Db2 Primary Key Auto Generated

DB2 set AUTOINCREMENT value How To Reset a auto generated Column in DB2 Table Auto-increment allows a unique number to be generated when a new record is inserted into a table. By default, the starting value for AUTOINCREMENT is 1, and it will increment by 1 for each new record. How can you tell if a table in DB2 has a Primary Key? Ask Question. We are using a tool that requires specific tables in our DB2 database to have a Primary Key defined. Is there a way using a select statement on the DB to see if a given table has one? Db2 primary-key. Composite vs auto incrementing integer as Primary key. Summary: in this tutorial, you will learn how to use the Db2 PRIMARY KEY constraint to define a primary key for a table. Db2 primary key overview. A primary key of a table is a column or group of columns whose values uniquely identify every row in the table. Each table has one and only one primary key. A primary key is optional. Jun 16, 2003  Hi Anil, From what I know of, database such as DB2, Oracle and SQL Server have these auto generated column. Therefore, you can use it in your CMP entity bean. If your database doesn't support this feature or you would like to have own control on the primary key feature, then TheServerSide.com have some good articles about it.

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:

Db2 Primary Key Constraint

Db2

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.

Key

Db2 Primary Key Auto Generated Form

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.

Db2 Add Primary Key

P: n/a
Hi!
While browsing the internet I've found an outline of a solution to
simulate 'ALTER TABLE ... RESTART WITH ...' on DB2 v7. It went like
this:
1. Instead of having IDENTITY column in your table, create another
table (counter) with a single column and define it as IDENTITY
2. Create a trigger in your table and have it: a) insert a record into
'counter' table, b) obtain the IDENTITY value, c) use this value in
your table, d) delete the record from the 'counter' table
This is transparent to the application and the advantage is that you
can always drop and re-create the 'counter' table with required initial
value of its IDENTITY counter.
So this is what I did:
1.
Instead of having normal table with IDENTITY column:
CREATE TABLE TAB (
ID INTEGER NOT NULL GENERATED ALWAYS AS IDENTITY CONSTRAINT PK
PRIMARY KEY,
PAYLOAD INTEGER)
I had two tables:
CREATE TABLE TAB (
ID INTEGER WITH DEFAULT NOT NULL CONSTRAINT PK PRIMARY KEY,
PAYLOAD INTEGER)
CREATE TABLE CNTR (
ID INTEGER GENERATED ALWAYS AS IDENTITY)
2.
Trigger:
CREATE TRIGGER TRG
NO CASCADE BEFORE
INSERT ON TAB
REFERENCING NEW AS NEW
FOR EACH ROW MODE DB2SQL
BEGIN ATOMIC
INSERT INTO CNTR VALUES (DEFAULT);
DELETE FROM CNTR;
UPDATE TAB
SET ID = IDENTITY_VAL_LOCAL();
END
Unfortunately this doesn't work because INSERTs are illegal in BEFORE
triggers. Rats! Why???
I could imagine that I can't do INSERTs to the same table but why not
some other table??? DB2 sucks! :-)
Ok, so another try:
CREATE TRIGGER TRG1
NO CASCADE BEFORE
INSERT ON TAB
REFERENCING NEW AS NEW
FOR EACH ROW MODE DB2SQL
BEGIN ATOMIC
SET NEW.ID = -1; --Mark the record somehow
END
CREATE TRIGGER TRG2
AFTER
INSERT ON TAB
FOR EACH ROW MODE DB2SQL
BEGIN ATOMIC
INSERT INTO CNTR VALUES (DEFAULT);
DELETE FROM CNTR;
UPDATE TAB
SET ID = IDENTITY_VAL_LOCAL()
WHERE ID = -1; --Find the marked record
END
Now it's starting to be really ugly and clumsy but works:
INSERT INTO TAB (PAYLOAD) VALUES (100)
SELECT * FROM TAB
ID PAYLOAD
-- -------
1 100
Unfortunately the application can't obtain the IDENTITY value of the
inserted record:
SELECT IDENTITY_VAL_LOCAL() FROM SYSIBM.SYSDUMMY1 yields NULL.
I found the explanation in the documentation of IDENTITY_VAL_LOCAL():
'The INSERT statement must also be issued at the same level; that is,
the value must available locally at the level it was assigned, until it
is replaced by the next assigned value. (A new level is initiated each
time a trigger or routine is invoked.)'
So I tried in vain.
Now the question: How do you think, did the original inventor of the
solution mentioned in the beginning of my post really had some working
solution or only THOUGHT they had a solution. (This resembles the known
case with Fermat's last theorem haha).
Is there any way to cross those 'levels' and pass the IDENTITY value
from within my trigger to the application???
I'll appreciate any comments. Thanks.
WhoReallyCares
P.S.
By the way. If I post this from Google Gropus, will it appear on Usenet
servers worldwide??