I created a foreign key in my sql by the following statemnt..
ALTER TABLE `users` ADD FOREIGN KEY ( `id`)
REFERENCES `user_login` (`user_id`)
ON DELETE CASCADE ;
The creation appears to succeed then after that I execute a delete statement
DELETE From user_login WHERE user_id = 1576;
yet in users the row still exists that is referencing that. I open up the mysql workbench and it doesn't show any signs that the foreign key was created. Does anyone know why this would be? Or what I am doing wrong? It is a one-to-one relationship in the two tables.
The table may be in MyISAM format, which does not support foreign keys.
Try converting it to InnoDB first:
alter table users engine=InnoDB;
You have to also make sure that both users.id and user_login.user_id have an index each.
Copy and paste this code in your Mysql script editor and run. You will have two tables categories and products these tables having cat_id as foreign key.
CREATE DATABASE IF NOT EXISTS dbdemo;
USE dbdemo;
CREATE TABLE categories(
cat_id int not null auto_increment primary key,
cat_name varchar(255) not null,
cat_description text
) ENGINE=InnoDB;
CREATE TABLE products(
prd_id int not null auto_increment primary key,
prd_name varchar(355) not null,
prd_price decimal,
cat_id int not null,
FOREIGN KEY fk_cat(cat_id)
REFERENCES categories(cat_id)
ON UPDATE CASCADE
ON DELETE RESTRICT
)ENGINE=InnoDB;
Related
I need to change order of rows in database table.
My table has 4 columns and 7 rows. I need to reorder these rows
pk_i_id int(10) unsigned Auto Increment
s_name varchar(255) NULL
s_heading varchar(255) NULL
s_order_type varchar(10) NULL
In Adminer, when I've changed pk_i_id value(number) something else, I'm getting this error...
Cannot delete or update a parent row: a foreign key constraint fails (`database_name`.`oc_t_item_custom_attr_categories`, CONSTRAINT `oc_t_item_custom_attr_categories_ibfk_1` FOREIGN KEY (`fk_i_group_id`) REFERENCES `oc_t_item_custom_attr_groups` (`pk_i_id`))
Do you know how to change it ? Thank you
Edit
oc_t_item_custom_attr_categories
fk_i_group_id int(10) unsigned
fk_i_category_id int(10) unsigned
indexes
PRIMARY fk_i_group_id, fk_i_category_id
INDEX fk_i_category_id
foregin keys
fk_i_group_id oc_t_item_custom_attr_groups_2(pk_i_id) RESTRICT RESTRICT
fk_i_category_id oc_t_category(pk_i_id) RESTRICT RESTRICT
You need to change your foreign key on table database_name.oc_t_item_custom_attr_categories so that it updates along with column it references.
ALTER TABLE database_name.oc_t_item_custom_attr_categories DROP CONSTRAINT oc_t_item_custom_attr_categories_ibfk_1;
ALTER TABLE database_name.oc_t_item_custom_attr_categories
ADD CONSTRAINT oc_t_item_custom_attr_categories_ibfk_1 FOREIGN KEY (fk_i_group_id)
REFERENCES oc_t_item_custom_attr_groups (pk_i_id)
ON UPDATE CASCADE;
Since MariaDB seem to not support ADDING foreign keys after table creation, this is how it should work for you, assuming description of tables is correct:
RENAME TABLE oc_t_item_custom_attr_categories TO oc_t_item_custom_attr_categories_2;
CREATE TABLE oc_t_item_custom_attr_categories (
fk_i_group_id int(10) unsigned,
fk_i_category_id int(10) unsigned,
PRIMARY KEY(fk_i_group_id, fk_i_category_id),
INDEX (fk_i_category_id),
CONSTRAINT `oc_t_item_custom_attr_categories_ibfk_1` FOREIGN KEY (fk_i_group_id)
REFERENCES oc_t_item_custom_attr_groups (pk_i_id)
ON UPDATE CASCADE,
CONSTRAINT `oc_t_item_custom_attr_categories_ibfk_2` FOREIGN KEY (fk_i_category_id)
REFERENCES oc_t_category (pk_i_id)
) ENGINE = XtraDB; --change engine to what you are using
INSERT INTO oc_t_item_custom_attr_categories SELECT * FROM oc_t_item_custom_attr_categories_2;
How it works on example data in MySQL database: http://rextester.com/ZAKR50399
I have the following tables (simplified):
create table dbo.Users
(
User_Id int identity not null
constraint PK_Users_Id primary key clustered (Id),
);
create table dbo.UsersSeals
(
UserId int not null,
SealId int not null,
constraint PK_UsersSeals_UserId_SealId primary key clustered (UserId, SealId)
);
create table dbo.Seals
(
Seal_Id int identity not null
constraint PK_Seals_Id primary key clustered (Id),
);
alter table dbo.UsersSeals
add constraint FK_UsersSeals_UserId foreign key (UserId) references Users(Id) on delete cascade on update cascade,
constraint FK_UsersSeals_SealId foreign key (SealId) references Seals(Id) on delete cascade on update cascade;
So I have a MANY to MANY relation between Users and Seals. One user can have many seals and on seal can have many users. I need a ONE to MANY where one user can have many seals but a seal has only one user.
