How to get child table's foreign key to have same value as parent's primary auto-increment key

Question

I have created these two tables:

CREATE TABLE Purchase(
purchaseID SERIAL,
custName VARCHAR(30) NOT null,
PRIMARY KEY (purchaseID));

CREATE TABLE PurchasedItem(         


        

Answer 1

You can use lastval()

INSERT INTO Purchase(custName) VALUES ('Lendl');
INSERT INTO PurchasedItem(purchaseID, itemNo) VALUES (lastval(), 111);
commit;

Alternatively query the underlying sequence directly:

INSERT INTO Purchase(custName) VALUES ('Lendl');
INSERT INTO PurchasedItem(purchaseID, itemNo) 
VALUES (currval('purchase_purchaseid_seq'), 111);
commit;

Or if you don't want to rely on the automatic naming of the sequence, use pg_get_serial_sequence to get the sequence associated with the column:

INSERT INTO Purchase(custName) VALUES ('Lendl');
INSERT INTO PurchasedItem(purchaseID, itemNo) 
VALUES (currval(pg_get_serial_sequence('purchase', 'purchaseid')), 111);
commit;

For more details see the manual: https://www.postgresql.org/docs/current/static/functions-sequence.html