How to make one request?

Question

MySQL

There are 3 tables:
1 - block (main_block_id, data, type)
2 - block_one (id, main_block_id, another_data)
3 - block_two (id, main_block_id, third_data)
1 and 2, and 1 and 3 are related
Each block has its own subblock in block_one or in block_two
This block is defined in block.type
I.e. if block.type == 'one', then JOIN with block_one will be taken
Now it's 2 queries
First determine what type, and then
look for Is it possible to do it all in one query?

Answer 1

Can. The case statement is here to help.

Answer 2

Or

select a.data, b.another_data
from block as a join block_one as b on (a.main_block_id=b.main_block_id)
where a.type = 'one'
union all
select a.data, b.third_data
from block as a join block_two as b on (a.main_block_id=b.main_block_id)
where a.type = 'two'

Or

select distinct a.data, 
(case when a.type='one' then b.another_data else c.third_data end) as some_data
from block as a left join block_one as b on (a.type = 'one' and a.main_block_id=b.main_block_id)
left join block_two as c on (a.type = 'two' and a.main_block_id=c.main_block_id)