A join here seems unnecessary - mysql

Question

TableA
------
id
Name
other_fields
TableB
------
A_id (foreign key to TableA.id)
other_fields
Select entries from TableB which reference entries in TableA with some specific property (e.g. Name = "Alice")
This can be easily done with a join:
SELECT TableB.*
FROM TableA INNER JOIN TableB on TableA.id = TableB.A_id
WHERE TableA.Name = "Alice"
Being used to procedural programming, the join seems overkill and unnecessary as we don't actually need any information from TableA other than the id of Alice.
So -- assuming that Alice is unique -- is there a way to do this (pseudocode):
variable alice_id = get id of Alice from TableA
SELECT *
FROM TableB
WHERE A_id = alice_id
If yes, should it be used in favor of the classical JOIN method? Is it faster? (in principle, of course)

Answer 1

You're asking if you can do this:
SELECT * FROM TableB WHERE A_id = (SELECT id FROM TableA WHERE Name = 'Alice');
It's a perfectly legitimate query, but MySQL will perform much better doing the join because the subquery is treated as a second separate query. Using the MySQL EXPLAIN command (just put it in front of your SELECT query) will show the indexes, temporary tables, and other resources that are used for a query. It should give you an idea when one query is faster or more efficient than another.

Answer 2

For your workload and indexes, you should try both queries' execution plan and runtime. In either case you would benefit from having an index on name.
I believe that both queries are going to end up with similar plans. Let's check that out.
Create the tables
create table tablea (id int primary key, nm as varchar(50));
create index idx_tablea_nm on tablea(nm);
create table tableb(a_id int, anotherfield varchar(100),
key idx_tableb_id(a_id),
constraint fk_tableb_tablea_id foreign key (a_id) references tablea (id));
Let's do an EXPLAIN on the first one:
explain select tableb.* from tablea inner join tableb on tablea.id = tableb.a_id where tablea.nm = 'Alice';
+----+-------------+--------+------+-----------------------+---------------+---------+-------------------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+-----------------------+---------------+---------+-------------------+------+--------------------------+
| 1 | SIMPLE | tablea | ref | PRIMARY,idx_tablea_nm | idx_tablea_nm | 53 | const | 1 | Using where; Using index |
| 1 | SIMPLE | tableb | ref | idx_tableb_id | idx_tableb_id | 5 | tablea.id | 1 | Using where |
+----+-------------+--------+------+-----------------------+---------------+---------+-------------------+------+--------------------------+
Let's do EXPLAIN on the second one:
explain select * from tableb where a_id = (select id from tablea where nm = 'Alice');
+----+-------------+--------+------+---------------+---------------+---------+-------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+---------------+---------+-------+------+--------------------------+
| 1 | PRIMARY | tableb | ref | idx_tableb_id | idx_tableb_id | 5 | const | 1 | Using where |
| 2 | SUBQUERY | tablea | ref | idx_tablea_nm | idx_tablea_nm | 53 | | 1 | Using where; Using index |
+----+-------------+--------+------+---------------+---------------+---------+-------+------+--------------------------+
I don't have much data in those tables and with little data you will notice identical performance. As the workload changes, the execution play may change.