Sql Insert Generate And Return Primary Key
- Sql Insert Generate And Return Primary Keyboard
- Sql Insert Generate And Return Primary Key 2017
- Sql Insert Generate And Return Primary Key Excel
- Sql Insert Generate And Return Primary Key Definition
A look at your options for generating unique sequential numbers
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The reason to use a surrogate key solution is to find an efficient way to generate unique sequential numbers. This article describes three implementations:
- Using the traditional method
- Using key manager
- Using DB2 UDB features
A surrogate key is also called an internal key. When you create a table, you can add an extra column to be the surrogate key. This column should be NOT NULL, and will have no business meaning. This surrogate column can be designated the primary key column. A simple example is to have a numeric surrogate column. The surrogate key values start with a number, for example '1', for the first row of the table, and is incremented by one for each row thereafter.
For example, if we have the table EMPLOYEE:
You can add a surrogate key column, SERIALNUMBER, and designate it as the primary key column. So the table definition becomes:
How do you assign unique values to the SERIALNUMBER column for each row? You need to generate unique values for the surrogate key. I will discuss three possible solutions below.
Using the traditional method
Solution idea
The traditional method is to use a simple SQL or trigger to generate unique values.
Examples
Let’s take the table EMPLOYEE as an example. You can put the surrogate key generation function in the INSERT statement:
The SQL statement '(SELECT MAX(SERIALNUMBER) FROM EMPLOYEE)+1' will find the maximum SERIALNUMBER and increase by 1, so that the new row will have a unique SERIALNUMBER.
The only problem is that when you insert the first row into the table, you will get the following error:
The reason you get the error is that the return of 'SELECT MAX(SERIALNUMBER) FROM EMPLOYEE' is NULL when the table is empty. So we have to use COALESCE() to handle this:
Another traditional way is to use a trigger to generate the surrogate key:
Benefits and concerns
The traditional way is easy to understand and easy to implement on all systems. But actually, this implementation will cause concurrency problems in a transaction system. It only allows one INSERT to occur on the table at a time.
The 'SELECT MAX(SERIALNUMBER) FROM EMPLOYEE' will have to wait for all the other transactions to finish the INSERT or UPDATE on the table EMPLOYEE before getting the maximum SERIALNUMBER. For example, if there are two transactions doing the INSERT on the EMPLOYEE table, one transaction will be blocked by the other one. Obviously, this kind of 'one-by-one insert' solution is not suitable for a multi-user transactional system.
Using a key manager
Solution idea
Many large applications use the key manager method to maintain the surrogate keys for all tables. The key manager can be a helper class. Every time you need to insert a row into a table, you call the key manager for the new key value, and then insert the row with the obtained key value.
Examples
First, you need to create the table KEYS to remember the current surrogate key values for each table. This table will be used by the key manager class to generate the new key values.
Second, register the new table, for example the EMPLOYEE table, into the table KEYS.
Third, write the KeyManger class to maintain the surrogate keys for each registered table. The KeyManager class will provide two methods:
For more detailed coding of the KeyManger, please refer to the appendix.
Fourth, call the KeyManger to get the primary key value:
Benefits and concerns
Obviously, the key manager is a good example of modular design. The key manager encapsulates the surrogate key generation function. It is also easy to customize. You can specify different values for SURROGATEKEYVALUE or INCREMENT in the KEYS table to get the different surrogate keys. And it will work on most databases systems. However, it requires a separate table and code to be maintained. So it may be more suitable for a large and cross-database system.
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Using DB2 UDB features
DB2 UDB provides three approaches to generate unique values. You can use them to implement the surrogate key.
- GENERATE_UNIQUE() SQL function from DB2 UDB Version 6.1
- IDENTITY option in the CREATE TABLE statement from DB2 UDB Version 7.2
- SEQUENCE object from DB2 UDB Version 7.2
GENERATE_UNIQUE()
Solution idea
The GENERATE_UNIQUE() is an SQL function originally provided in DB2 UDB Version 6.1. It returns the current system timestamp. It can be used to generate unique values for the surrogate key column.
Examples
Then you can insert a row with the following SQL:
Benefits and concerns
There are a couple of things you should be aware of.
