使用C#对SQLite创建自定义函数

SQLite是轻量级的数据库，关于它的常规用法就不一一介绍了，本文主要介绍了如何创建自定义函数，对其进行扩展，以满足开发中的个性化需求。

SQLiteFunction简单介绍

SQLiteFunction是一个抽象类，主要用于处理用户自定义函数。主要成员如下：

自定义函数

有三种类型函数可以自定义，分别是：Scalar,Aggregate,Collation。
Scalar：标量（对单条数据进行计算的函数）
Aggregate：聚合（对多条数据进行计算的函数）
Collation：集合（用于排序）
下面分别创建一种函数表示其简单用法，注意要引用命名空间 System.Data.SQLite

    /// 

    /// 求平方根
    /// 
    [SQLiteFunction(Name = "sqrt", Arguments = 1, FuncType = FunctionType.Scalar)]
    public class Sqrt : SQLiteFunction
    {
        public override object Invoke(object[] args)
        {
            double d = Convert.ToDouble(args[0]);
            return Math.Sqrt(d);
        }
    }

    /// 

    /// 求平均
    /// 
    [SQLiteFunction(Name = "mean", Arguments = -1, FuncType = FunctionType.Aggregate)]
    public class Mean : SQLiteFunction
    {
        int step = 0;
        public override void Step(object[] args, int stepNumber, ref object contextData)
        {
            double sum = Convert.ToDouble(contextData);
            sum += Convert.ToDouble(args[0]);
            contextData = sum;
            step++;
        }
        public override object Final(object contextData)
        {
            double sum = Convert.ToDouble(contextData);
            double mean = sum / step;
            return mean;
        }
    }

    /// 

    /// 中文排序
    /// 
    [SQLiteFunction(FuncType = FunctionType.Collation, Name = "pinyin")]
    public class PinYin : SQLiteFunction
    {
        public override int Compare(string x, string y)
        {
            return string.Compare(x, y);
        }
    }

Arguments为参数个数，等于-1可为任意个参数。

使用自定义函数

 static void Main(string[] args)
        {
            //注册自定义函数
            SQLiteFunction.RegisterFunction(typeof(Sqrt));
            SQLiteFunction.RegisterFunction(typeof(Mean));
            SQLiteFunction.RegisterFunction(typeof(PinYin));

            using (var connection = new SQLiteConnection(string.Format("Data source={0}", "test.db")))
            {
                connection.Open();
                using (var com = connection.CreateCommand())
                {
                    com.CommandText = "select sqrt(id) as value from student ";
                    using (SQLiteDataReader dr = com.ExecuteReader())
                    {
                        while (dr.Read())
                        {
                            double value = Convert.ToDouble(dr["value"]);
                            Console.WriteLine(value);
                        }
                        dr.Close();
                    }

                    com.CommandText = "select mean(high) as val from student ";
                    using (SQLiteDataReader dr = com.ExecuteReader())
                    {
                        if (dr.Read())
                        {
                            double value = Convert.ToDouble(dr["val"]);
                            Console.WriteLine(value);
                        }
                        dr.Close();
                    }

                    com.CommandText = "select name from student ORDER BY name COLLATE  pinyin";
                    using (SQLiteDataReader dr = com.ExecuteReader())
                    {
                        while (dr.Read())
                        {
                            Console.WriteLine(dr["name"].ToString());
                        }
                        dr.Close();
                    }
                }
            }
        }

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