Selecting data from another table using a foreign key
Joining tables is a fundamental principle of relational databases. In your case, A and B are related with the id column, which means that you can use a syntax similar to this one:

SELECT a.id, a.name, a.num, b.date, b.roll
FROM a
INNER JOIN b ON a.id=b.id;


INNER JOIN means that you'll only see rows where there are matching records in A and B. If you want all the rows in A and matching records in B, you could change INNER JOIN to LEFT JOIN. Conversely, if you want all the records from B and only the matching ones from A, use RIGHT JOIN. Finally, if you need everything from both tables, matching or not, you can use FULL JOIN.