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

Answer 2

DEFAULT will work for SERIAL as it sets default value for column. So

INSERT INTO Purchase VALUES (DEFAULT,'Lendl');

should work. But PurchasedItem.purchaseID has no default value set, so it it tries to insert NULL (and null is not in referenced column yet), so it fails.

try:

INSERT INTO Purchase(custName) VALUES ('Lendl') RETURNING purchaseID;

you will see the value of inserted purchaseID, use it in next query:

INSERT INTO PurchasedItem(purchaseID, itemNo) VALUES (_the_value_above_, 111);
commit;

If you want it to be used without interactivity, use DO block with returning purchaseID into _value

update:

or cte, smth like

WITH i AS (
  INSERT INTO Purchase(custName, orderedDate) 
  VALUES ('Lendl', '2016-09-28') 
  RETURNING purchaseID
)
insert into PurchasedItem
select i.purchaseID,'smth',3
from i