How to add data to table and update user if condition is met?

Question

PostgreSQL

INSERT INTO inventory (itemid, created_at, status, userid) 
SELECT UNNEST ($1::bigint[]), $2, 0, $3
WHERE EXISTS (  
        SELECT balance FROM users 
        WHERE id = $3 AND balance >= $4::numeric
)

Here the query works, so if the user has enough money on the balance, we add items to the inventory
. And after adding the balance, we also need to reduce the balance by the amount of the purchase

UPDATE users 
SET balance -= $4::numeric

How? Where to add this request?
I just started learning sql.
And if it's not difficult, you can comment on the request if something is wrong in it

Answer 1

Where to add this request?

into a transaction.

BEGIN;
INSERT ...;
UPDATE ...;
COMMIT;