Yes, I could remove the UsersSeals table and add a UserId into Seals table. However, I am using seals, the same way, with other tables.
I would like to have only one Seals tables with One to Many relation with Users tables and other tables.
Can this be done?
Add a separate unique constraint on the UsersSeals table on your SealID column
You then guarantee that this table is unique on SealID, which means that one seal can be associated with only one user but a user can have many seals.
I'm not sure why these have to be unique, but from reading the MySQL forums it appears that they do. However, I think it has something more to do with the INDEX name. I have two tables that have foreign key constraints referencing the same primary key on a third table. If it helps, I'm using MySQL workbench to design the schema.
I usually name my foreign key on each table the same name as the primary key it references. I guess this isn't possible. It will create the first table with the foreign key constraint, but when it tries to create the second table it throws an error. Here is the second table it throws the error on:
CREATE TABLE IF NOT EXISTS `joe`.`products_to_categories` (
`product_to_category_id` INT NOT NULL AUTO_INCREMENT ,
`category_id` INT NOT NULL ,
`product_id` INT NOT NULL ,
PRIMARY KEY (`product_to_category_id`) ,
INDEX `category_id` (`category_id` ASC) ,
INDEX `product_id` (`product_id` ASC) ,
CONSTRAINT `category_id`
FOREIGN KEY (`category_id` )
REFERENCES `joe`.`categories` (`category_id` )
ON DELETE CASCADE
ON UPDATE NO ACTION,
CONSTRAINT `product_id`
FOREIGN KEY (`product_id` )
REFERENCES `joe`.`products` (`product_id` )
ON DELETE CASCADE
ON UPDATE NO ACTION)
ENGINE = InnoDB;
I want the foreign key names to be the same as the primary key in both of the other tables. What should I remove here so that I can use these names. What is the best practice here.
It is not possible because you would have a conflict in the filename for the file that is used for the index IIRC. I probably would name the key < tablename >_< column_name > or something like that.
You are creating an index (constraint) by the name of product_id via:
INDEX product_id
Then you are going and creating another constraint (for the foreign key) with the same name:
CONSTRAINT product_id
What you need to do is allow the server to provide a default, unique constraint name by removing the
CONSTRAINT product_id
See this URL: http://dev.mysql.com/doc/refman/5.1/en/innodb-foreign-key-constraints.html
"If the CONSTRAINT symbol clause is given, the symbol value must be unique in the database. If the clause is not given, InnoDB creates the name automatically."
In PostgreSQL, the default for naming indexes is to append "_pkey" and "_fkey" to the name of the primary and foreign key, respectively. So your case would look like:
INDEX `product_id_fkey` (`product_id` ASC) ,
UPDATE: I just tried this and it worked. See if that's what you had in mind.
use test;
create table if not exists test.product
(
product_id int not null auto_increment,
name varchar(80) not null,
primary key(product_id)
);
create table if not exists test.category
(
category_id int not null auto_increment,
name varchar(80) not null,
primary key(category_id)
);
create table if not exists test.product_category
(
product_id int,
category_id int,
primary key(product_id, category_id),
constraint product_id_fkey
foreign key(product_id) references product(product_id)
on delete cascade
on update no action,
constraint category_id_fkey
foreign key(category_id) references category(category_id)
on delete cascade
on update no action
);
insert into test.product(name) values('teddy bear');
insert into test.category(name) values('toy');
insert into test.product_category
select p.product_id, c.category_id from product as p, category as c
where p.name = 'teddy bear' and c.name = 'toy';
I have a table containing 2 entries.
