如何向表中添加检查约束？

Question

我无法与此表

CREATE TABLE `Participants` (
    `meetid` int(11) NOT NULL, 
    `pid` varchar(15) NOT NULL, 
    `status` char(1) DEFAULT NULL, 
    PRIMARY KEY (`meetid`,`pid`), 
    CONSTRAINT `participants_ibfk_1` FOREIGN KEY (`meetid`) REFERENCES `Meetings` (`meetid`) ON DELETE CASCADE 
    CONSTRAINT `participants_ibfk_2` CHECK (status IN ('a','d','u')) 
    CONSTRAINT `participants_ibfk_3` CHECK (pid IN (SELECT name FROM Rooms) OR pid IN (SELECT userid FROM People)) 
); 

我想有一个外键约束，而工作。然后我想为属性status添加一个约束，所以它只能使用值'a'，'d'和'u'。我不能将字段设置为Enum或set。

谁能告诉我为什么这段代码在MySQL中不起作用吗？

Answer 1

CHECK约束不受MySQL支持。你可以定义它们，但它们什么也不做（如MySQL 5.7）。

从manual：

的CHECK子句解析，但所有的存储引擎忽略。

解决方法是创建triggers，但它们不是最简单的工作。

如果您想要一个支持CHECK约束条件的开源​​RDBMS，请尝试PostgreSQL。它实际上是一个非常好的数据库。

Answer 2

除了触发器，对于像一个简单的限制，你必须：

CONSTRAINT `participants_ibfk_2` 
    CHECK status IN ('a','d','u') 

你可以使用一个Foreign Key从status的参考表（ParticipantStatus 3行：'a','d','u'）：

CONSTRAINT ParticipantStatus_Participant_fk 
    FOREIGN KEY (status) 
    REFERENCES ParticipantStatus(status) 

Answer 3

以下是一种快速方便地获得所需支票的方法：

drop database if exists gtest; 

create database if not exists gtest; 
use gtest; 

create table users (
    user_id  integer unsigned not null auto_increment primary key, 
    username  varchar(32) not null default '', 
    password  varchar(64) not null default '', 
    unique key ix_username (username) 
) Engine=InnoDB auto_increment 10001; 

create table owners (
    owner_id  integer unsigned not null auto_increment primary key, 
    ownername  varchar(32) not null default '', 
    unique key ix_ownername (ownername) 
) Engine=InnoDB auto_increment 5001; 

create table users_and_owners (
    id integer unsigned not null primary key, 
    name varchar(32) not null default '', 
    unique key ix_name(name) 
) Engine=InnoDB; 

create table p_status (
    a_status  char(1) not null primary key 
) Engine=InnoDB; 

create table people (
    person_id integer unsigned not null auto_increment primary key, 
    pid  integer unsigned not null, 
    name  varchar(32) not null default '', 
    status char(1) not null, 
    unique key ix_name (name), 
    foreign key people_ibfk_001 (pid) references users_and_owners(id), 
    foreign key people_ibfk_002 (status) references p_status (a_status) 
) Engine=InnoDB; 

create or replace view vw_users_and_owners as 
select 
    user_id id, 
    username name 
from users 
union 
select 
    owner_id id, 
    ownername name 
from owners 
order by id asc 
; 

create trigger newUser after insert on users for each row replace into users_and_owners select * from vw_users_and_owners; 
create trigger newOwner after insert on owners for each row replace into users_and_owners select * from vw_users_and_owners; 

insert into users (username, password) values 
('fred Smith', password('fredSmith')), 
('jack Sparrow', password('jackSparrow')), 
('Jim Beam', password('JimBeam')), 
('Ted Turner', password('TedTurner')) 
; 

insert into owners (ownername) values ('Tom Jones'),('Elvis Presley'),('Wally Lewis'),('Ted Turner'); 

insert into people (pid, name, status) values (5001, 'Tom Jones', 1),(10002,'jack Sparrow',1),(5002,'Elvis Presley',1); 

Answer 4

我不明白，为什么这里没有人提到，WITH CHECK OPTION VIEW可以在MySQL的一个很好的替代CHECK约束：

CREATE VIEW name_of_view AS SELECT * FROM your_table 
WHERE <condition> WITH [LOCAL | CASCADED] CHECK OPTION; 

有上的MySQL网站文档：The View WITH CHECK OPTION Clause

DROP TABLE `Participants`; 

CREATE TABLE `Participants` (
    `meetid` int(11) NOT NULL, 
    `pid` varchar(15) NOT NULL, 
    `status` char(1) DEFAULT NULL check (status IN ('a','d','u')), 
    PRIMARY KEY (`meetid`,`pid`) 
); 

-- should work 
INSERT INTO `Participants` VALUES (1,1,'a'); 
-- should fail but doesn't because table check is not implemented in MySQL 
INSERT INTO `Participants` VALUES (2,1,'x'); 

DROP VIEW vParticipants; 
CREATE VIEW vParticipants AS 
    SELECT * FROM Participants WHERE status IN ('a','d','u') 
    WITH CHECK OPTION; 

-- should work 
INSERT INTO vParticipants VALUES (3,1,'a'); 
-- will fail because view uses a WITH CHECK OPTION 
INSERT INTO vParticipants VALUES (4,1,'x');