如何將列更新爲外鍵？

Question

這裏是我創建的表：

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

現在我想更新聲明爲從一個叫做類表的外鍵的CCODE列。我怎麼做？

Answer 1

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

但是這個工作要記住，你必須有一個唯一索引上class.ccode列：

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