Question

我有两张桌子

CREATE TABLE public.city_url
(
  id bigint NOT NULL DEFAULT nextval('city_url_id_seq'::regclass),
  url text,
  city text,
  state text,
  country text,
  common_name text,
  CONSTRAINT city_url_pkey PRIMARY KEY (id)
)

和

CREATE TABLE public.email_account
(
  id bigint NOT NULL DEFAULT nextval('email_accounts_id_seq'::regclass),
  email text,
  password text,
  total_replied integer DEFAULT 0,
  last_accessed timestamp with time zone,
  enabled boolean NOT NULL DEFAULT true,
  deleted boolean NOT NULL DEFAULT false,
  city_url_id bigint,
  CONSTRAINT email_accounts_pkey PRIMARY KEY (id),
  CONSTRAINT email_account_city_url_id_fkey FOREIGN KEY (city_url_id)
      REFERENCES public.city_url (id) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE NO ACTION
)

我想提出一个查询，只有当email_account中没有指向city_url列的行时才会在city_url_id中提取行。

Answer 1

脑海中浮现出

NOT EXISTS：

select c.*
from city_url c
where not exists (select 1
                  from email_account ea
                  where ea.city_url_id = c.id
                 );

Answer 2

还有这个选项：

SELECT city_url.*
FROM city_url
LEFT JOIN email_account ON email_account.city_url_id = city_url.id
WHERE email_account.id IS NULL

Answer 3

NOT EXISTS绝对是＆＃34;的答案......如果没有行......＆＃34;。
尽管如此，最好通过选择差异量来实现这一点。

原则上是：

SELECT a.*  
FROM table1 a
LEFT JOIN table2 b
ON a.[columnX] = b.[columnY]
WHERE b.[columnY] IS NULL

在这里使用表名，这将是：

SELECT c.*
FROM city_url c
LEFT JOIN email_account e
ON c.id = e.city_url
WHERE e.city_url IS NULL

获取没有外键指向的行

3 个答案: