com.alilitech.mybatis.dialect.pagination.support.DB2PaginationDialect Maven / Gradle / Ivy

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/*
 *    Copyright 2017-2022 the original author or authors.
 *
 *    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.alilitech.mybatis.dialect.pagination.support;


import com.alilitech.mybatis.dialect.pagination.PaginationDialect;


/**
 * DB2 Pagination
 * @author Zhou Xiaoxiang
 * @since 1.0
 */
public class DB2PaginationDialect implements PaginationDialect {

    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, int offset, int 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_ ");

        // add the restriction to the outer select
        if (offset > 0) {
            String endString = offset + "+" + limit;
            pagingSelect.append("between ").append(offset).append("+1 and ").append(endString);
        } else {
            pagingSelect.append("<= ").append(limit);
        }
        return pagingSelect.toString();
    }
}