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SQL Primary Key - is it necessary?
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In database systems, should every table have a primary key?
For example I have a table table1(foreignkey1,foreignkey2,attribute) like this.table1 does not have a primary key.
Should I define a primary key for this table like table1id?
This is a subjective question, so I hope you don't mind me answering with some opinion :)
In the vast majority of tables I've made – I'm talking 95%+ – I've added a primary key, and been glad I did. This is either the most critical unique field in my table (think "social security number") or, more often than not, just an auto-incrementing number that allows me to quickly and easily refer to a field when querying.
This latter use is the most common, and it even has its own name: a "surrogate" or "synthetic" key. This is a value auto-generated by the database and not derived from your application data. If you want to add relations between your tables, this surrogate key is immediately helpful as a foreign key. As someone else answered, these keys are so common that MySQL likes to add one even if you don't, so I'd suggest that means the consensus is very heavily biased towards adding primary keys.
One other thing I like about primary keys is that they help convey your intent to others reading your table schemata and also to your DMBS: "this bit is how I intend to identify my rows uniquely, don't let me try to break that rule!"
To answer your question specifically: no, a primary key is not necessary. But realistically if you intend to store data in the table for any period of time beyond a few minutes, I would very strongly recommend you add one.
No, it is not required for every table to have a primary key. Whether or not a table should have a primary key is based on requirements of your database.
Even though this is allowed it is bad practice because it allows for one to add duplicate rows further preventing the unique identification of rows. Which contradicts the underline purposes of having a database.
I am a strong fan of synthetic primary keys. These are auto-incremented columns that uniquely identify each row.
These provide functionality such as:
Ability to see the order of insertion of rows. Which were inserted most recently?
Ability to create a foreign key relationship to the table. You might not need one now, but it might be useful in the future.
Ability to rename "data" columns without affecting other tables.
Presumably, for your table, you can define a primary key on (foreignkey1, foreighkey2). Composite primary keys are also sensible, but they are cumbersome for foreign key relationships and joins. And, when there are foreign key relationships, they may cause additional storage, because the composite key ends up being stored across multiple tables.
It's a good practise to have a primary key/composite primary key for a table:
it helps to join tables,
clustered tables will need primary key.
Database design should have primary key for a table.
In MySQL storage engine always creates a PRIMARY KEY if you didn't specify it explicitly, thus making an extra column you don't have access to.
You can create Composite Primary key like:
CREATE TABLE table1(
FK1 INT,
FK2 INT,
ATTRIBUTE INT,
PRIMARY KEY (FK1, FK2)
)
or create a constraint on table1:
ALTER TABLE table_name
ADD CONSTRAINT pk_table1 PRIMARY KEY (FK1,FK2)
Related
I have table department that has two column (dept_ID and dept_name)
my another table is login which i create a foreign key for the column dept_name referencing dept_name at table department. so i create an index named index_department, now in login table, i want to create another foreign key for the column eadd which will reference to DIFFERENT table named info_table.
should i create another index for the second foreign key??
another scenario, i want to create a dept_name column at info_table too. can i use the same index 'index_department'??
The general answer is, "It depends."
As #gordon-Linoff commented "You create indexes to meet performance requirements for queries."
Indexes take up space and and take processing time as the have to be maintained. So the case for any given index depends on the trade off between cost and usage. For example if you the data rarely changes, but you look it up a lot you will prefer to have more indexes.
My educated guess is that on the scale you are probably working you do want the indexes on all your foreign keys.
Specifically in mysql you seem to get the index is you formally add the FK constraint. It is discussed here does mysql index foreign key columns automatically
I say formally, because you can have implied foreign key relationships without actually declaring/enforcing the constraints. People sometimes do that to avoid even the cost of the checking/enforcing constraint. The risk is in updates that violate referential integrity. But I'm drifting onto a tangent.
As a side note, there is some pertinent discussion here does a foreign key automatically create an index
In MySQL (at least 5.6, which I am using), indices are automatically created for foreign keys.
I'm putting together a new database and I have a few tables that contain temp data.
e.g.: user requests to change password - a token is stored and then later removed.
Currently I have a primary key on these tables that will auto-increment from 1 upwards.
AUTO_INCREMENT = 1;
I don't really see any use for this primary key... I will never reference it and it will just get larger.
Should tables like this have a primary key or not?
Short answer: yes.
Long answer:
You need your table to be joinable on something If you want your table
to be clustered, you need some kind of a primary key. If your table
design does not need a primary key, rethink your design: most
probably, you are missing something. Why keep identical records? In
MySQL, the InnoDB storage engine always creates a PRIMARY KEY if you
didn't specify it explicitly, thus making an extra column you don't
have access to.
Note that a PRIMARY KEY can be composite.
