parameters, those are passed as array.

参数:

value - the value(s) for this row

抛出:

SQLException - on failure

getResult

Object getResult()
          throws SQLException

This method returns the computed aggregate value. This method must preserve previously added values and must be able to reevaluate result if more values were added since its previous invocation.

返回:

the aggregated value

抛出:

SQLException - on failure