Add conditional extension methods for SelectQuery

- Implemented `If` method to apply a function to `SelectQuery` if a boolean condition is true.
- Added `IfNotNull` method to apply a function when the provided object is not null.
- Introduced `IfNotNullOrEmpty` method to apply a function when a string is not null or empty.

These methods improve readability and allow for cleaner conditional query modifications.

src/Carbunql/Fluent/SelectQueryParameterExtensions.cs

@@ -8,4 +8,18 @@ public static SelectQuery AddParameter(this SelectQuery query, string name, obje
         query.Parameters.Add(prm);
         return query;
     }
+
+    public static SelectQuery FluentParameter(this SelectQuery query, string name, object? value)
+    {
+        var prm = new QueryParameter(name, value);
+        query.Parameters.Add(prm);
+        return query;
+    }
+
+    public static SelectQuery FluentParameter(this SelectQuery query, string name, object? value, Func<string, SelectQuery> func)
+    {
+        var prm = new QueryParameter(name, value);
+        query.Parameters.Add(prm);
+        return func(prm.ParameterName);
+    }
 }

src/Carbunql/Fluent/SelectQueryWhereExtensions.cs

@@ -7,6 +7,48 @@ public static class SelectQueryWhereExtensions
 {
     internal static char[] ParameterSymbols = { '@', ':', '$' };
 
+    /// <summary>
+    /// Conditionally applies the specified function to the query if the condition is true.
+    /// </summary>
+    /// <param name="query">The query to apply the function to.</param>
+    /// <param name="condition">The condition to evaluate.</param>
+    /// <param name="func">The function to apply if the condition is true.</param>
+    /// <returns>The modified query if the condition is true; otherwise, the original query.</returns>
+    public static SelectQuery If(this SelectQuery query, bool condition, Func<SelectQuery, SelectQuery> func)
+    {
+        return condition
+            ? func(query)
+            : query;
+    }
+
+    /// <summary>
+    /// Applies the specified function to the query if the value is not null.
+    /// </summary>
+    /// <param name="query">The query to apply the function to.</param>
+    /// <param name="value">The value to check for null.</param>
+    /// <param name="func">The function to apply if the value is not null.</param>
+    /// <returns>The modified query if the value is not null; otherwise, the original query.</returns>
+    public static SelectQuery IfNotNull(this SelectQuery query, object? value, Func<SelectQuery, SelectQuery> func)
+    {
+        return value != null
+            ? func(query)
+            : query;
+    }
+
+    /// <summary>
+    /// Applies the specified function to the query if the string value is not null or empty.
+    /// </summary>
+    /// <param name="query">The query to apply the function to.</param>
+    /// <param name="value">The string value to check for null or emptiness.</param>
+    /// <param name="func">The function to apply if the string value is not null or empty.</param>
+    /// <returns>The modified query if the string value is not null or empty; otherwise, the original query.</returns>
+    public static SelectQuery IfNotNullOrEmpty(this SelectQuery query, string? value, Func<SelectQuery, SelectQuery> func)
+    {
+        return !string.IsNullOrEmpty(value)
+            ? func(query)
+            : query;
+    }
+
     private static (string, string) GenerateComparison(string operatorSymbol, object? value)
     {
         if (value == null)

test/Carbunql.Building.Test/QuerySourceFilterTest.cs

@@ -1271,4 +1271,153 @@ sale AS s
 
         Assert.Equal(expect, query.ToText(), true, true, true);
     }
+
+    [Fact]
+    public void IfTest()
+    {
+        var sql = """
+            select 
+                s.unit_price * s.amount as price
+            from
+                sale as s
+            """;
+
+        int? price = null;
+
+        var query = SelectQuery.Parse(sql)
+            .If(price != null, x => x.Equal(nameof(price), price));
+
+        Monitor.Log(query, exportTokens: false);
+
+        var expect = """
+            SELECT
+                s.unit_price * s.amount AS price
+            FROM
+                sale AS s
+            """;
+
+        Assert.Equal(expect, query.ToText(), true, true, true);
+
+        price = 10;
+
+        query = SelectQuery.Parse(sql)
+            .If(price != null, x => x.Equal(nameof(price), price));
+
+        Monitor.Log(query, exportTokens: false);
+
+        expect = """
+            SELECT
+                s.unit_price * s.amount AS price
+            FROM
+                sale AS s
+            WHERE
+                s.unit_price * s.amount = 10
+            """;
+
+        Assert.Equal(expect, query.ToText(), true, true, true);
+    }
+
+    [Fact]
+    public void IfNotNullTest()
+    {
+        var sql = """
+            select 
+                s.unit_price * s.amount as price
+            from
+                sale as s
+            """;
+
+        int? price = null;
+
+        var query = SelectQuery.Parse(sql)
+            .IfNotNull(price, x => x.Equal(nameof(price), price));
+
+        Monitor.Log(query, exportTokens: false);
+
+        var expect = """
+            SELECT
+                s.unit_price * s.amount AS price
+            FROM
+                sale AS s
+            """;
+
+        Assert.Equal(expect, query.ToText(), true, true, true);
+
+        price = 10;
+
+        query = SelectQuery.Parse(sql)
+            .IfNotNull(price, x => x.Equal(nameof(price), price));
+
+        Monitor.Log(query, exportTokens: false);
+
+        expect = """
+            SELECT
+                s.unit_price * s.amount AS price
+            FROM
+                sale AS s
+            WHERE
+                s.unit_price * s.amount = 10
+            """;
+
+        Assert.Equal(expect, query.ToText(), true, true, true);
+    }
+
+    [Fact]
+    public void IfNotNullOrEmptyTest()
+    {
+        var sql = """
+            select 
+                s.product_name
+            from
+                sale as s
+            """;
+
+        string? product_name = null;
+
+        var query = SelectQuery.Parse(sql)
+            .IfNotNullOrEmpty(product_name, x =>
+            {
+                x.FluentParameter(":product_name"
+                    , product_name
+                    , name => x.Equal(nameof(product_name), name)
+                );
+                return x;
+            });
+
+        Monitor.Log(query, exportTokens: false);
+
+        var expect = """
+            SELECT
+                s.product_name
+            FROM
+                sale AS s
+            """;
+
+        Assert.Equal(expect, query.ToText(), true, true, true);
+
+        product_name = "test";
+
+        query = SelectQuery.Parse(sql)
+            .IfNotNullOrEmpty(product_name, x =>
+            {
+                x.FluentParameter(":product_name"
+                    , product_name
+                    , name => x.Equal(nameof(product_name), name)
+                );
+                return x;
+            });
+
+        Monitor.Log(query, exportTokens: false);
+
+        expect = """
+            SELECT
+                s.product_name
+            FROM
+                sale AS s
+            WHERE
+                s.product_name = :product_name
+            """;
+
+        Assert.Equal(expect, query.ToText(), true, true, true);
+    }
 }