dialect/sql: add option to prefix select queries (#1598)

dialect/sql/builder.go

@@ -1731,6 +1731,7 @@ type Selector struct {
 	offset   *int
 	distinct bool
 	union    []union
+	prefix   Queries
 }
 
 // WithContext sets the context into the *Selector.
@@ -1905,7 +1906,7 @@ func (s *Selector) join(kind string, t TableView) *Selector {
 	return s
 }
 
-// unionType describes a union type.
+// unionType describes an UNION type.
 type unionType string
 
 const (
@@ -1945,6 +1946,12 @@ func (s *Selector) UnionDistinct(t TableView) *Selector {
 	return s
 }
 
+// Prefix prefixes the query with list of queries.
+func (s *Selector) Prefix(queries ...Querier) *Selector {
+	s.prefix = append(s.prefix, queries...)
+	return s
+}
+
 // C returns a formatted string for a selected column from this statement.
 func (s *Selector) C(column string) string {
 	if s.as != "" {
@@ -2081,6 +2088,7 @@ func (s *Selector) Having(p *Predicate) *Selector {
 // Query returns query representation of a `SELECT` statement.
 func (s *Selector) Query() (string, []interface{}) {
 	b := s.Builder.clone()
+	s.joinPrefix(&b)
 	b.WriteString("SELECT ")
 	if s.distinct {
 		b.WriteString("DISTINCT ")
@@ -2153,6 +2161,13 @@ func (s *Selector) Query() (string, []interface{}) {
 	return b.String(), b.args
 }
 
+func (s *Selector) joinPrefix(b *Builder) {
+	if len(s.prefix) > 0 {
+		b.join(s.prefix, " ")
+		b.WriteByte(' ')
+	}
+}
+
 func (s *Selector) joinUnion(b *Builder) {
 	for _, union := range s.union {
 		b.WriteString(" UNION ")
@@ -2234,6 +2249,13 @@ func (w *WithBuilder) As(s *Selector) *WithBuilder {
 	return w
 }
 
+// C returns a formatted string for the WITH column.
+func (w *WithBuilder) C(column string) string {
+	b := &Builder{dialect: w.dialect}
+	b.Ident(w.name).WriteByte('.').Ident(column)
+	return b.String()
+}
+
 // Query returns query representation of a `WITH` clause.
 func (w *WithBuilder) Query() (string, []interface{}) {
 	w.WriteString("WITH ")