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base on h2database 1.4.196. for more information, see http://h2database.com/

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/*
 * Copyright 2004-2018 H2 Group. Multiple-Licensed under the MPL 2.0,
 * and the EPL 1.0 (http://h2database.com/html/license.html).
 * Initial Developer: H2 Group
 */
package org.h2.api;

import java.sql.Connection;
import java.sql.SQLException;

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

    /**
     * This method is called when the aggregate function is used.
     * A new object is created for each invocation.
     *
     * @param conn a connection to the database
     */
    void init(Connection conn) throws SQLException;

    /**
     * This method must return the H2 data type, {@link org.h2.value.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.
     *
     * @param inputTypes the H2 data type of the parameters,
     * @return the H2 data type of the result
     * @throws SQLException if the number/type of parameters passed is incorrect
     */
    int getInternalType(int[] inputTypes) throws SQLException;

    /**
     * This method is called once for each row.
     * If the aggregate function is called with multiple parameters,
     * those are passed as array.
     *
     * @param value the value(s) for this row
     */
    void add(Object value) throws SQLException;

    /**
     * This method returns the computed aggregate value.
     *
     * @return the aggregated value
     */
    Object getResult() throws SQLException;

}