inner join for a query?

Question

I want to do a sql query and have some problems:

1. I want to select from table_1 the ID's Where parent_id is the value I have:

   SELECT ID 
     FROM table_1 
    WHERE parent_ID = 'x'
   

2. I want to use the ID'S I got in 1. and

   SELECT 
     FROM table_2 
    WHERE ID = 'The ID's from Query 1.'
   

Answer 1

Like this?

select ...
  from table_1 a
  join table_2 b on(a.id = b.id)
 where a.parent_id = 'x';

Edit
Note: the query will potentially produce duplicate rows depending on the keys and relation between the tables. For example, you will get duplicates if, for a given table_1.parent_id = X, there can be multiple occurrences of the same table_1.ID. Another example is when table_2.ID isn't unique.

In those cases you would want to remove the duplicates (using distinct, group by, partitioned @row_number, etc) or, not produce the duplicates in the first place using a semi-join instead (exists, in). Have a look @OMG Ponies answer for reference.