foreign-keys

I have a table user with userID as the primary key. I have another table called Friends . In the Friends table, I have two Users as friends represented by the columns UserID and FrndID where both UserID and FrndID should be a userID in table user . I want to enforce data integrity. Could I use something like this? ADD CONSTRAINT `ufd_users_fk` FOREIGN KEY (`userId`, `friendId`) REFERENCES `users` (`userId`, `userId`) ON DELETE CASCADE ON UPDATE CASCADE; I want to know is REFERENCES users ( userId , userId ) referencing a column multiple times correctly? The reason I am not creating 2 separate

I have these models (I have limited the number of fields to just those needed) class unit(models.Model): name = models.CharField(max_length=200) class project(models.Model): name = models.CharField(max_length=200) class location(address): project = models.ForeignKey(project) class project_unit(models.Model): project = models.ForeignKey(project) unit = models.ForeignKey(unit) class location_unit(models.Model): project = models.ForeignKey(project) #Limit the selection of locations based on which project has been selected location = models.ForeignKey(location) #The same here for unit. But I have

I'm extremely new to SQL and I'm trying to perform a simple ALTER TABLE to create a new column and use it as a foreign key to reference another simple table in my database. I've altered both tables to be InnoDB However, when executing the ALTER TABLE code, I get the following error: Error 1452 Cannot add or update a child row: a foreign key constraint fails (`toys`.<result 2 when explaining filename '#sql-6d4_6'>, CONSTRAINT `#sql-6d4_6_ibfk_1` FOREIGN KEY (`toy_id`) REFERENCES `toys` (`toy_id`)) Below are the DESC of both tables: Table 1: FIELD TYPE NULL KEY EXTRA toy_id int(11) NO PRI auto

I have two MySQL tables: collections and privacy_level . I define them with a foreign key relationship as such: CREATE TABLE collections ( coll_id smallint NOT NULL AUTO_INCREMENT UNSIGNED, name varchar(30) NOT NULL, privacy tinyint NOT NULL UNSIGNED DEFAULT '0', PRIMARY KEY(coll_id), INDEX(privacy), FOREIGN KEY fk_priv (privacy) REFERENCES privacy_level (level) ON UPDATE CASCADE ON DELETE RESTRICT ) ENGINE=InnoDB; CREATE TABLE privacy_level ( level tinyint NOT NULL UNSIGNED, name varchar(20) NOT NULL, PRIMARY KEY (level) ) ENGINE InnoDB; My question is about the ON DELETE RESTRICT clause and

I need to make some changes to a SQL Server 2008 database. This requires the creation of a new table, and inserting a foreign key in the new table that references the Primary key of an already existing table. So I want to set up a relationship between my new tblTwo, which references the primary key of tblOne. However when I tried to do this (through SQL Server Management Studio) I got the following error: The columns in table 'tblOne' do not match an existing primary key or UNIQUE constraint I'm not really sure what this means, and I was wondering if there was any way around it? It means that

I have been slowly learning SQL the last few weeks. I've picked up all of the relational algebra and the basics of how relational databases work. What I'm trying to do now is learn how it's implemented. A stumbling block I've come across in this, is foreign keys in MySQL. I can't seem to find much about the other than that they exist in the InnoDB storage schema that MySQL has. What is a simple example of foreign keys implemented in MySQL? Here's part of a schema I wrote that doesn't seem to be working if you would rather point out my flaw than show me a working example. CREATE TABLE `posts` (