refs #48 #49 #53: add new method SQL in all builders

All builders add a new method called SQL to enable us to add any
arbitrary SQL when building. It's quite useful when we are using some
kinds of extension syntax or add special comments in SQL.

injection.go

@@ -0,0 +1,56 @@
+// Copyright 2018 Huan Du. All rights reserved.
+// Licensed under the MIT license that can be found in the LICENSE file.
+
+package sqlbuilder
+
+import (
+	"bytes"
+	"strings"
+)
+
+// injection is a helper type to manage injected SQLs in all builders.
+type injection struct {
+	markerSQLs map[injectionMarker][]string
+}
+
+type injectionMarker int
+
+type injectedSQL struct {
+	marker injectionMarker
+	sql    string
+}
+
+// newInjection creates a new injection.
+func newInjection() *injection {
+	return &injection{
+		markerSQLs: map[injectionMarker][]string{},
+	}
+}
+
+// SQL adds sql to injection's sql list.
+// All sqls inside injection is ordered by marker in ascending order.
+func (injection *injection) SQL(marker injectionMarker, sql string) {
+	injection.markerSQLs[marker] = append(injection.markerSQLs[marker], sql)
+}
+
+// WriteTo joins all SQL strings at the same marker value with blank (" ")
+// and writes the joined value to buf.
+func (injection *injection) WriteTo(buf *bytes.Buffer, marker injectionMarker) {
+	sqls := injection.markerSQLs[marker]
+	notEmpty := buf.Len() > 0
+
+	if len(sqls) == 0 {
+		return
+	}
+
+	if notEmpty {
+		buf.WriteRune(' ')
+	}
+
+	s := strings.Join(sqls, " ")
+	buf.WriteString(s)
+
+	if !notEmpty {
+		buf.WriteRune(' ')
+	}
+}