Question

I want to select from 2 table where 2nd id is equal with first 4 character from first id

SELECT a.*, b.*, substring(a.my_id from 1 for 4)::integer as number
   FROM table1 as a
INNER Join table2 as b ON(b.id_2=number) where my_id = 101
                                 ^
It produces an error here        | 

ERROR: column “number” does not exist

SQL state: 42703

Character: 189

Answer 1

You can’t use an alias like that, you need a derived table:

select *
from (
   SELECT t1.*, 
          substring(t1.my_id::text from 1 for 4)::integer as number
   FROM table1 t1
) as a 
inner join table2 as b ON (b.id_2 = a.number) 
where my_id = 101

Storing a number that is used as a foreign key as a part in a varchar column is a really, really ugly design. That number should be a column of its own in table1.