postgresql – 在数据库中强制执行“至少一个”或“恰好一个”的

在users表中包含活动电子邮件的ID,从而产生相互引用.有人可能会认为我们需要一个DEFERRABLE约束来解决插入用户及其活动电子邮件的鸡与蛋问题,但使用数据修改CTE我们甚至不需要它.

这会始终为每个用户强制执行一封有效电子邮件：

CREATE TABLE users (
  user_id  serial PRIMARY KEY,username text NOT NULL,email_id int NOT NULL  -- FK to active email,constraint added below
);

CREATE TABLE email (
  email_id serial PRIMARY KEY,user_id  int NOT NULL REFERENCES users ON DELETE CASCADE ON UPDATE CASCADE,email    text NOT NULL,CONSTRAINT email_fk_uni UNIQUE(user_id,email_id)  -- for FK constraint below
);

ALTER TABLE users ADD CONSTRAINT active_email_fkey
FOREIGN KEY (user_id,email_id) REFERENCES email(user_id,email_id);

从users.email_id中删除NOT NULL约束,使其“最多只能有一个活动的电子邮件”. (您仍然可以为每个用户存储多封电子邮件,但这些电子邮件都不是“活动”.)

您可以使active_email_fkey DEFERRABLE允许更多余地(在同一事务的单独命令中插入用户和电子邮件),但这不是必需的.

我将user_id放在UNIQUE约束email_fk_uni中以优化索引覆盖率.细节：

> Is a composite index also good for queries on the first field?

可选视图：

CREATE VIEW user_with_active_email AS
SELECT * FROM users JOIN email USING (user_id,email_id);

以下是使用活动电子邮件插入新用户的方式(根据需要)：

WITH new_data(username,email) AS (
   VALUES
      ('usr1','abc@d.com')   -- new users with *1* active email,('usr2','def3@d.com'),('usr3','ghi1@d.com')
   ),u AS (
   INSERT INTO users(username,email_id)
   SELECT n.username,nextval('email_email_id_seq'::regclass)
   FROM   new_data n
   RETURNING *
   )
INSERT INTO email(email_id,user_id,email)
SELECT u.email_id,u.user_id,n.email
FROM   u
JOIN   new_data n USING (username);

具体的困难是我们既没有user_id也没有email_id.两者都是从各自的SEQUENCE提供的序列号.单个RETURNING子句(另一个鸡和蛋问题)无法解决它.解决方案是nextval(),如explained in detail in the linked answer below.

如果您不知道串行列email.email_id的附加序列的名称,则可以替换：

nextval('email_email_id_seq'::regclass)

同

nextval(pg_get_serial_sequence('email','email_id'))

以下是添加新“有效”电子邮件的方式：

WITH e AS (
   INSERT INTO email (user_id,email)
   VALUES  (3,'new_active@d.com')
   RETURNING *
   )
UPDATE users u
SET    email_id = e.email_id
FROM   e
WHERE  u.user_id = e.user_id;

SQL Fiddle.

如果一些头脑简单的ORM不够聪明,无法应对,那么您可以将SQL命令封装在服务器端函数中.

密切相关,有充分的解释：

> How to deal with mutually recursive inserts
> Complex foreign key constraint in SQLAlchemy

还有关系：

> Enforcing constraints “two tables away”

关于DEFERRABLE约束：

> Constraint defined DEFERRABLE INITIALLY IMMEDIATE is still DEFERRED?

关于nextval()和pg_get_serial_sequence()：

> PostgreSQL SELECT primary key as “serial” or “bigserial”
> Duplicate row with Primary Key in PostgreSQL