问如何在PostgreSQL中使用ALTER table向同一个表添加外键约束

EN

-- create some fake data for testing
--
DROP SCHEMA tmp CASCADE;
CREATE SCHEMA tmp ;
SET search_path=tmp;
CREATE TABLE category

(
  cat_id serial NOT NULL,
  cat_name character varying NOT NULL,
  parent_id integer NOT NULL,
  CONSTRAINT cat_id PRIMARY KEY (cat_id)
);

INSERT INTO category(cat_name,parent_id)
SELECT 'Name_' || gs::text
        , gs % 3
FROM generate_series(0,9) gs
        ;

        -- find the records with the non-existing parents
SELECT ca.parent_id , COUNT(*)
FROM category ca
WHERE NOT EXISTS (
        SELECT *
        FROM category nx
        WHERE nx.cat_id = ca.parent_id
        )
GROUP BY ca.parent_id
        ;

        -- if all is well: proceed
        -- make parent pointer nullable
ALTER TABLE category
        ALTER COLUMN parent_id DROP NOT NULL
        ;

        -- set non-existing parent pointers to NULL
UPDATE category ca
SET parent_id = NULL
WHERE NOT EXISTS (
        SELECT *
        FROM category nx
        WHERE nx.cat_id = ca.parent_id
        )
        ;

        -- Finally, add the FK constraint
ALTER TABLE category
        ADD CONSTRAINT cat_cat_id_fkey FOREIGN KEY (parent_id)
              REFERENCES category (cat_id) MATCH SIMPLE
              ON UPDATE CASCADE ON DELETE CASCADE
        ;