Something like
CREATE TABLE `db`.`main` (
`id` int(10) unsigned NOT NULL,
PRIMARY KEY (`id`)
);
The id for these 2 entries are automatically generated primary keys.
I have another table with a rule linking
CREATE TABLE `db`.`day` (
`main_id` int(10) unsigned NOT NULL,
`day` tinyint(4) NOT NULL,
CONSTRAINT `fk_db_main` FOREIGN KEY (`main_id`) REFERENCES `main` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION
);
now I can successfully get a result using
SELECT * FROM main where id='9';
but when I try to run
INSERT INTO day (main_id, day) VALUES (9, 0);
I get
"Cannot add or update a child row: a foreign key constraint fails (db.day, CONSTRAINT fk_db_main FOREIGN KEY (main_id) REFERENCES main (id) ON DELETE NO ACTION ON UPDATE NO ACTION) (1452)"
Any suggestions on what I am missing with the insert?
**I hadn't listed the actual cause of the issue while asking the question. The actual cause was that the main db table was in MyISAM, and the InnoDB tables couldn't create a foreign key connecting to it. In short, MyISAM doesn't support foreign keys, even when they are coming from other tables.
The insert works for me if I remove the db. parts in the CREATE TABLE statements (and insert into main a row with an id of 9). Maybe the problem is that you're using that db. prefix inconsistently, i.e. after TABLE but not in the CONSTRAINT clause...?
The FOREIGN KEY constraint says "there shall be an entry in the 'main` table with an ID value that matches the newly inserted 'main_id' value in the 'day' table".
When you INSERT the value 9 into 'day', is there already a row in 'main' with ID = 9?
The DBMS doesn't think so - that's why it complained.
I hadn't listed the actual cause of the issue while asking the question. The actual cause was that the main db table was in MyISAM, and the InnoDB tables couldn't create a foreign key connecting to it. In short, MyISAM doesn't support foreign keys, even when they are coming from other tables.
I would like to know how you can prevent to use of two same tags in a database table.
One said me that use two private keys in a table. However, W3Schools -website says that it is impossible.
My relational table
alt text http://files.getdropbox.com/u/175564/db/db7.png
My logical table
alt text http://files.getdropbox.com/u/175564/db/db77.png
The context of tables
alt text http://files.getdropbox.com/u/175564/db/db777.png
How can you prevent the use of duplicate tags in a question?
I have updated my NORMA model to more closely match your diagram. I can see where you've made a few mistakes, but some of them may have been due to my earlier model.
I have updated this model to prevent duplicate tags. It didn't really matter before. But since you want it, here it is (for Postgres):
START TRANSACTION ISOLATION LEVEL SERIALIZABLE, READ WRITE;
CREATE SCHEMA so;
SET search_path TO SO,"$user",public;
CREATE DOMAIN so.HashedPassword AS
BIGINT CONSTRAINT HashedPassword_Unsigned_Chk CHECK (VALUE >= 0);
CREATE TABLE so."User"
(
USER_ID SERIAL NOT NULL,
USER_NAME CHARACTER VARYING(50) NOT NULL,
EMAIL_ADDRESS CHARACTER VARYING(256) NOT NULL,
HASHED_PASSWORD so.HashedPassword NOT NULL,
OPEN_ID CHARACTER VARYING(512),
A_MODERATOR BOOLEAN,
LOGGED_IN BOOLEAN,
HAS_BEEN_SENT_A_MODERATOR_MESSAGE BOOLEAN,
CONSTRAINT User_PK PRIMARY KEY(USER_ID)
);
CREATE TABLE so.Question
(
QUESTION_ID SERIAL NOT NULL,
TITLE CHARACTER VARYING(256) NOT NULL,
WAS_SENT_AT_TIME TIMESTAMP NOT NULL,
