SUBSTRING on an Inner Join in PostgreSQL - postgresql

Question

One of my tables looks like the following-
One of the columns ordertype in the table has data that always has 4 characters in the beginning and then the rest of the characters are separated by ; The length of the characters vary after the ;
I am trying to use SUBSTRING to pull all the characters after the ; (semi-colon) regardless of their length. Also trying to do that for an INNER JOIN i am trying to execute. The substring in the INNER JOIN aims to pull the first 4 characters.
For some reason my query errors out. Not sure if I am doing something wrong.
Below is the sample query that I am trying to run-
SELECT
ind_id,
substring(ordertype FROM 6)
FROM
order_table
INNER JOIN
transaction_table
ON
transaction_table.fileid = order_table.substring(ordertype FROM 1 FOR 4;

Answer 1

Functions in SQL are generally not called as predicates of the objects they act on. Instead, we pass database objects to functions. Try this version:
SELECT
ind_id,
SUBSTRING(o.ordertype FROM 6)
FROM order_table o
INNER JOIN
transaction_table t
ON t.fileid = SUBSTRING(o.ordertype FROM 1 FOR 4); -- chamge is here
Note that a possibly more robust solution here would be to use the SPLIT_PART function:
SELECT
ind_id,
SPLIT_PART(o.ordertype, ';', 2)
FROM order_table o
INNER JOIN
transaction_table t
ON t.fileid = SPlIT_PART(o.ordertype, ';', 1);