SQL: how to limit a join on the first found row?

The key word here is FIRST. You can use analytic function FIRST_VALUE or aggregate construct FIRST.
For FIRST or LAST the performance is never worse and frequently better than the equivalent FIRST_VALUE or LAST_VALUE construct because we don't have a superfluous window sort and as a consequence a lower execution cost:

select table_A.id, table_A.name, firstFromB.city from table_A join (    select table_B.id2, max(table_B.city) keep (dense_rank first order by table_B.city) city    from table_b    group by table_B.id2    ) firstFromB on firstFromB.id2 = table_A.id where 1=1 /* some conditions here */;

Since 12c introduced operator LATERAL, as well as CROSS/OUTER APPLY joins, make it possible to use a correlated subquery on right side of JOIN clause:

select table_A.id, table_A.name, firstFromB.city from table_A cross apply (    select max(table_B.city) keep (dense_rank first order by table_B.city) city    from table_b    where table_B.id2 = table_A.id     ) firstFromBwhere 1=1 /* some conditions here */;

If you want just single value a scalar subquery can be used:

SELECT    id, name, (SELECT city FROM table_B WHERE id2 = table_A.id AND ROWNUM = 1) cityFROM    table_A

select table_A.id, table_A.name,FIRST_VALUE(table_B.city) IGNORE NULLS          OVER (PARTITION BY table_B.id2 ORDER BY table_B.city) AS "city"from table_A join table_B on table_A.id = table_B.id2where ..