BODY CHARACTER VARYING NOT NULL,
USER_ID INTEGER NOT NULL,
FLAGGED_FOR_MODERATOR_REMOVAL BOOLEAN,
WAS_LAST_CHECKED_BY_MODERATOR_AT_TIME TIMESTAMP,
CONSTRAINT Question_PK PRIMARY KEY(QUESTION_ID)
);
CREATE TABLE so.Tag
(
TAG_ID SERIAL NOT NULL,
TAG_NAME CHARACTER VARYING(20) NOT NULL,
CONSTRAINT Tag_PK PRIMARY KEY(TAG_ID),
CONSTRAINT Tag_UC UNIQUE(TAG_NAME)
);
CREATE TABLE so.QuestionTaggedTag
(
QUESTION_ID INTEGER NOT NULL,
TAG_ID INTEGER NOT NULL,
CONSTRAINT QuestionTaggedTag_PK PRIMARY KEY(QUESTION_ID, TAG_ID)
);
CREATE TABLE so.Answer
(
ANSWER_ID SERIAL NOT NULL,
BODY CHARACTER VARYING NOT NULL,
USER_ID INTEGER NOT NULL,
QUESTION_ID INTEGER NOT NULL,
CONSTRAINT Answer_PK PRIMARY KEY(ANSWER_ID)
);
ALTER TABLE so.Question
ADD CONSTRAINT Question_FK FOREIGN KEY (USER_ID)
REFERENCES so."User" (USER_ID) ON DELETE RESTRICT ON UPDATE RESTRICT;
ALTER TABLE so.QuestionTaggedTag
ADD CONSTRAINT QuestionTaggedTag_FK1 FOREIGN KEY (QUESTION_ID)
REFERENCES so.Question (QUESTION_ID) ON DELETE RESTRICT ON UPDATE RESTRICT;
ALTER TABLE so.QuestionTaggedTag
ADD CONSTRAINT QuestionTaggedTag_FK2 FOREIGN KEY (TAG_ID)
REFERENCES so.Tag (TAG_ID) ON DELETE RESTRICT ON UPDATE RESTRICT;
ALTER TABLE so.Answer
ADD CONSTRAINT Answer_FK1 FOREIGN KEY (USER_ID)
REFERENCES so."User" (USER_ID) ON DELETE RESTRICT ON UPDATE RESTRICT;
ALTER TABLE so.Answer
ADD CONSTRAINT Answer_FK2 FOREIGN KEY (QUESTION_ID)
REFERENCES so.Question (QUESTION_ID) ON DELETE RESTRICT ON UPDATE RESTRICT;
COMMIT WORK;
Note that there is now a separate Tag table with TAG_ID as the primary key. TAG_NAME is a separate column with a uniqueness constraint over it, preventing duplicate tags. The QuestionTaggedTag table now has (QUESTION_ID, TAG_ID), which is also its primary key.
I hope I didn't go too far in answering this, but when I tried to write smaller answers, I kept having to untangle my earlier answers, and it seemed simpler just to post this.
You can create a unique constraint on (question_id, tag_name) in the tags table, which will ensure that the pair is unique. That would mean that the same question may not have the same tag attached more than once. However, the same tag could still apply to different questions.
You cannot create two primary keys, but you can place a uniqueness constraint on an index.
You can only have one primary key (I assume that's what you mean by "private" key), but that key can be a composite key consisting of the question-id and tag-name. In SQL, it would look like (depending on your SQL dialect):
CREATE TABLE Tags
(
question_id int,
tag_name varchar(xxx),
PRIMARY KEY (question_id, tag_name)
);
This will ensure you cannot have the same tag against the same question.
I will use PostgreSQL or Oracle.
I feel that the following is correspondent to Ken's code which is for MySQL.
CREATE TABLE Tags
(
QUESTION_ID integer FOREIGN KEY REFERENCES Questions(QUESTION_ID)
CHECK (QUESTION_ID>0),
TAG_NAME nvarchar(20) NOT NULL,
CONSTRAINT no_duplicate_tag UNIQUE (QUESTION_ID,TAG_NAME)
)
I added some extra measures to the query. For instance, CHECK (USER_ID>0) is to ensure that there is no corrupted data in the database.
I dropped out the AUTO_INCREMENT from this QUESTION_ID because I see that it would break our system, since one question cannot then have two purposely-selected tags. In other, tags would go mixed up.
I see that we need to give a name for the constraint. Its name is no_duplicate_tag in the command.