Add StringColumn type and expression

Add Projection type
Alias refactoring
More numeric operations

sqlbuilder/projection.go

@@ -0,0 +1,26 @@
+package sqlbuilder
+
+import "bytes"
+
+type Projection interface {
+	SerializeForProjection(out *bytes.Buffer) error
+}
+
+//------------------------------------------------------//
+// Dummy type for select * AllColumns
+type ColumnList []Column
+
+func (cl ColumnList) SerializeForProjection(out *bytes.Buffer) error {
+	for i, column := range cl {
+		err := column.SerializeSql(out, FOR_PROJECTION)
+
+		if err != nil {
+			return err
+		}
+
+		if i != len(cl)-1 {
+			out.WriteString(", ")
+		}
+	}
+	return nil
+}