org.h2.api

接口 Aggregate

public interface Aggregate

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

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.
int	getInternalType(int[] inputTypes) This method must return the H2 data type, Value, of the aggregate function, given the H2 data type of the input data.
Object	getResult() This method returns the computed aggregate value.
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

getInternalType

int getInternalType(int[] inputTypes)
             throws SQLException

This method must return the H2 data type, Value, of the aggregate function, given the H2 data 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 H2 data type of the parameters,

返回:

the H2 data type of the result

抛出:

SQLException - if the number/type of parameters passed is incorrect

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