Is it possible to have a table in a database that has no primary foreign key match up?
The table will be holding file paths to particular files and directories that I want to upload which doesn't relate to any of the other tables in the database. Is this good practise or should all tables have a primary foreign key match up? If so should I create a separate database with just this table?
Every table should have a key but that doesn't mean it has to have a foreign key referencing it. The purpose of the key is to ensure that data is uniquely identifiable and guarantee that the expected set of dependencies is enforced.
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What is the difference between Primary and Unique key (in MySQL)? How these can be considered as a foreign key? please explain.
I tried creating a database table and don't know how to make a primary key as a foreign key. Does it take Joins concept where the separating attribute automatically create a foreign key?
A table in MySQL can only have one Primary Key at most, while you can create as many unique Keys or indexes as you want.
Also a primary key is not nullable while a unique key can have NULL as value.
But the biggest difference is the purpose:
You want to have a primary key because you need an identifier
The unique Key/Index on the other hand is usefull to controll values that are automatically getting inserted into your table (e.g. to avoid duplicates where none are allowed)
If you want to use a column as a forgein key, you need to define it as a primary key first. Unique Constraint can not be related with other tables as a foreign key.
Today I found that some database tables in my Magento 1.9.0.1 installation don't have primary keys. I found out during a search and replace in the database.
Are the primary keys important to run Magento properly? If so, how do I add them to the tables? I've checked out the Magento Database Repair Tool, however that only fixed foreign keys, not primary keys.
Any thoughts?
Thanks.
Missing primary keys:
The table "api_session" has no primary key. Changes will have to be made manually.
The table "catalog_category_anc_categs_index_idx" has no primary key. Changes will have to be made manually.
The table "catalog_category_anc_categs_index_tmp" has no primary key. Changes will have to be made manually.
The table "catalog_category_anc_products_index_idx" has no primary key. Changes will have to be made manually.
The table "catalog_category_anc_products_index_tmp" has no primary key. Changes will have to be made manually.
The table "catalog_category_product_index_enbl_idx" has no primary key. Changes will have to be made manually.
The table "catalog_category_product_index_enbl_tmp" has no primary key. Changes will have to be made manually.
The table "catalog_category_product_index_idx" has no primary key. Changes will have to be made manually.
The table "catalog_category_product_index_tmp" has no primary key. Changes will have to be made manually.
The table "log_url" has no primary key. Changes will have to be made manually.
The table "shipment_carriers" has no primary key. Changes will have to be made manually.
The table "shipment_export" has no primary key. Changes will have to be made manually.
The table "weee_discount" has no primary key. Changes will have to be made manually.
The following table in your list should have a primary key:
log_url
The following tables are not in out of the box Magento:
shipment_carriers
shipment_export
The rest of the tables do not have primary keys. This is okay, not all tables need to have primary keys, for example tables that join two tables or temporary tables. All the index tables are temporary and get cleared out pretty frequently, therefore no primary key needed.
I am currently creating a database using SQL but I have found the need to use a foreign key to in 3 different fields in a single table.
I have CourseID1, CourseID2 and CourseID3 in students courses table. Each of those 3 fields need to be foreign and reference to the CourseID field in the course table which is a primary key.
Is this possible? how do I go about doing this?
Thank you
This is possible. You would do:
foreign key (CourseId1) references Courses(CourseId),
foreign key (CourseId2) references Courses(CourseId),
foreign key (CourseId3) references Courses(CourseId),
That said, you don't want to do this. Having multiple columns with numeric appendages usually means that you want an association/junction table. In this case, you want a table named something like StudentCourses which would have one row per student and per course that the student takes.
I have two tables:
User (username, password)
Profile (profileId, gender, dateofbirth, ...)
Currently I'm using this approach: each Profile record has a field named "userId" as foreign key which links to the User table. When a user registers, his Profile record is automatically created.
