如何将列更新为外键？

Question

这是我创建的表格：

CREATE TABLE enrolled
(
  sid integer NOT NULL,
  ccode character varying(6) NOT NULL,
  CONSTRAINT enrolled_pkey1 PRIMARY KEY (sid, ccode)
);

现在我想更新 ccode 列，从名为Class的表中声明为外键。我该怎么做？

Answer 1

ALTER TABLE ONLY enrolled
    ADD CONSTRAINT enrolled_ccode_fkey FOREIGN KEY (ccode)
        REFERENCES class(ccode)
    ON UPDATE CASCADE ON DELETE RESTRICT;

但是要记住这一点，请务必在class.ccode列上添加唯一索引：

CREATE UNIQUE INDEX class_ccode_idx ON class USING btree (ccode);