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Plugin for DataNucleus providing persistence to RDBMS datastores.
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/**********************************************************************
Copyright (c) 2009 Andy Jefferson and others. All rights reserved.
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.

Contributors:
    ...
**********************************************************************/
package org.datanucleus.store.rdbms.sql.method;

import java.util.List;

import org.datanucleus.ClassLoaderResolver;
import org.datanucleus.exceptions.NucleusException;
import org.datanucleus.query.compiler.CompilationComponent;
import org.datanucleus.store.rdbms.mapping.java.JavaTypeMapping;
import org.datanucleus.store.rdbms.sql.SQLStatement;
import org.datanucleus.store.rdbms.sql.SelectStatement;
import org.datanucleus.store.rdbms.sql.expression.AggregateNumericExpression;
import org.datanucleus.store.rdbms.sql.expression.NumericSubqueryExpression;
import org.datanucleus.store.rdbms.sql.expression.SQLExpression;
import org.datanucleus.store.rdbms.sql.expression.SQLExpressionFactory;
import org.datanucleus.util.Localiser;

/**
 * Expression handler to invoke the SQL AVG aggregation function.
 * For use in evaluating AVG({expr}) where the RDBMS supports this function.
 * Returns a NumericExpression "AVG({numericExpr})".
 */
public class AvgFunction extends SimpleNumericAggregateMethod
{
    protected String getFunctionName()
    {
        return "AVG";
    }

    /* (non-Javadoc)
     * @see org.datanucleus.store.rdbms.sql.method.SQLMethod#getExpression(org.datanucleus.store.rdbms.sql.expression.SQLExpression, java.util.List)
     */
    public SQLExpression getExpression(SQLStatement stmt, SQLExpression expr, List args)
    {
        if (expr != null)
        {
            throw new NucleusException(Localiser.msg("060002", getFunctionName(), expr));
        }
        if (args == null || args.size() != 1)
        {
            throw new NucleusException(getFunctionName() + " is only supported with a single argument");
        }

        // Set the return type (double, for JDOQL and JPQL)
        Class returnType = Double.class;

        SQLExpressionFactory exprFactory = stmt.getSQLExpressionFactory();
        if (stmt.getQueryGenerator().getCompilationComponent() == CompilationComponent.RESULT ||
            stmt.getQueryGenerator().getCompilationComponent() == CompilationComponent.HAVING)
        {
            // FUNC(argExpr)
            JavaTypeMapping m = exprFactory.getMappingForType(returnType, true);

             return getAggregateExpression(stmt, args, m);
        }

        // Handle as Subquery "SELECT AVG(expr) FROM tbl"
        ClassLoaderResolver clr = stmt.getQueryGenerator().getClassLoaderResolver();
        SQLExpression argExpr = (SQLExpression)args.get(0);
        SelectStatement subStmt = new SelectStatement(stmt, stmt.getRDBMSManager(), argExpr.getSQLTable().getTable(), argExpr.getSQLTable().getAlias(), null);
        subStmt.setClassLoaderResolver(clr);

        JavaTypeMapping mapping = stmt.getRDBMSManager().getMappingManager().getMappingWithDatastoreMapping(String.class, false, false, clr);
        SQLExpression aggExpr = getAggregateExpression(stmt, args, mapping);
        subStmt.select(aggExpr, null);

        JavaTypeMapping subqMapping = exprFactory.getMappingForType(returnType, false);
        SQLExpression subqExpr = new NumericSubqueryExpression(stmt, subStmt);
        subqExpr.setJavaTypeMapping(subqMapping);
        return subqExpr;
    }

    protected SQLExpression getAggregateExpression(SQLStatement stmt, List args, JavaTypeMapping m)
    {
        return new AggregateNumericExpression(stmt, m, getFunctionName(), args);
    }

    /* (non-Javadoc)
     * @see org.datanucleus.store.rdbms.sql.method.SimpleAggregateMethod#getClassForMapping()
     */
    @Override
    protected Class getClassForMapping()
    {
        return double.class;
    }
}