How get selected rows where user id is not in table record

the table like this

table\_a

```
id subid userid
1  2     123
2  4     123

```

table\_b

```
id title
1  like
2  liked
3  bookmark
4  bookmarked

```

i use join table to get the result 
if WHERE userid=123 then get result

```
id title
2  liked
4  bookmarked

```

the query

```
select b.id,b.title from
table_a a
LEFT JOIN table_b b
ON b.id = a.subid
WHERE a.userid = '123'

```

but, how if userid = '345' and not in table\_a get the result too?.. the result is must like this (selected rows)

```
id title
1  like
3  bookmark

```

Thankyou