mySQL Subquery - Single pass if possible - mysql

Question

I am looking to perform a query that will return data from a number of tables in mySQL that will require some subqueries however I am not sure if what I am trying to achieve is even possible in a single pass.
I have the below as an example (i know this doesn't work but gives you the idea)
SELECT field_id, field_1,
(SELECT subfield_1 FROM tableb WHERE subfield_id = field_3) AS field_a,
(SELECT subfield_1 FROM tableb WHERE subfield_id = field_4) as field_b
FROM tablea WHERE field_2 = 123
This will pull 2 entries from a second table based on 2 different columns in the primary table.
tablea
----------
field_id, bigint
field_1, varchar(50)
field_2, int
field_3, bigint
field_4, bigint
tableb
----------
subfield_id, bigint
subfield_1, varchar(50)

Answer 1

Use JOINs. Depending on whether field_3 and field_4 are NULLable or not use either LEFT JOIN (as in example below) or INNER JOIN
SELECT field_id, field_1, b1.subfield_1 field_a, b2.subfield_1 field_b
FROM tablea a LEFT JOIN tableb b1
ON a.field_3 = b1.subfield_id LEFT JOIN tableb b2
ON a.field_4 = b2.subfield_id
WHERE a.field_2 = 123
Sample output:
| FIELD_ID | FIELD_1 | FIELD_A | FIELD_B |
|----------|---------|-----------|-----------|
| 1 | 11 | subvalue1 | subvalue5 |
| 2 | 22 | subvalue4 | subvalue2 |
| 3 | 33 | subvalue2 | (null) |
Note: Make sure that you have indices on field_3 and field_4
Here is SQLFiddle demo

Answer 2

What you are trying to achieve is possible only if your sub query returns one row .
for example here
(SELECT subfield_1 FROM tableb WHERE subfield_id = field_3)
field 3 should be a primary key for tableb

Answer 3

You could perform an outer join, where you will see the same result without having the delay of having to use a correlated subquery.
The outer join has the advantage that all rows from tablea will be displayed, regardless of whether there is an accompanying value within the tableb subqueries.
A normal join would instead only return rows where all three tables contained field_3 and field_4, and as a result the row for field_id = 3 would not be returned.
Example:
SELECT field_id,
field_1,
s1.subfield_1 as field_a,
s2.subfield_1 as field_b
FROM tablea AS a
LEFT JOIN tableb AS s1 ON a.field_3 = s1.subfield_id
LEFT JOIN tableb AS s2 ON a.field_4 = s2.subfield_id
WHERE field_2 = 123;