Suppose I have two tables (primary key -> foreign key) where one record in primary key table corresponds to many record in foreign key, so if I want to make insert page (using ASP.NET for instance) for both primary key table and foreign key table, how can I get the primary key value to insert it into the foreign key table?
(usually I took the max(of PK) but I am curious to find better solution)
I assume you are using SQL Server, since you mention the use of ASP.NET. Using Max(PK) is not a good idea in a multi-user system because eventually you will end up getting an incorrect primary key value. There are also resource issues to consider. In general, this approach should be avoided.
There are several ways you can accomplish what you want to do. One way is to do your inserts via stored procedures. The stored procedure for creating the primary key row would return the Id value of the new row, which then can be used for inserting your foreign key rows. But don't use Max(PK Value). Use the SCOPE_IDENTITY() value. This assumes you are using an Identity type for your PK values. You don't state what you are actually using.
Related
I have used natural keys of transactional tables as primary keys but I was told to use surrogate key as a primary key. But this table has a dependent child table. I want to know what is the best practice to choose primary key of a table, when it has a child table- between natural key vs surrogate key.
If surrogate key is preferred, how would I derive this surrogate key into child table to make a foreign key relationship? I know we do this is in Data warehousing environment to load dimension tables first and then fact tables by doing the look up on the dimension table keys.
Please give me your inputs on what is the best approach for transactional relational database design?
I appreciate your help in this regard.
Thank you!
Varun
In Microsoft SQL Server there is no great difference between a key defined as PRIMARY KEY and a key defined as a UNIQUE constraint. Therefore there is no special advantage to making the primary key a surrogate.
A disadvantage to having a surrogate key referenced by a foreign key constraint is that you will usually need extra joins and lookups in code. Because any searches will usually be based on the natural key(s) you will need an extra join - or at least an extra index seek operation - to find the surrogate key value first. You will typically need to do that kind of lookup or join whenever you want to modify referencing rows in a child table.
If you insert rows to the child table immediately after inserting to the parent table then there you can retrieve the surrogate values without any extra operation. If the surrogate is an IDENTITY column you can use the SCOPE_IDENTITY function to retrieve the last-inserted value. Alternatively, you can also use the OUTPUT clause on an INSERT statement to retrieve the inserted values - useful when you are inserting more than one row.
I'm not sure what is the best way to structure my question.
I have a table with foreign key column in it. By default the foreign key is set to NOT NULL and I want to keep it that way for now, because maybe this will be the final result. But for now there could be records that don't need (and have) foreign keys values and I want to distinct them somehow so it is as clear as possible that these records are something different from the other.
I tried but as it seems I can not use negative numbers for bigint which is the value of the foreign key in my SQL Server table. I guess this is pretty standard stuff so what is the best thing to do in this situation besides making the foreign key to NULL?
Not sure why HABO didn't make that the answer, because that's pretty much your only option.
Make it NULL
Create a row in the referenced table and set it to TBD or whatever moniker you prefer and use the ID from that instead of NULL
If you have records that do not need an FK and never will, then you should set the column to NULL, else use a temp value.
You cannot use a negative value because you MUST reference something in the foreign table if you have a foreign key constraint.
Foreign key constraints enforces you to refer an existing PK of the other table.
One way not mentioned yet is to drop the constraint for now :
ALTER TABLE YourTable
DROP CONSTRAINT fk_something
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.
What meaning does the concept of a primary key have to the database engine of SQL Server? I don't mean the clustered/nonclustered index created on the "ID" column, i mean the constraint object "primary key". Does it matter if it exists or not?
Alternatives:
alter table add primary key clustered
alter table create clustered index
Does it make a difference?
In general, a KEY is a column (or combination of columns) that uniquely identifies each row in the table. It is possible to have multiple KEYs in a table (for example, you might have a Person table where both the social security number as well as an auto-increasing number are both KEYs).
The database designer chooses one of theses KEYs to be the PRIMARY KEY. Conceptually, it does not matter which KEY is chosen as the PRIMARY KEY. However, since the PRIMARY KEY is usually used to refer to entries in this table from other tables (through FOREIGN KEYs), choosing a good PRIMARY KEY can be relevant w.r.t. (a) performance and (b) maintainability:
(a) Since the primary key will usually be used in JOINs, the index on the primary key (its size, its distribution, ...) is much more relevant to performance than other indexes.
(b) Since the primary key is used as a foreign key in other tables, changing the primary key value is always a hassle, since all the foreign key values in the other tables need to be modified as well.
A PRIMARY KEY is a constraint - this is a logical object that says something about the rules that your data must adhere to. An index is an access structure - it says something about the way the machine can search through the data. To implement a PRIMARY KEY, most RDBMS-es use an index.
Some RDBMS-es (fe. MySQL) do not make the distinction between PRIMARY KEY or UNIQUE constraint and the index that is used to help implement it. But for example, Oracle does: in oracle you can do something like: ALTER TABLE t DROP pk KEEP INDEX. This is useful if you want to change the definition of the primary key (for example, you are replacing a natural primary key with a surrogate primary key) but you still want to have a unique constraint on the original primary key columns without rebuilding the index. That makes sense if the index is very large and would take considerable table and resources to rebuild.
From what I can see, MS SQL does not make the distinction. I mean a tool like Management studio does display "Keys", "Indexes" and "Constraints" in differrent folders, but changing the name of one immediately changes the name of the corresponding objects in the other folders. So I think here the distinction is not really present in this case.
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.