/// <summary>
/// 缓存计数
/// </summary>
/// <param name="sqlTool">SQL操作工具</param>
/// <param name="getKey">缓存关键字获取器</param>
/// <param name="group">数据分组</param>
protected Cache(Sql.Table <valueType, modelType> sqlTool, int group, Expression <Func <modelType, keyType> > getKey)
: this(sqlTool, group, getKey == null ? null : getKey.Compile())
{
sqlTool.SetSelectMember(getKey);
}