UpdateStmt supports IncrBy and DecrBy (#198)

update.go

@@ -56,6 +56,7 @@ func (b *UpdateStmt) Build(d Dialect, buf Buffer) error {
 		buf.WriteString(placeholder)
 
 		buf.WriteValue(v)
+
 		i++
 	}
 
@@ -174,6 +175,18 @@ func (b *UpdateStmt) SetMap(m map[string]interface{}) *UpdateStmt {
 	return b
 }
 
+// IncrBy increases column by value
+func (b *UpdateStmt) IncrBy(column string, value interface{}) *UpdateStmt {
+	b.Value[column] = Expr("? + ?", I(column), value)
+	return b
+}
+
+// DecrBy decreases column by value
+func (b *UpdateStmt) DecrBy(column string, value interface{}) *UpdateStmt {
+	b.Value[column] = Expr("? - ?", I(column), value)
+	return b
+}
+
 func (b *UpdateStmt) Limit(n uint64) *UpdateStmt {
 	b.LimitCount = int64(n)
 	return b

update_test.go

@@ -48,3 +48,15 @@ func TestPostgresUpdateReturning(t *testing.T) {
 	require.Equal(t, 1, sess.EventReceiver.(*testTraceReceiver).finished)
 	require.Equal(t, 0, sess.EventReceiver.(*testTraceReceiver).errored)
 }
+
+func TestUpdateIncrBy(t *testing.T) {
+	buf := NewBuffer()
+	builder := Update("table").IncrBy("a", 1).Where(Eq("b", 2))
+	err := builder.Build(dialect.MySQL, buf)
+	require.NoError(t, err)
+
+	sqlstr, err := InterpolateForDialect(buf.String(), buf.Value(), dialect.MySQL)
+	require.NoError(t, err)
+
+	require.Equal(t, "UPDATE `table` SET `a` = `a` + 1 WHERE (`b` = 2)", sqlstr)
+}