First, when multiple transactions insert rows at the same time, GENERATE_UNIQUE() may return the same timestamp. In this case GENERATE_UNIQUE() cannot guarantee a unique return value for each transaction, making it not suitable for a high-volume transaction system.
Second, if the system clock is adjusted backwards, GENERATE_UNIQUE() will return duplicate values.
Because of the above limitations, I would never use GENERATE_UNIQUE() in my production system. But it may be an alternative when you have to finish a prototype in limited time.
IDENTITY option in CREATE TABLE statement
Solution idea
The IDENTITY is an option in the CREATE TABLE statement provided in DB2 UDB Version 7.1and later. A column can be specified as an IDENTITY column when the table is created. DB2 will be responsible for generating a unique value for the column for each INSERT statement.
Example
Then you can insert a row with the following statement:
The INSERT statement does not need to specify the value for the SERIALNUMBER column. DB2 UDB will automatically generate unique values according to the column definition, 'GENERATED ALWAYS AS IDENTITY (START WITH 1, INCREMENT BY 1)'.
Benefits and concerns
The IDENTITY function is a good solution for a surrogate key function in most cases. DB2 import and export utilities also support the IDENTITY option.
However, it might not be convenient in one case. The application never knows what value is put into the primary key column after running the INSERT statement. If the application has to continue to insert rows into the child tables, it has to run a SELECT statement on the parent table to get the primary key value. But if this is not a concern in your system, using the IDENTITY option is a good idea.
SEQUENCE object
Solution idea
The SEQUENCE object is a feature that was introduced in DB2 UDB Version 7.2. Users can create a SEQUENCE object in the database, just like creating a table object or a view object, and then request the values from the SEQUENCE. DB2 guarantees that users get a unique sequence value every time.
Example
You can create a SEQUENCE object in the database:
If you have the EMPLOYEE table as the following:
You can insert a row with the following statement:
Here you use the 'NEXTVAL FOR EMPSERIAL' to obtain the unique value from the SEQUENCE.
You can use 'PREVVAL FOR EMPSERIAL' to get the most recently generated sequence value in your current connection session. The application will be able to know what value is put into the primary key column, and continue to insert rows in the child tables. Here, the 'in your current connection session' is important, which means the 'PREVVAL' will only return the value that is generated in the same connection sessions.
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For example, let's consider a situation where there are two applications connecting to the database and running the following SQL statements in the following order.
(Let's assume the current value of the SEQUENCE 'EMPSERIAL' is 3.)
Application 1:
The 'NEXTVAL' generated from the EMPSERIAL is 4.
Application 2:
Sql Insert Generate And Return Primary Keyboard
The 'NEXTVAL' generated from the EMPSERIAL is 5.
Application 1:
The 'PREVVAL' will return 4, not 5.
Furthermore, the PREVVAL and NEXTVAL values are not impacted by the transaction rollback.
For example, let's assume the current value of the SEQUENCE 'EMPSERIAL' is 30. An application starts a transaction:
Sql Insert Generate And Return Primary Key 2017
The application called ROLLBACK is started.
And then if you run:
The 'PREVVAL' will return 31, not 30.
Benefits and concerns
The SEQUENCE is the latest function implemented by DB2 UDB for generating unique values. It also has a caching function to improve performance (For details, please refer to the IBM DB2 UDB SQL Reference). It is more flexible than the IDENTITY function, because it is a separate object in your database. You can change its settings by running ALTER SEQUENCE statement any time when it is necessary.
If your system is going to run on DB2 UDB only, the SEQUENCE may be the best solution, because it is easy to use, it does not need extra code like the key manager, and it is easy to change as your requirements change.
Conclusion
Sql Insert Generate And Return Primary Key Excel
This article described three approaches to implementing a surrogate key as primary key. It focused on how to generate unique consequent values for the surrogate key.
The traditional method is suitable for a simple, single user (non-concurrent) system. The key manager implementation is a good choice for large and cross-platform systems. But if your project will only work on DB2 UDB, you can consider the features that DB2 UDB provides. The IDENTITY and SEQUENCE functions provide an easy and flexible solution.
There are many options that you can use when you create the IDENTITY column and SEQUENCE object. Please refer to the IBM DB2 UDB Administration Guide and IBM DB2 UDB SQL Reference for full details.