Right now I'm working on a small blog project and I have 2 tables in my database: users and posts. I want to display name of the author of the post so I thought I should make foreign key for user in post table. But what if I would create normal column called for example user_id and just save there id of the author. Then while I would like to display post i could join both tables and display content of post and name of the author.
Does creating foreign key have some adventages?
Either way, you are going to need a "normal" column, such as user_id.
The advantage of making the user_id a foreign key is that then, the database will enforce referential integrity. This means it won't allow you to set on a post a user that does not exist, nor will it allow you to delete a user who has one or more posts (without also deleting the relevant posts).
Related
I want to create a posting system to a profile. I created a database for storing all users posts each user have a table.
Ihad created another database for storing the comments of each posts. My logic is to create each table in the comments database and store each comment in that.
Is there a logic to link the post and the comments. I thought to use mysql last insert id but it will return last id which will create error because one of the post will not have a table.
Is there any other way?
Another way would be to have a single table for posts, and identify a user post in the table using a userid column. To find all posts by a particular user, simply query by the user's ID. By doing so, you have a single table to manage, and you can do a lookup easily. If you create separate tables for each user, you have to create additional logic to first figure out which table to use. If a user is removed, you delete a table, rather than simply removing some rows from a common table.
The same logic applies to the comments table - add columns for postid',commentid,userid`. Again, a single table contains all the comments. To find comments on a particular post, you would do a simple query such as
select comment_text
from comments_table
where postid = ?
The whole purpose of using MySQL is to leverage relationships between entities, i.e. a user owns posts, a post is linked to comments.
If you do not want to use a relational schema like this, take a look at NoSQL DBs.
You have a couple options here:
Add a user_id column to your posts table, and a post_id, and user_id column to your comments table. You can then setup foreign keys with one-to-many relationships.
Only use a single table that has (in addition to your existing) a user_id, and type column. Type will define comment/post/etc. This can be defined with intermediary tables as a number mapped to a CONST, string, or any other way that you see fit (intermediary best option imho).
Vary the above example and use 2 intermediary tables to match users to posts and comments to posts (possibly also users to comments).
Creating a comment system with a simple rating system for each comment.
tables : 1.For the comments and it is called comments and it has three columns : id, name, comment
2. for the IP of the user that did the rating and it is called voted_ipand it has three columns id, comment_id, user_ip
The purpose of the voted_ip table is that i need to save the IP address for each rate to validate it that it cannot rate again if it exists.
I created a foreign key from the child table voted_ip in the column comment_id connecting it to the parent table comments in the column id following the steps at this link and this video on how to create a working foreign key except that the child table still do not update after a comment or a rate is inserted.
as follow :
I thought about that there might be another step or I have to do something in the php side of the project. What am I missing?
Data is not inserted in the other table "voted_ip" on insertion in "comment" by itself you have to add it explicitly this constraints are just for checking not for adding data in other table automatically.
I wish to know what SQL is needed to be passed to implement multiple categories for an article.
I have created 3 tables.
ARTICLES:
id, title, content
CATEGORIES:
id, name
RELATION:
article_id, cat_id
I am successfully able to create the first two tables, and store data in them when user submits the form. But, I have no clue how to update the RELATION table. I searched on stackoverflow, and I learned that I need to use a many to many relationship. I have idea about it. But, I do not know how to do it practically i.e. the SQL.
The Categories are obviously added while the post is published, So i need to update this table only after the first two tables have been updated.
If someone can guide me to a tutorial or in the right direction I shall be greatful.
Assuming that post and article are synonyms, then each time a new post is published and its category is determined you need to 'INSERT' a record into 'RELATION' table.
When you originally create tables you will need to identify Primary and Foreign keys (CONSTRAINTS) and (if so desired) specify whether CASCADE should be enabled.
Apparently you already know how to CREATE tables and INSERT rows.
You may want to Google for PRIMARY KEY, FOREIGN KEY, CASCADE ON DELETE in conjunction with MYSQL and PHP.
Also see if the following helps any: How do I use on delete cascade in mysql?.
This should be taken care of by your SQL DBMS. For example, if you set your relationship's foreign keys to cascade delete or update, when something changes in the parent, the children will also be deleted/updated. Can you give me an example of an update that you would expect to make to the first two tables and the resulting update to the RELATION table?
I am trying to understand relationships fully, and have gone through alot of tutorials. Still i linger on a few things (using MySQLWORKBENCH):
1. Every user can upload a product
2. Every product can have multiple categories, bids, ratings, etc
3. Every user can have multiple phonenumbers
(there are more, but this is the basic setup)
Is this correct?:
1 - I used a 1:n relationship, since every user can upload multiple products.
2 and 3. I used n:m relationship, since there can be multiple products with multiple categories, bids, ratings, etc.
DELETE/UPDATE:
I used ON UPDATE CASCADE and ON DELETE CASCADE everywhere where there is a foreign key...that being 'product', 'category_tags_has_products', 'bid_price_has_product', 'phone_has_user'.
I tried to delete a product like this (php): mysql_query("DELETE FROM product WHERE id='$id'");
I get a 1054 error, which is a foreign key error.
What are the best practises here? It is to my understanding that i shouldn't need to do deletions in any other than the parent-table?
Thanx!
You have a lot of identifying relationships, which mean that the foreign key form part of the primary key on the second table. This is not necessary in most instances, and is only really useful in instances such as link tables.
To this end I would change the user->product link to be non-identifying, which will make user_id a Foreign Key instead of being part of the Primary Key. In workbench the shortcut for a 1:n non-identifying relationship is key '2' (where as '4' is identifying). This in turn should remove the user_id fields from the tables which product links onto.
When you delete a product, it should cascade to the 3 link tables that it links to. At present it may be that it is trying to delete from users also depending on how the FK is set up. The following should cascade deletions (assuming a deletion is permanent and you just want to clear out all linked records)
DELETE FROM product -> deletes from any table with product_id in
DELETE FROM user -> deletes from any table with user_id in
The same applies for phone, rating, bid_price, category_tags.
I hope this if of use, if you need any further pointers feel free to shout :)
The relations look correct.
To find out what is wrong with your query check the $id variable.
Or check the whole query and then run it in the console/phpMyAdmin/etc:
$query = "DELETE FROM product WHERE id='$id'";
var_dump($query);
mysql_query($query);
P.S.: and don't forget to escape all data got from the users! Like this:
$id = mysql_real_escape_string($_GET['id']);
using php and mysql
I have two tables, a users table and a profiles table.
When a new account is created on my site the user and profile rows are created at the same time.
I want a foreign key to reference from the users table to the profiles table so that the users profile information will be stored in a seperate table.
My questions is, how do I ensure that the foreign key links to the correct profiles table.
The current way I would go about doing this is, after the new user and profiles row was created I would query the profiles in descending order to get the last row created and grab the profile id and insert it into the users table profile id foreign key reference. I'm sure that there must be a better way to do this though.. isn't there?
Create user with mysql_query();
Assuming the primary key of the users table is an auto-increment field, call mysql_insert_id();
Use that as the foreign key when you insert the profile.