I'm confused with my friend suggestion: to have the "userId" field as the foreign and primary key and delete the "profileId" field. Which approach is better?
Foreign keys are almost always "Allow Duplicates," which would make them unsuitable as Primary Keys.
Instead, find a field that uniquely identifies each record in the table, or add a new field (either an auto-incrementing integer or a GUID) to act as the primary key.
The only exception to this are tables with a one-to-one relationship, where the foreign key and primary key of the linked table are one and the same.
Primary keys always need to be unique, foreign keys need to allow non-unique values if the table is a one-to-many relationship. It is perfectly fine to use a foreign key as the primary key if the table is connected by a one-to-one relationship, not a one-to-many relationship. If you want the same user record to have the possibility of having more than 1 related profile record, go with a separate primary key, otherwise stick with what you have.
Yes, it is legal to have a primary key being a foreign key. This is a rare construct, but it applies for:
a 1:1 relation. The two tables cannot be merged in one because of different permissions and privileges only apply at table level (as of 2017, such a database would be odd).
a 1:0..1 relation. Profile may or may not exist, depending on the user type.
performance is an issue, and the design acts as a partition: the profile table is rarely accessed, hosted on a separate disk or has a different sharding policy as compared to the users table. Would not make sense if the underlining storage is columnar.
Yes, a foreign key can be a primary key in the case of one to one relationship between those tables
I would not do that. I would keep the profileID as primary key of the table Profile
A foreign key is just a referential constraint between two tables
One could argue that a primary key is necessary as the target of any foreign keys which refer to it from other tables. A foreign key is a set of one or more columns in any table (not necessarily a candidate key, let alone the primary key, of that table) which may hold the value(s) found in the primary key column(s) of some other table. So we must have a primary key to match the foreign key.
Or must we? The only purpose of the primary key in the primary key/foreign key pair is to provide an unambiguous join - to maintain referential integrity with respect to the "foreign" table which holds the referenced primary key. This insures that the value to which the foreign key refers will always be valid (or null, if allowed).
http://www.aisintl.com/case/primary_and_foreign_key.html
It is generally considered bad practise to have a one to one relationship. This is because you could just have the data represented in one table and achieve the same result.
However, there are instances where you may not be able to make these changes to the table you are referencing. In this instance there is no problem using the Foreign key as the primary key. It might help to have a composite key consisting of an auto incrementing unique primary key and the foreign key.
I am currently working on a system where users can log in and generate a registration code to use with an app. For reasons I won't go into I am unable to simply add the columns required to the users table. So I am going down a one to one route with the codes table.
It depends on the business and system.
If your userId is unique and will be unique all the time, you can use userId as your primary key. But if you ever want to expand your system, it will make things difficult. I advise you to add a foreign key in table user to make a relationship with table profile instead of adding a foreign key in table profile.
Short answer: DEPENDS.... In this particular case, it might be fine. However, experts will recommend against it just about every time; including your case.
Why?
Keys are seldomly unique in tables when they are foreign (originated in another table) to the table in question. For example, an item ID might be unique in an ITEMS table, but not in an ORDERS table, since the same type of item will most likely exist in another order. Likewise, order IDs might be unique (might) in the ORDERS table, but not in some other table like ORDER_DETAILS where an order with multiple line items can exist and to query against a particular item in a particular order, you need the concatenation of two FK (order_id and item_id) as the PK for this table.
I am not DB expert, but if you can justify logically to have an auto-generated value as your PK, I would do that. If this is not practical, then a concatenation of two (or maybe more) FK could serve as your PK. BUT, I cannot think of any case where a single FK value can be justified as the PK.
It is not totally applied for the question's case, but since I ended up on this question serching for other info and by reading some comments, I can say it is possible to only have a FK in a table and get unique values.
You can use a column that have classes, which can only be assigned 1 time, it works almost like and ID, however it could be done in the case you want to use a unique categorical value that distinguish each record.