If you have a many-to-many link table, you create the PRIMARY KEY on
all fields involved in the link. Thus you ensure that you don't have
two or more records describing one link.
Besides the logical consistency issues, most RDBMS engines will
benefit from including these fields in an UNIQUE index.
And since any PRIMARY KEY involves creating a UNIQUE index, you should
declare it and get both logical consistency and performance.
Here is a SO thread already have same discussion.
Some people still loves to go with your opinion. Have a look here
My personal opinion is that you should have primary keys, to identify or to make a row unique. The logic can be your program logic. Can be an auto-increment or composite or whatever it can be.
If I add a foreign key between two tables, am I allowed to add orphan rows afterwards? Also when I'm creating a foreign key between two tables, is there any way to create it ignoring the orphan rows?
My next question is about the efficiency of foreign keys. I had always thought that they created an index between one key in a table and the corresponding key in another table which essentially made it a linear lookup when doing a join.
Is a foreign key much more efficient then simply having an index or are they the same?
Thanks.
foreign key relationship is often between a foreign key in one table and a primary key in another.
primary key do implicitly create an index.
Not foreign keys. Mostly it's good practice to add index on a foreign key column.
keys are constraints to guarantee data consistency and index can be used improve access performance on the data. So these are different things that you often combine in practices and thus are often confused.
Regarding orphan rows I think the concept of keys is to prevent this. But I'm not completely sure If I understand exactly what you are asking here. I think adding orphan rows is not possible and creating keys if orphan rows exist sounds kind of impossible.
see also other questions on SO dealing with foreign key / index topic.
Here some more good answers related to primary key / key / index
Will it ever happen that we design a table that doesn't need a primary key?
No.
The primary key does a lot of stuff behind-the-scenes, even if your application never uses it.
For example: clustering improves efficiency (because heap tables are a mess).
Not to mention, if ANYONE ever has to do something on your table that requires pulling a specific row and you don't have a primary key, you are the bad guy.
Yes.
If you have a table that will always be fetched completely, and is being referred-to by zero other tables, such as some kind of standalone settings or configuration table, then there is no point having a primary key, and the argument could be made by some that adding a PK in this situation would be a deception of the normal use of such a table.
It is rare, and probably when it is most often done it is done wrongly, but they do exist, and such instances can be valid.
Depends.
What is primary key / unique key?
In relational database design, a unique key can uniquely identify each row in a table, and is closely related to the Superkey concept. A unique key comprises a single column or a set of columns. No two distinct rows in a table can have the same value (or combination of values) in those columns if NULL values are not used. Depending on its design, a table may have arbitrarily many unique keys but at most one primary key.
So, when you don't have to differentiate (uniquely identify) each row,
you don't have to use primary key
For example, a big table for logs,
without using primary key, you can have fairly smaller size of data and faster for insertion
Primary key not mandatory but it is not a good practice to create tables without primary key. DBMS creates auto-index on PK, but you can make a column unique and index it, e.g. user_name column in users table are usually made unique and indexed, so you may choose to skip PK here. But it is still a bad idea because PK can be used as foreign key for referential integrity.
In general, you should almost always have PK in a table unless you have very strong reason to justify not having a PK.
Link tables (in many to many relationship) may not have a primary key. But, I personally like to have PK in those tables as well.
I have to work with a database to do reporting
The DB is quite big : 416 055 104 rows
Each row is very light though, just booleans and int ids.
Each row is identify by 3 columns, but at my surprise, there is no Primary Key on it.
Only a Clustered Index with a unique constraint.
So Knowing that, I have 2 question.
Could there be ANY good reason for that?
Is there any way I can turn this into a primary key.
Regarding question 2
Creating a new primary key also creates a non-clustered index to associate with (there is already an existing clustered one).
This is not what I am looking for. I want to keep that same index, but also make it a primary key.
Is it possible?
Would that be faster that creating the whole index again? (I hope so)
What could be the consequences? (locks? crash? corrupted data?)
There is little or no difference between a PRIMARY KEY and a UNIQUE constraint on non-nullable columns. So if the columns in question are non-nullable then I suggest you do nothing. The main reason to make a candidate key into a primary key is if you have some software (such as a data modelling tool or other development tool) that expects the key to be identified with a PRIMARY KEY constraint.
Good question.
If you already have a unique index on non nullable columns then you have a candidate key. I'm not aware of any particular benefit of making this an "official" primary key. In fact I have a feeling that not making it a PK will give greater flexibility.
A unique index can allow null
values. A primary key can't.
I believe you can't "mark" an existing index as the primary key. You'd have to drop it and recreate. To avoid stuff, I'd say it'd be good to place a TABLOCKX, HOLDLOCK on the table before doing that.