Primary Key Generator In Python
16.04.2020by admin
What is Primary Key in MongoDB?
In MongoDB, _id field as the primary key for the collection so that each document can be uniquely identified in the collection. The _id field contains a unique ObjectID value.
The model primary key Field can be obtained via the StdModel.pk class method. A model can have one and only one primary key which is specified by passing primarykey=True during model definition: class MyModel ( odm. I would like a model to generate automatically a random alphanumeric string as its primary key when I create a new instance of it. Example: from django.db import models class MyTemporaryObject(m.
By default when inserting documents in the collection, if you don't add a field name with the _id in the field name, then MongoDB will automatically add an Object id field as shown below
When you query the documents in a collection, you can see the ObjectId for each document in the collection.
If you want to ensure that MongoDB does not create the _id Field when the collection is created and if you want to specify your own id as the _id of the collection, then you need to explicitly define this while creating the collection.
When explicitly creating an id field, it needs to be created with _id in its name.
Let's look at an example on how we can achieve this.
Code Explanation:
- We are assuming that we are creating the first document in the collection and hence in the above statement while creating the collection, we explicitly define the field _id and define a value for it.
If the command is executed successfully and now use the find command to display the documents in the collection, the following Output will be shown
Output:
The output clearly shows that the _id field we defined while creating the collection is now used as the primary key for the collection.
Summary: in this tutorial, you will learn how to use MySQL primary key constraint to create the primary key for a table.
Introduction to MySQL primary key
A primary key is a column or a set of columns that uniquely identifies each row in the table. The primary key follows these rules:
- A primary key must contain unique values. If the primary key consists of multiple columns, the combination of values in these columns must be unique.
- A primary key column cannot have
NULLvalues. Any attempt to insert or updateNULLto primary key columns will result in an error. Note that MySQL implicitly adds aNOT NULLconstraint to primary key columns. - A table can have one an only one primary key.
Because MySQL works faster with integers, the data type of the primary key column should be the integer e.g., INT, BIGINT. And you should ensure sure that value ranges of the integer type for the primary key are sufficient for storing all possible rows that the table may have.
A primary key column often has the AUTO_INCREMENT attribute that automatically generates a sequential integer whenever you insert a new row into the table.
When you define a primary key for a table, MySQL automatically creates an index called PRIMARY.
MySQL PRIMARY KEY examples
The PRIMARY KEY constraint allows you to define a primary key of a table when you create or alter table.
1) Define a PRIMARY KEY constraint in CREATE TABLE
Typically, you define the primary key for a table in the CREATE TABLE statement.
If the primary key has one column, you can use the PRIMARY KEY constraint as a column constraint:
When the primary key has more than one column, you must use the PRIMARY KEY constraint as a table constraint.
In this syntax, you separate columns in the column_list by commas (,).
The PRIMARY KEY table constraint can be used when the primary key has one column:
The following example creates a table named users whose primary key is the user_id column:
This statement creates the roles table that has the PRIMARY KEY constraint as the table constraint:
In case the primary key consists of multiple columns, you must specify them at the end of the CREATE TABLE statement. You put a comma-separated list of primary key columns inside parentheses followed the PRIMARY KEY keywords.
The following example creates the user_roles table whose primary key consists of two columns: user_id and role_id. It defines the PRIMARY KEY constraint as the table constraint:
Note that the statement also created two foreign key constraints.
2) Define PRIMARY KEY constraints using ALTER TABLE
If a table, for some reasons, does not have a primary key, you can use the ALTER TABLEstatement to add a primary key to the table as follows:
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The following example adds the id column to the primary key.
Primary Key Generator In Python Code
First, create the pkdemos table without a primary key.
Second, add a primary key to the pkdemos table using the ALTER TABLE statement:
If you add a primary key to a table that already has data. The data in the column(s), which will be included in the primary key, must be unique and not NULL.
PRIMARY KEY vs. UNIQUE KEY vs. KEY
KEY is the synonym for INDEX. You use the KEY when you want to create an index for a column or a set of columns that is not the part of a primary key or unique key.
A UNIQUE index ensures that values in a column must be unique. Unlike the PRIMARY index, MySQL allows NULL values in the UNIQUE index. In addition, a table can have multiple UNIQUE indexes.
Suppose that email and username of users in the users table must be unique. To enforce thes rules, you can define UNIQUE indexes for the email and username columns as the following statement:
Add a UNIQUE index for the username column:
Primary Key Generator In Python Pdf
Add a UNIQUE index for the email column:
In this tutorial, you have learned how to create a primary key for a new table or add a primary key to an existing table.
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