/**
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* Copyright © 2015-2020 <a href="http://www.jeeplus.org/">JeePlus</a> All rights reserved.
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*/
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package com.jeeplus.common.persistence.dialect.db;
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import com.jeeplus.common.persistence.dialect.Dialect;
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/**
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* DB2的分页数据库方言实现
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*
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* @author poplar.yfyang
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* @version 1.0 2010-10-10 下午12:31
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* @since JDK 1.5
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*/
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public class DB2Dialect implements Dialect {
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@Override
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public boolean supportsLimit() {
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return true;
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}
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private static String getRowNumber(String sql) {
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StringBuilder rownumber = new StringBuilder(50)
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.append("rownumber() over(");
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int orderByIndex = sql.toLowerCase().indexOf("order by");
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if (orderByIndex > 0 && !hasDistinct(sql)) {
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rownumber.append(sql.substring(orderByIndex));
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}
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rownumber.append(") as rownumber_,");
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return rownumber.toString();
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}
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private static boolean hasDistinct(String sql) {
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return sql.toLowerCase().contains("select distinct");
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}
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@Override
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public String getLimitString(String sql, int offset, int limit) {
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return getLimitString(sql, offset, Integer.toString(offset), Integer.toString(limit));
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}
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/**
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* 将sql变成分页sql语句,提供将offset及limit使用占位符号(placeholder)替换.
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* <pre>
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* 如mysql
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* dialect.getLimitString("select * from user", 12, ":offset",0,":limit") 将返回
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* select * from user limit :offset,:limit
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* </pre>
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*
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* @param sql 实际SQL语句
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* @param offset 分页开始纪录条数
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* @param offsetPlaceholder 分页开始纪录条数－占位符号
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* @param limitPlaceholder 分页纪录条数占位符号
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* @return 包含占位符的分页sql
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*/
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public String getLimitString(String sql, int offset, String offsetPlaceholder, String limitPlaceholder) {
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int startOfSelect = sql.toLowerCase().indexOf("select");
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StringBuilder pagingSelect = new StringBuilder(sql.length() + 100)
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.append(sql.substring(0, startOfSelect)) //add the comment
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.append("select * from ( select ") //nest the main query in an outer select
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.append(getRowNumber(sql)); //add the rownnumber bit into the outer query select list
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if (hasDistinct(sql)) {
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pagingSelect.append(" row_.* from ( ") //add another (inner) nested select
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.append(sql.substring(startOfSelect)) //add the main query
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.append(" ) as row_"); //close off the inner nested select
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} else {
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pagingSelect.append(sql.substring(startOfSelect + 6)); //add the main query
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}
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pagingSelect.append(" ) as temp_ where rownumber_ ");
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//add the restriction to the outer select
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if (offset > 0) {
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// int end = offset + limit;
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String endString = offsetPlaceholder + "+" + limitPlaceholder;
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pagingSelect.append("between ").append(offsetPlaceholder)
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.append("+1 and ").append(endString);
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} else {
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pagingSelect.append("<= ").append(limitPlaceholder);
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}
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return pagingSelect.toString();
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}
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}
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