sql - Substring in postgres -

i have 2 tables in postgres trying compare , having trouble accessing substring 1 , comparing one. i'm new postgres (specifically sql through pgadmin), suggestions helpful.

i trying count number of id numbers similar between 2 tables, 1 set of numbers longer other due concatenated fips code. don't want trim data if don't have to, right have:

select count(id_a) server.table1  where(id_a not null  , id_a in (select (substring(id_b 3 8) server.table 2))) 

i answer because little catch on query. if typo delete after update original question.

select count(id_a)    server.table1  (     id_a not null          , id_a in (select (substring(id_b 3 8)                          server.table2)                      )                    ^        )                                  |_ parenthesis in wrong place 

note either missing parenthesis or excessive one.

the right query be:

select count(id_a)    server.table1  id_a not null    , id_a in (select substring(id_b 3 8)                    server.table2)                     

as @ziggycrueltyfreezeitgeister pointed out, don't need id_a not null if don't have null values on it.