Getting Single values

得到单一值

Sometimes all you need from a data base is a single value, which could be a count, sum, average, or other aggregated value from a data set.  Performing an ExecuteReader and calculating the result in your code is not the most efficient way to do this.  The best choice is to let the data base perform the work and return just the single value you need.  The following example shows how to do this with the ExecuteScalar method:

某些时候你想从数据库中只取一个值，它可能是关于数据集的计数、和、平均值或者其他聚合数值。使用ExecuteReader方法并计算代码中的结果并不是做这些事情的有效方式。最好的选择就是让数据库能够执行并且只返回你所需要的单独的值。下面的示例说明了如何使用ExecuteScalar方法来实现：

// 1. Instantiate a new command
 SqlCommand cmd = new SqlCommand("select count(*) from Categories", conn);
 
 // 2. Call ExecuteNonQuery to send command
 int count = (int)cmd.ExecuteScalar();

The query in the SqlCommand constructor obtains the count of all records from the Categories table.  This query will only return a single value.  The ExecuteScalar method in step 2 returns this value.  Since the return type of ExecuteScalar is type object, we use a cast operator to convert the value to int.

在SqlComand构造函数中的查询语句要求从Categories表中得到所有所有记录的计数。这些查询将致返回单独的值。在第2步中的ExecuteScalar方法返回这个值。因为ExecuteScalar方法返回类型是object，我们使用转换操作符将它转换为int

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