/// <summary>
/// Extendable method: 查询该对象化数据源中总共多少个元素
/// </summary>
/// <typeparam name="TEntity">对象类型</typeparam>
/// <param name="source">数据源</param>
/// <returns>元素个数</returns>
public static long LongCount <TEntity>(this IDbQuery <TEntity> source)
where TEntity : class
{
//检查数据源是否为空
Check.ArgumentNull(source, nameof(source));
//创建DbScaler
IDbScalar dbScaler = source.Factory.CreateScalar(source, "COUNT");
//执行统计查询获取元素数量
return(TypeHelper.ConvertTo <long>(dbScaler.Execute()));
}
/// <summary>
/// Extendable method: 统计数据源中所有元素某属性的最小值
/// </summary>
/// <typeparam name="TEntity">实体类型</typeparam>
/// <typeparam name="TProperty">属性类型</typeparam>
/// <param name="source">数据源</param>
/// <param name="selector">指定元素某属性的表达式</param>
/// <returns>数据源中所有元素某属性的最小值</returns>
public static TProperty Min <TEntity, TProperty>(this IDbQuery <TEntity> source, Expression <Func <TEntity, TProperty> > selector)
where TEntity : class
{
//非空检查
Check.ArgumentNull(source, nameof(source));
Check.ArgumentNull(selector, nameof(selector));
//创建DbScaler
IDbScalar dbScaler = source.Factory.CreateScalar(source, "MIN", selector);
//执行统计查询获取元素属性最大值
return(TypeHelper.ConvertTo <TProperty>(dbScaler.Execute()));
}
/// <summary>
/// Internal Extendable method: 执行对数据源中所有元素的某属性的统计,获取统计值
/// </summary>
/// <typeparam name="TEntity">实体类型</typeparam>
/// <typeparam name="TProperty">属性类型</typeparam>
/// <param name="source">数据源</param>
/// <param name="methodName">源方法名</param>
/// <param name="selector">指定实体某属性表达式</param>
/// <returns>数据源中所有元素某属性的统计值</returns>
private static TProperty Function <TEntity, TProperty>(this IDbQuery <TEntity> source, string methodName, Expression <Func <TEntity, TProperty> > selector)
where TEntity : class
where TProperty : struct
{
//非空检查
Check.ArgumentNull(source, nameof(source));
Check.ArgumentNull(selector, nameof(selector));
IDbScalar dbScaler = source.Factory.CreateScalar(source, methodName.ToUpper(), selector);
//执行获取第一行第一列的统计值
return(TypeHelper.ConvertTo <TProperty>(dbScaler.Execute()));
}
