Add support for USING clauses in DELETE statements

This commit adds the ability to join in DELETE statements with the USING
clause.

Sample code:

    _, err = z.
        DeleteFrom("one").
        Using("two", "three").
        Where(
            sqlz.Eq("two.fk_id", sqlz.Indirect("one.id")),
            sqlz.Eq("three.fk_id", sqlz.Indirect("one.id")),
        ).
        Exec()

delete.go

@@ -9,10 +9,11 @@ import (
 
 // DeleteStmt represents a DELETE statement
 type DeleteStmt struct {
-	Table      string
-	Conditions []WhereCondition
-	Return     []string
-	execer     sqlx.Ext
+	Table       string
+	Conditions  []WhereCondition
+	UsingTables []string
+	Return      []string
+	execer      sqlx.Ext
 }
 
 // DeleteFrom creates a new DeleteStmt object for the
@@ -33,6 +34,12 @@ func (tx *Tx) DeleteFrom(table string) *DeleteStmt {
 	}
 }
 
+// Using adds a USING clause for joining in a delete statement
+func (stmt *DeleteStmt) Using(tables ...string) *DeleteStmt {
+	stmt.UsingTables = append(stmt.UsingTables, tables...)
+	return stmt
+}
+
 // Where creates one or more WHERE conditions for the DELETE statement.
 // If multiple conditions are passed, they are considered AND conditions.
 func (stmt *DeleteStmt) Where(conds ...WhereCondition) *DeleteStmt {
@@ -55,6 +62,10 @@ func (stmt *DeleteStmt) Returning(cols ...string) *DeleteStmt {
 func (stmt *DeleteStmt) ToSQL(rebind bool) (asSQL string, bindings []interface{}) {
 	var clauses = []string{"DELETE FROM " + stmt.Table}
 
+	if len(stmt.UsingTables) > 0 {
+		clauses = append(clauses, "USING "+strings.Join(stmt.UsingTables, ", "))
+	}
+
 	if len(stmt.Conditions) > 0 {
 		whereClause, whereBindings := parseConditions(stmt.Conditions)
 		bindings = append(bindings, whereBindings...)

delete_test.go

@@ -1,6 +1,8 @@
 package sqlz
 
-import "testing"
+import (
+	"testing"
+)
 
 func TestDelete(t *testing.T) {
 	runTests(t, func(dbz *DB) []test {
@@ -25,6 +27,13 @@ func TestDelete(t *testing.T) {
 				"DELETE FROM table WHERE id = ? RETURNING name",
 				[]interface{}{2},
 			},
+
+			test{
+				"delete using join",
+				dbz.DeleteFrom("table").Using("other", "another").Where(Eq("other.fk_id", Indirect("table.id")), Eq("another.fk_id", Indirect("table.id"))),
+				"DELETE FROM table USING other, another WHERE other.fk_id = table.id AND another.fk_id = table.id",
+				[]interface{}{},
+			},
 		}
 	})
 }