Question

How can I do something like this in a Single MySQL Query?

select `value_a` 
from `table_1` 
where `value_b` = (select `value_b` from `table_2` where `value_c` = `x`);

I use CodeIgniter so I can use Active Records.

Answer 1

You can also use a JOIN for this:

select t1.value_a
from table_1 t1
inner join table_2 t2
  on t1.value_b = t2.value_b
where t2.value_c = 'x'

You can also use your existing query, but the x is surrounded by backticks and not single quotes:

select `value_a` 
from `table_1` 
where `value_b` = (select `value_b` from `table_2` where `value_c` = 'x);