fix #213 support typed nil in interpolation

Author: huandu

interpolate.go

@@ -618,12 +618,22 @@ func encodeValue(buf []byte, arg interface{}, flavor Flavor) ([]byte, error) {
 			buf = append(buf, "', 'YYYY-MM-DD HH24:MI:SS.FF')"...)
 		}
 
-	case fmt.Stringer:
-		buf = quoteStringValue(buf, v.String(), flavor)
-
 	default:
 		primative := reflect.ValueOf(arg)
 
+		// Handle typed nil values (e.g. (*string)(nil), (*time.Time)(nil))
+		// This check must come before fmt.Stringer check since nil pointers may implement interfaces
+		if !primative.IsValid() || (primative.Kind() == reflect.Ptr && primative.IsNil()) {
+			buf = append(buf, "NULL"...)
+			return buf, nil
+		}
+
+		// Check for fmt.Stringer after nil pointer check
+		if stringer, ok := arg.(fmt.Stringer); ok {
+			buf = quoteStringValue(buf, stringer.String(), flavor)
+			return buf, nil
+		}
+
 		switch k := primative.Kind(); k {
 		case reflect.Bool:
 			switch flavor {

interpolate_test.go

@@ -389,3 +389,109 @@ func TestFlavorInterpolate(t *testing.T) {
 		})
 	}
 }
+
+func TestTypedNilInterpolation(t *testing.T) {
+	a := assert.New(t)
+
+	// Test various typed nil pointers
+	cases := []struct {
+		name     string
+		flavor   Flavor
+		sql      string
+		args     []interface{}
+		expected string
+	}{
+		{
+			name:     "string pointer nil",
+			flavor:   MySQL,
+			sql:      "SELECT ?",
+			args:     []interface{}{(*string)(nil)},
+			expected: "SELECT NULL",
+		},
+		{
+			name:     "int pointer nil",
+			flavor:   MySQL,
+			sql:      "SELECT ?",
+			args:     []interface{}{(*int)(nil)},
+			expected: "SELECT NULL",
+		},
+		{
+			name:     "float64 pointer nil",
+			flavor:   MySQL,
+			sql:      "SELECT ?",
+			args:     []interface{}{(*float64)(nil)},
+			expected: "SELECT NULL",
+		},
+		{
+			name:     "time.Time pointer nil",
+			flavor:   MySQL,
+			sql:      "SELECT ?",
+			args:     []interface{}{(*time.Time)(nil)},
+			expected: "SELECT NULL",
+		},
+		{
+			name:     "byte slice pointer nil",
+			flavor:   MySQL,
+			sql:      "SELECT ?",
+			args:     []interface{}{(*[]byte)(nil)},
+			expected: "SELECT NULL",
+		},
+		{
+			name:     "PostgreSQL string pointer nil",
+			flavor:   PostgreSQL,
+			sql:      "SELECT $1",
+			args:     []interface{}{(*string)(nil)},
+			expected: "SELECT NULL",
+		},
+		{
+			name:     "SQLite int pointer nil",
+			flavor:   SQLite,
+			sql:      "SELECT ?",
+			args:     []interface{}{(*int)(nil)},
+			expected: "SELECT NULL",
+		},
+		{
+			name:     "Oracle float pointer nil",
+			flavor:   Oracle,
+			sql:      "SELECT :1",
+			args:     []interface{}{(*float64)(nil)},
+			expected: "SELECT NULL",
+		},
+		{
+			name:     "mixed nil types",
+			flavor:   MySQL,
+			sql:      "SELECT ?, ?, ?, ?",
+			args:     []interface{}{(*string)(nil), nil, (*int)(nil), (*[]byte)(nil)},
+			expected: "SELECT NULL, NULL, NULL, NULL",
+		},
+		{
+			name:     "interface pointer nil",
+			flavor:   MySQL,
+			sql:      "SELECT ?",
+			args:     []interface{}{(*fmt.Stringer)(nil)},
+			expected: "SELECT NULL",
+		},
+		{
+			name:     "slice pointer nil",
+			flavor:   MySQL,
+			sql:      "SELECT ?",
+			args:     []interface{}{(*[]int)(nil)},
+			expected: "SELECT NULL",
+		},
+		{
+			name:     "map pointer nil",
+			flavor:   MySQL,
+			sql:      "SELECT ?",
+			args:     []interface{}{(*map[string]int)(nil)},
+			expected: "SELECT NULL",
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			query, err := tc.flavor.Interpolate(tc.sql, tc.args)
+			a.NilError(err)
+			a.Equal(tc.expected, query)
+		})
+	}
+}