You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: db_integration_test.go
    path of file: ./repos/tidb/pkg/ddl
    
    the code of the file until where you have to start completion: // Copyright 2018 PingCAP, Inc.
//
// License
    
func TestNullGeneratedColumn(t *testing.T) {
	store := testkit.CreateMockStore(t, mockstore.WithDDLChecker())

	tk := testkit.NewTestKit(t, store)

	tk.MustExec("use test")
	tk.MustExec("drop table if exists t")
	tk.MustExec("CREATE TABLE `t` (" +
		"`a` int(11) DEFAULT NULL," +
		"`b` int(11) DEFAULT NULL," +
		"`c` int(11) GENERATED ALWAYS AS (`a` + `b`) VIRTUAL," +
		"`h` varchar(10) DEFAULT NULL," +
		"`m` int(11) DEFAULT NULL" +
		") ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin")

	tk.MustExec("insert into t values()")
	tk.MustExec("alter table t add index idx_c(c)")
	tk.MustExec("drop table t")
}