re: REFERENCES

Hi!

I am just a newbie in postgresql.  What if:

CREATE TABLE purchase (id int, po int, date date, PRIMARY KEY (id));

CREATE TABLE lcopen (lc numeric, po int, PRIMARY KEY (lc));

CREATE TABLE lcreceive (id int, lc numeric REFERENCES lcopen, purid int REFERENCES purchase);

How do I refer the keywords po in table lcopen to table purchase the po keywords?  I am quite confuse, since I have
announceid in table purchase as PRIMARY KEY.  I do not know how to key another keywords. 

Best regards,

Terence
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