I've been told that if I foreign key two tables, that SQL Server will create something akin to an index in the child table. I have a hard time believing this to be true, but can't find much out there related specifically to this.
My real reason for asking this is because we're experiencing some very slow response time in a delete statement against a table that has probably 15 related tables. I've asked our database guy and he says that if there is a foreign key on the fields, then it acts like an index. What is your experience with this? Should I add indexes on all foreign key fields or are they just unnecessary overhead?
A foreign key is a constraint, a relationship between two tables - that has nothing to do with an index per se.
However, it makes a lot of sense to index all the columns that are part of any foreign key relationship. An FK-relationship will often need to look up a relating table and extract certain rows based on a single value or a range of values.
So it makes good sense to index any columns involved in an FK, but an FK per se is not an index.
Check out Kimberly Tripp's excellent article "When did SQL Server stop putting indexes on Foreign Key columns?".
Wow, the answers are all over the map. So the Documentation says:
A FOREIGN KEY constraint is a candidate for an index because:
Changes to PRIMARY KEY constraints are checked with FOREIGN KEY constraints in related tables.
Foreign key columns are often used in join criteria when the data from related tables is combined in queries by matching the column(s) in the FOREIGN KEY constraint of one table with the primary or unique key column(s) in the other table. An index allows Microsoft® SQL Server™ 2000 to find related data in the foreign key table quickly. However, creating this index is not a requirement. Data from two related tables can be combined even if no PRIMARY KEY or FOREIGN KEY constraints are defined between the tables, but a foreign key relationship between two tables indicates that the two tables have been optimized to be combined in a query that uses the keys as its criteria.
So it seems pretty clear (although the documentation is a bit muddled) that it does not in fact create an index.
No, there is no implicit index on foreign key fields, otherwise why would Microsoft say "Creating an index on a foreign key is often useful". Your colleague may be confusing the foreign key field in the referring table with the primary key in the referred-to table - primary keys do create an implicit index.
Foreign keys do not create indexes. Only alternate key constraints(UNIQUE) and primary key constraints create indexes. This is true in Oracle and SQL Server.
In PostgeSql you can check for indexes yourself if you hit \d tablename
You will see that btree indexes have been automatically created on columns with primary key and unique constraints, but not on columns with foreign keys.
I think that answers your question at least for postgres.
Say you have a big table called orders, and a small table called customers. There is a foreign key from an order to a customer. Now if you delete a customer, Sql Server must check that there are no orphan orders; if there are, it raises an error.
To check if there are any orders, Sql Server has to search the big orders table. Now if there is an index, the search will be fast; if there is not, the search will be slow.
So in this case, the slow delete could be explained by the absence of an index. Especially if Sql Server would have to search 15 big tables without an index.
P.S. If the foreign key has ON DELETE CASCADE, Sql Server still has to search the order table, but then to remove any orders that reference the deleted customer.
SQL Server autocreates indices for Primary Keys, but not for Foreign Keys. Create the index for the Foreign Keys. It's probably worth the overhead.
It depends. On MySQL an index is created if you don't create it on your own:
MySQL requires that foreign key columns be indexed; if you create a table with a foreign key constraint but no index on a given column, an index is created.
Source: https://dev.mysql.com/doc/refman/8.0/en/constraint-foreign-key.html
The same for MySQL 5.6 eh.
Strictly speaking, foreign keys have absolutely nothing to do with indexes, yes. But, as the speakers above me pointed out, it makes sense to create one to speed up the FK-lookups. In fact, in MySQL, if you don't specify an index in your FK declaration, the engine (InnoDB) creates it for you automatically.
Not to my knowledge. A foreign key only adds a constraint that the value in the child key also be represented somewhere in the parent column. It's not telling the database that the child key also needs to be indexed, only constrained.
I notice that Entity Framework 6.1 pointed at MSSQL does automatically add indexes on foreign keys.