Much to the frustration of database administrators worldwide, prior to Oracle version 12c in mid-2014, Oracle simply had no inherent ability to inherently generate auto incrementing columns within a table schema. Windows 7 key generator download. While the reasons for this design decision can only be guessed at, the good news is that even for users on older Oracle systems, there is a possible workaround to circumnavigate this pitfall and create your own auto incremented primary key column.

1. Pengertian Primary Key
2. Db2 Primary Key Auto Generated Parts
3. Db2 Primary Key Auto Increment

Creating a Sequence

The first step is to create a SEQUENCE in your database, which is a data object that multiple users can access to automatically generate incremented values. As discussed in the documentation, a sequence in Oracle prevents duplicate values from being created simultaneously because multiple users are effectively forced to “take turns” before each sequential item is generated.

The first is PRIMARY KEY, which as the name suggests, forces the specified column to behave as a completely unique index for the table, allowing for rapid searching and queries. While SQL Server only allows one PRIMARY KEY constraint assigned to a single table, that PRIMARY KEY can be defined for more than one column. By simply setting our id column as SERIAL with PRIMARY KEY attached, Postgres will handle all the complicated behind-the-scenes work and automatically increment our id column with a unique, primary key value for every INSERT. Using a Custom Sequence. In some rare cases, the standard incremental nature built into the SERIAL and BIGSERIAL data types may not suit your needs.

For the purposes of creating a unique primary key for a new table, first we must CREATE the table we’ll be using:

Next we need to add a PRIMARY KEY constraint:

Finally, we’ll create our SEQUENCE that will be utilized later to actually generate the unique, auto incremented value.

Adding a Trigger

While we have our table created and ready to go, our sequence is thus far just sitting there but never being put to use. This is where TRIGGERS come in.

Similar to an event in modern programming languages, a TRIGGER in Oracle is a stored procedure that is executed when a particular event occurs.

Typically a TRIGGER will be configured to fire when a table is updated or a record is deleted, providing a bit of cleanup when necessary.

In our case, we want to execute our TRIGGER prior to INSERT into our books table, ensuring our SEQUENCE is incremented and that new value is passed onto our primary key column.

Here we are creating (or replacing if it exists) the TRIGGER named books_on_insert and specifying that we want the trigger to fire BEFORE INSERT occurs for the books table, and to be applicable to any and all rows therein.

The ‘code’ of the trigger itself is fairly simple: We SELECT the next incremental value from our previously created books_sequenceSEQUENCE, and inserting that into the :new record of the books table in the specified .id field.

Note: The FROM dual part is necessary to complete a proper query but is effectively irrelevant. The dual table is just a single dummy row of data and is added, in this case, just so it can be ignored and we can instead execute the system function of our trigger rather than returning data of some kind.

IDENTITY Columns

IDENTITY columns were introduced in Oracle 12c, allowing for simple auto increment functionality in modern versions of Oracle.

Pengertian Primary Key

Using the IDENTITY column is functionally similar to that of other database systems. Recreating our above books table schema in modern Oracle 12c or higher, we’d simply use the following column definition.

Summary: in this tutorial, you will learn how to use DB2 identity column to define an identity column for a table.

Introduction to Db2 identity column

When you create a new table and use the GENERATED AS IDENTITY option for a column, this column will become an identity column.

An identity column contains a unique integer for each row in the table. When you insert a new row into the table, Db2 automatically generates a sequential integer for the identity column. Thus, identity columns are ideal for the primary key columns such as book id (book_id) or publisher id (publisher_id).

The following shows the syntax of declaring an identity column:

In this syntax:

First, specify the data type for the identity column. The data type can be SMALLINT, INT, and BIGINT.

Second, use either GENERATED ALWAYS or GENERATED BY DEFAULT option.

• For the GENERATED ALWAYS option, Db2 will always generate a sequential integer for the identity column. Any attempt to insert a value into the identity column with GENERATED ALWAYS option will result in an error.
• On the other hand, for the GENERATED BY DEFAULT option, DB2 will only generate the sequential integer when you don’t provide the value for the identity column. If you insert a value into the identity column with the GENERATED BY DEFAULT option, Db2 will use your value instead of using the system generated one.

Third, specify the identity column’s options:

The identity option allows you to specify the starting value in START WITH clause and increment value in the INCREMENT BY.

If the increment value is positive, you will have an ascending sequence like 1, 2, 3, … In case it is negative, then you will have a descending sequence e.g., -1, -2, -3, …

The MINVALUE and MAXVALUE options allow you to specify the minimum and maximum values that Db2 will generate.

The CYCLE or NOCYCLE option determines whether Db2 should restart the values when it has generated all the possible values.

For example, if you use CYCLE option and the sequence is 1, 2, 3, then Db2 will return 1 if it has generated 3. However, if you use the NO CYCLE option, Db2 will raise an error instead.

Notice that a table can have one and only one identity column in a table in Db2.

Db2 identity column examples

Let’s take some examples of using identity columns to get a better understanding.

1) Db2 identity column example

First, create a new table named t1 with the id column as an identity column.

The value of the id column will start with 10 and increment by 10.

Second, use the following INSERT statement to insert three rows into the t1 table:

Third, view data from the t1 table using the following SELECT statement:

Here is the output:

2) Db2 identity column with CYCLE example

First, create a new table named t2 whose id column is an identity column.

Second, insert seven rows into the t2 table:

Db2 Primary Key Auto Generated Parts

Third, query data from the t2 table:

Here is the output:

In this example, the id column’s value starts with -1 and has an increment of one.

Because the MAXVALUE is 2 and CYCLE option is specified, the sequence is -1, 0, 1 ,2, -1, 0, 1 …

Db2 Primary Key Auto Increment

In this tutorial, you have learned how to use the Db2 identity column to define an identity column for a table.