Learn how to define an auto increment primary key in Oracle. Prior to Oracle’s version 12c, there was no way to generate auto incrementing columns within a t.
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For a relational database like PostgreSQL, it could widely be considered a sin among developers not to include a primary key in every table. Norton key generator activation code. It is therefore crucial that you do your utmost to add that all-important primary key column to every table, and thankfully Postgres provides two methods for accomplishing this task.
Using the Serial Data Type
By far the simplest and most common technique for adding a primary key in Postgres is by using the SERIAL or BIGSERIAL data types when CREATING a new table. As indicated in the official documentation, SERIAL is not a true data type, but is simply shorthand notation that tells Postgres to create a auto incremented, unique identifier for the specified column.
Below we’ll create our simple books table with an appropriate SERIAL data type for the primary key.
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. In these cases, you can perform the same auto incremented primary key functionality for your column by creating a custom SEQUENCE, similar to the method used in older version of Oracle.
Perhaps we’re particularly fond of even numbers but also have a strong distaste for anything smaller than 100, so we only want our primary key to be incremented by two starting at 100 for every insert. This can be accomplished with a custom SEQUENCE like so:
Now when we INSERT a new record into our books table, we need to evaluate the the next value of our sequence with nextval('books_sequence') and use that as our id.
SEQUENCES can be spiced up even more if desired, with options like minvalue and maxvalue to of course indicate extreme values, and even CYCLE, which allows the sequence to “loop around” once it reaches the maxvalue, returning back to the start value and beginning the climb all over again. Far more information can be found in the official documentation.
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. 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.
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.
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.
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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.
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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.