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An enhanced toolkit of Mybatis to simplify development.

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/*
 * Copyright (c) 2011-2014, hubin ([email protected]).
 * 
 * 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.
 */
package com.baomidou.mybatisplus.extension.plugins.pagination.dialects;

import com.baomidou.mybatisplus.core.toolkit.StringPool;

/**
 * 

 * DB2 数据库分页方言
 * 
 *
 * @author hubin
 * @since 2016-11-10
 */
public class DB2Dialect implements IDialect {


    private static String getRowNumber(String originalSql) {
        StringBuilder rownumber = new StringBuilder(50).append("rownumber() over(");
        int orderByIndex = originalSql.toLowerCase().indexOf("order by");
        if (orderByIndex > 0 && !hasDistinct(originalSql)) {
            rownumber.append(originalSql.substring(orderByIndex));
        }
        rownumber.append(") as rownumber_,");
        return rownumber.toString();
    }

    private static boolean hasDistinct(String originalSql) {
        return originalSql.toLowerCase().contains("select distinct");
    }

    @Override
    public String buildPaginationSql(String originalSql, long offset, long limit) {
        int startOfSelect = originalSql.toLowerCase().indexOf("select");
        StringBuilder pagingSelect = new StringBuilder(originalSql.length() + 100)
            .append(originalSql, 0, startOfSelect).append("select * from ( select ")
            .append(getRowNumber(originalSql));
        if (hasDistinct(originalSql)) {
            pagingSelect.append(" row_.* from ( ").append(originalSql.substring(startOfSelect)).append(" ) as row_");
        } else {
            pagingSelect.append(originalSql.substring(startOfSelect + 6));
        }
        pagingSelect.append(" ) as temp_ where rownumber_ ");

        // 20180829 modify by hepengju
        // https://github.com/baomidou/mybatis-plus/issues/450
        if (offset > 0) {
            String endString = offset + StringPool.PLUS + limit;
            pagingSelect.append(" fetch first ").append(endString).append(" rows only) as temp_ where rownumber_ ")
                .append("> ").append(offset);
        } else {
            pagingSelect.append(" fetch first ").append(limit).append(" rows only) as temp_ ");
        }
        return pagingSelect.toString();
    }
}