![]() The concepts used in this tutorial are as follows: If you need a better understanding of the Foreign Key to Primary Key MySQL concepts i would take a look here " How to Use MySQL Foreign Keys for Quicker Database Development". In this tutorial we will make use of phpMyAdmin however please feel free to use the concepts within and relate them through to any database client you prefer. This is the fourth video in the Wappler v1.2.3 "Fruit Juice International" training set, in it I am going to only create the first 3 tables needed for a website I already created, in great detail as usual. Luckily it is fairly simple as we have already designed the entire database in MindNode so we just have to follow that. If your web service supports InnoDB, you should definitely use foreign key constraints to preserve the integrity of your data.Let's make our first 3 database tables! Part 4īefore we can get to the fun part of creating the backend in Wappler we are first going to have to create some database tables for Wappler to use and reference back to. phpMyAdmin makes it easy to set up foreign key constraints as you'll see in the rest of this chapter. A foreign key constraint would stop you from inserting 18 as a value in the make id column of the cars table without first adding a new record to the parent table. For example, the highest value of make id in the makes table is currently 17. In addition, foreign key constraints reject any insert or update operation that attempts to use a foreign key a value that doesn't exist in the parent table. In MySQL, checks are performed immediately, so it has the same effect as RESTRICT. In some database systems, this allows the foreign key checks to be delayed. Technically speaking, there's a fourth option: NO ACTION. For this to work, the foreign key column in the child table must accept NULL values. SET NULL updates or deletes the row in the parent table and sets the foreign key in matching rows of the child table to NULL. ![]() CASCADE updates or deletes the row in the parent table and automatically updates or deletes the matching rows in the child table. RESTRICT rejects the update or delete operation. When setting up foreign key constraints, you need to specify what happens when you update or delete a record in the parent table that has dependent records in the child table. Although MyISAM tables support foreign keys, the MyISAM storage engine doesn't support foreign key constraints. And at the time of recording this video, XtraDB was aliased as InnoDB. This has been the default since MySQL 5.5 In MariaDB, the storage engine can either be XtraDB or InnoDB. In MySQL, both tables must use the InnoDB storage engine. In the cars table, make id has an ordinary index. In the makes table, make id is indexed as the primary key. ![]() To establish foreign key constraints, both columns must be indexed. In our example, that means the cars table. You always set up the foreign key constraints in the child table. So in the example database, makes is the parent table, and cars is the child. The parent is the table that contains a primary key, which is used as a foreign key in the child table. Using foreign keys establishes a parent-child relationship between tables. In other words, they prevent inconsistencies between related tables. By doing so foreign key constraints maintain referential integrity. Foreign key constraints avoid this problem by detecting related records in another table and taking preventative action such as canceling the operation to prevent records from becoming orphaned. ![]() You can no longer search them by make because there's no way of knowing what make id 5 stands for. But if you delete make id 5 from the makes table, all cars that use that as the foreign key become orphaned. Make id 5 is still associated with Chrysler. If you delete either of these rows, for example when the car is sold, it has no effect on the makes table. After importing the SQL file from the exercise files, the first and seventh rows of the cars table became associated with make id 5 or Chrysler. In the database we've been using in this course, the primary key in the makes table is used as a foreign key in the cars table. Foreign key constraints control what happens when records are inserted, updated, or deleted. In a relational database like MySQL, foreign keys are used to link records in different tables.
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