Sql Generate Primary Key Value

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Pengertian Primary Key
How To Set Primary Keys In Sql

What is a Primary Key?

Create table dbo.CustomerNumberGenerator ( company varchar(8) not null, currvalue int not null default(1), constraint CustomerNumberGeneratorPK primary key clustered ( company ), ) Second, you'll need a stored procedure like this (in fact, you might want to integrate this logic into the stored procedure responsible for inserting the.

A Primary Key is a field or a combination of fields that identify a record uniquely. The Primary key is a column or set of columns which are unique. In other words, every value is unique for Primary Key.

Rules for Primary Key

Each table can have only one Primary Key.
All the values are unique and Primary key value can uniquely identify each row.
The system will not allow inserting a row with a primary key which already exists in the table.
Primary Key cannot be NULL.

Below diagram summarizes all the above point for the primary key.

In this tutorial, you will learn

How to Create Primary Key

We can Create a Primary Key in 2 ways:

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Kingdom come deliverance licence key generator. 1. SQL Server Management Studio.

2. T-SQL: Create Primary while creating a New Table.

SQL Server Management Studio

Step 1) Right Click on the Table name. Click on Design.

Step 2) Right-click on the Column name. Click on 'Set Primary Key'

Result: Course_Id is now a Primary Key.

T-SQL: Create a Primary key while creating a New Table.

Below is the syntax to create the table from T-SQL

Syntax:

Let's create a Table with One Column as a PRIMARY KEY.

Query:

Step 1) Run the Query by clicking on 'Execute.'

Result: Course_Id is now a Primary Key.

Example: Let's see if it allows entering Multiple Records with Same Course ID.

Step 1) Insert 4 rows with different Course_ID

Step 2) Verify all Data Inserted successfully by running the Select query.

Note: We can insert duplicate values in the Non-Primary key.

Step 3) Now let's try Inserting New records with an existing Course_ID which is Primary Key.

Result: System does not allow inserting new value as 4 is there Course_ID column which is a Primary Key.

T-SQL: Add a Primary key to existing table using Alter Table

You can use the ALTER statement to create a primary key. However, the primary key can only be created on columns that are defined as NOT NULL. You cannot create a primary key on a column that allows NULLs. If you need to do, you have to drop and recreate the table.

Here is the syntax:

For example:

We have added a primary key constraint to an already existing table. The constraint has been added on the admission column and assigned the name students_pk.

Interesting Facts!

The Primary key can be a combination of multiple columns. This combination is known as the Composite primary key.
The Primary key can have a maximum of 16 columns.

Summary

Each Table can have only one Primary Key
All the values are unique which can uniquely identify each row.

Primary Key Generation Using Oracle's Sequence

Oracle provides the sequence utility to automatically generate unique primary keys. To use this utility to auto-generate primary keys for a CMP entity bean, you must create a sequence table and use the @AutomaticKeyGeneration annotation to point to this table.

In your Oracle database, you must create a sequence table that will create the primary keys, as shown in the following example:

This creates a sequences of primary key values, starting with 1, followed by 2, 3, and so forth. The sequence table in the example uses the default increment 1, but you can change this by specifying the increment keyword, such as increment by 3. When you do the latter, you must specify the exact same value in the cacheSize attribute of the @AutomaticKeyGeneration annotation:

If you have specified automatic table creation in the CMP bean's project settings, the sequence table will be created automatically when the entity bean is deployed. For more information, see @JarSettings Annotation. For more information on the definition of a CMP entity bean, see below.

Primary Key Generation Using SQL Server's IDENTITY

In SQL Server you can use the IDENTITY keyword to indicate that a primary-key needs to be auto-generated. The following example shows a common scenario where the first primary key value is 1, and the increment is 1:

In the CMP entity bean definition you need to specify SQLServer(2000) as the type of automatic key generator you are using. You can also provide a cache size:

Primary Key Generation Using a Named Sequence Table

A named sequence table is similar to the Oracle sequence functionality in that a dedicated table is used to generate primary keys. However, the named sequence table approach is vendor-neutral. To auto-generate primary keys this way, create a named sequence table using the two SQL statements shown in the example:

In the CMP entity bean definition you need to specify the named sequence table as the type of automatic key generator you are using. You can also provide a cache size:

Note. When you specify a cacheSize value for a named sequence table, a series of unique values are reserved for entity bean creation. When a new cache is necessary, a second series of unique values is reserved, under the assumption that the first series of unique values was entirely used. This guarantees that primary key values are always unique, although it leaves open the possibility that primary key values are not necessarily sequential. For instance, when the first series of values is 10..20, the second series of values is 21-30, even if not all values in the first series were actually used to create entity beans.

Defining the CMP Entity Bean

When defining a CMP entity bean that uses one of the primary key generators, you use the the @AutomaticKeyGeneration annotation to point to the name of the primary key generator table to obtain primary keys. Also, you must define a primary key field of type Integer or Long to set and get the auto-generated primary key. However, the ejbCreate method does not take a primary key value as an argument. Instead the EJB container adds the correct primary key to the entity bean record.

The following example shows what the entity bean might look like. Notice that the bean uses the named sequence option described above, and that ejbCreate

Pengertian Primary Key

method does not take a primary key: