动机
业务系统对数据库进行查询通常需要分页查询(否则数据量太大无法返回)，所以需要通用的分页查询。分页查询一般需要做两件事情：
- 生成分页查询语句，进行分页查询。
- 生成求总数的语句，查询所有符合条件的数量，以求一共有多少页。
在开发过程中能力强的程序员可以直接写这两个语句，作为先腾框架提供了自动转换的函数，传入一个正常查询的语句，函数会自动转换为上述的两个语句。
源码详解
所有源码参见QueryUtils中的buildLimitQuerySQL 方法和buildGetCountSQL 方法。
buildLimitQuerySQL
不同的数据库分页方法是不一样的， 所有的方法都用到了编译原理中的lexer词法分析器。
/* 参数 asParameter 表示分页参数 直接作为参数 还是直接 拼接到sql语句中 */
public static String buildLimitQuerySQL(String sql,int offset,int maxsize,
boolean asParameter, DBType dbType){
switch (dbType){
case Oracle:
return buildOracleLimitQuerySQL(sql,offset, maxsize,asParameter);
case DB2:
return buildDB2LimitQuerySQL(sql,offset, maxsize);
case SqlServer:
return buildSqlServerLimitQuerySQL(sql,offset, maxsize);
case MySql:
return buildMySqlLimitQuerySQL(sql,offset, maxsize,asParameter);
case H2:
return buildMySqlLimitQuerySQL(sql,offset, maxsize,asParameter);
case Access:
default:
throw new PersistenceException(PersistenceException.ORM_METADATA_EXCEPTION,
"不支持的数据库类型："+dbType.toString());
}
}buildOracleLimitQuerySQL
public static String buildOracleLimitQuerySQL(String sql,int offset,int maxsize,boolean asParameter) {
final StringBuilder pagingSelect = new StringBuilder( sql.length()+100 );
if(asParameter){
if (offset>0) {
pagingSelect.append( "select * from ( select row_.*, rownum rownum_ from ( " );
}
else {
pagingSelect.append( "select * from ( " );
}
pagingSelect.append( sql );
if (offset>0) {
pagingSelect.append( " ) row_ ) where rownum_ <= ? and rownum_ > ?" );
}
else {
pagingSelect.append( " ) where rownum <= ?" );
}
}else{
if (offset>0) {
pagingSelect.append( "select * from ( select row_.*, rownum rownum_ from ( " );
}
else {
pagingSelect.append( "select * from ( " );
}
pagingSelect.append( sql );
if (offset>0) {
pagingSelect.append( " ) row_ ) where rownum_ <= ")
.append(offset + maxsize)
.append(" and rownum_ > ")
.append(offset);
}
else {
pagingSelect.append( " ) where rownum <= " ).append(maxsize);
}
}
return pagingSelect.toString();
}buildDB2LimitQuerySQL
public static String buildDB2LimitQuerySQL(String sql,int offset,int maxsize/*,boolean asParameter*/)
/*throws SQLException*/{
/*if(asParameter)*/
//throw new SQLException("DB2 unsupported parameter in fetch statement.");
if ( offset == 0 ) {
return maxsize>1?sql + " fetch first " + maxsize + " rows only":
" fetch first 1 row only";
}
//nest the main query in an outer select
return "select * from ( select inner2_.*, rownumber() over(order by order of inner2_) as rownumber_ from ( "
+ sql + " fetch first " + String.valueOf(offset+maxsize)
+ + " rows only ) as inner2_ ) as inner1_ where rownumber_ > "
+ offset + " order by rownumber_";
} buildSqlServerLimitQuerySQL
public static String buildSqlServerLimitQuerySQL(String sql,int offset,int maxsize){
if ( offset > 0 ) {
// SQL SERVER 2012 才支持
String alias_list = StringBaseOpt.objectToString( getSqlFiledNames(sql));
return "WITH query AS ("
+"SELECT inner_query.* "
+", ROW_NUMBER() OVER (ORDER BY CURRENT_TIMESTAMP) as __row_nr__ "
+" FROM ( " + sql + ") inner_query"
+" ) "
+" SELECT "+ alias_list +" FROM query WHERE __row_nr__ >=" + String.valueOf(offset)
+ " AND __row_nr__ < " + String.valueOf(offset + maxsize);
}else{
int selectIndex = sql.toLowerCase(Locale.ROOT).indexOf( "select" );
int selectDistinctIndex = sql.toLowerCase(Locale.ROOT).indexOf( "select distinct" );
selectIndex = selectIndex + (selectDistinctIndex == selectIndex ? 15 : 6);
return new StringBuilder( sql.length() + 8 )
.append( sql )
.insert( selectIndex, " top " + maxsize )
.toString();
}
}buildMySqlLimitQuerySQL
public static String buildMySqlLimitQuerySQL(String sql,int offset,int maxsize,boolean asParameter) {
if(asParameter)
return sql + (offset>0 ? " limit ?, ?" : " limit ?");
else
return sql + (offset>0 ? " limit "+String.valueOf(offset)+","+String.valueOf(maxsize) :
" limit "+String.valueOf(maxsize));
}buildGetCountSQL
获取总数的语句转换有两种方式，一种比较保险的方法就是子查询方法，还有一种式将select语句替换为select count(*) 但是这个在有group分组时就不能正确返回结果了。附带码：
/**
* 通过子查询来实现获取计数语句
* @param sql sql 或者 hql 语句
* @return sql
*/
public static String buildGetCountSQLBySubSelect(String sql) {
List<String> sqlPieces = splitSqlByFields(sql);
if (sqlPieces == null || sqlPieces.size() < 3)
return "";
if(StringUtils.isBlank(sqlPieces.get(0))) {
sqlPieces.set(0, "select");
}
//这个仅仅为了兼容hibernate
if("from".equalsIgnoreCase(sqlPieces.get(1).trim())) {
sqlPieces.set(1, " * from");
}
return sqlPieces.get(0) + " count(*) as rowCount from (select "+
sqlPieces.get(1) + sqlPieces.get(2) + ") a";
}
/**
* 将查询语句转换为相同条件的查询符合条件的记录数的语句, 需要考虑with语句
* 即将 select 的字段部分替换为 count(*) 并去掉 order by排序部分
* 对查询语句中有distinct的sql语句不使用
* @param sql sql
* @return sql
*/
public static String buildGetCountSQLByReplaceFields(String sql) {
List<String> sqlPieces = splitSqlByFields(sql);
if (sqlPieces == null || sqlPieces.size() < 3)
return "";
if(StringUtils.isBlank(sqlPieces.get(0))) {
sqlPieces.set(0, "select");
}
String groupByField = QueryUtils.getGroupByField(sqlPieces.get(2));
if(groupByField==null)
return sqlPieces.get(0) + " count(*) as rowcount from " +
removeOrderBy(sqlPieces.get(2));
return sqlPieces.get(0) + " count(*) as rowcount from (select "+
groupByField + " from " + removeOrderBy(sqlPieces.get(2)) + ") a";
}