This article demonstrates how to “roll your own” surrogate keys and sequences in a platform-independent way, using standard SQL.

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The only significance of the surrogate key is to act as the primary key. It is also possible that the surrogate key exists in addition to the database-generated UUID (for example, an HR number for each employee other than the UUID of each employee). Computer-generated files could be assigned, e.g., more metallic sounds than user-generated files. When the user deletes a file by putting it in the trash can, the computer can play a dramatic crashing sound if the file was large, and a puny sound if the file was small, just as physical trash cans sound different, depending on what is thrown away. Oct 14, 2017  Why Surrogate Keys are used in Data Warehouse. Slowly changing dimensions explained with real examples - Duration: 25:43. Tech Coach 58,796 views. Database Design 25 - Surrogate Key. Surrogate keys are just simple sequential number. Surrogate keys are only used to act as a primary key. Example: BranchId is a Surrogate Key in BranchInfo table and StudentId is a Surrogate key of StudentInformation table. Foreign Keys: Foreign key is used to generate the relationship between the tables.

Surrogate keys

Relational theory talks about something called a “candidate key.” In SQL terms, a candidate key is any combination of columns that uniquely identifies a row (SQL and the relational model aren’t the same thing, but I’ll put that aside for this article). The data’s primary key is the minimal candidate key. Many people think a primary key is something the DBA defines, but that’s not true. The primary key is a property of the data, not the table that holds the data.

Unfortunately, the minimal candidate key is sometimes not a good primary key in the real world. For example, if the primary key is 6 columns wide and I need to refer to a row from another table, it’s impractical to make a 6-column wide foreign key. For this reason, database designers sometimes introduce a surrogate key, which uniquely identifies every row in the table and is “more minimal” than the inherently unique aspect of the data. The usual choice is a monotonically increasing integer, which is small and easy to use in foreign keys.

Every RDBMS of which I’m aware offers a feature to make surrogate keys easier by automatically generating the next larger value upon insert. In SQL Server, it’s called an IDENTITY column. In MySQL, it’s called AUTO_INCREMENT. It’s possible to generate the value in SQL, but it’s easier and generally safer to let the RDBMS do it instead. This does lead to some issues itself, such as the need to find out the value that was generated by the last insertion, but those are usually not hard to solve (LAST_INSERT_ID() and similar functions, for example).

It’s sometimes desirable not to use the provided feature. For instance, I might want to be sure I always use the next available number. In that case, I can’t use the built-in features, because they don’t generate the next available number under some circumstances. For example, SQL Server doesn’t decrement the internal counter when transactions are rolled back, leaving holes in the data (see my article on finding missing numbers in a sequence). Neither MySQL nor SQL Server decrements the counter when rows are deleted.

In these cases, it’s possible to generate the next value in the insert statement. Suppose my table looks like this:

The next value for c1 is simply the maximum value + 1. If there is no maximum value, it is 1, which is the same as 0 + 1.

There are platform-dependent ways to write that statement as well, such as using SQL Server’s ISNULL function or MySQL’s IFNULL. This code can be combined into an INSERT statement, such as the following statement to insert 3 into the second column:

The code above is a single atomic statement and will prevent any two concurrent inserts from getting the same value for c1. It is not safe to find the next value in one statement and use it in another, unless both statements are in a transaction. I would consider that a bad idea, though. There’s no need for a transaction in the statement above.

Downsides to this approach are inability to find the value of c1 immediately after inserting, and inability to insert multiple rows at once. The first problem is inherently caused by inserting meaningless data, and is always a problem, even with the built-in surrogate keys where the RDBMS provides a mechanism to retrieve the value.

Sequences: a better surrogate key

Surrogate keys are often considered very bad practice, for a variety of good reasons I won’t discuss here. Sometimes, though, there is just nothing for it but to artificially unique-ify the data. In these cases, a sequence number can often be a less evil approach. A sequence is just a surrogate key that restarts at 1 for each group of related records. For example, consider a table of log entries related to records in my t1 table:

At this point I might want to enter some more records (0, 11) into t1:

Now suppose I want the following three log entries for the first row in t1:

There’s no good primary key in this data. I will have to add a surrogate key. It might seem I could add a date-time column instead, but that’s a dangerous design. It breaks as soon as two records are inserted within a timespan less than the maximum resolution of the data type. It also breaks if two records are inserted in a single transaction where the time is consistent from the first to the last statement. I’m much happier with a sequence column. The following statement will insert the log records as desired:

If I want to enter a log record on another record in t1, the sequence will start at 1 for it:

MySQL actually allows an AUTO_INCREMENT value to serve as a sequence for certain table types (MyISAM and BDB). To do tihs, just make the column the last column in a multi-column primary key. I’m not aware of any other RDBMS that does this.

Summary: in this tutorial, you will learn how to use the Oracle identity column to easily define an automatic generated numeric column for a table.

Introduction to Oracle identity column

Oracle 12c introduced a new way that allows you to define an identity column for a table, which is similar to the AUTO_INCREMENT column in MySQL or IDENTITY column in SQL Server.

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The identity column is very useful for the surrogate primary key column. When you insert a new row into the identity column, Oracle auto-generates and insert a sequential value into the column.

To define an identity column, you use the identity clause as shown below:

First, the GENERATED keyword is mandatory.

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Second, you can specify an option to generate identity values:

• GENERATED ALWAYS: Oracle always generates a value for the identity column. Attempt to insert a value into the identity column will cause an error.
• GENERATED BY DEFAULT: Oracle generates a value for the identity column if you provide no value. If you provide a value, Oracle will insert that value into the identity column. For this option, Oracle will issue an error if you insert a NULL value into the identity column.
• GENERATED BY DEFAULT ON NULL: Oracle generates a value for the identity column if you provide a NULL value or no value at all.

Third, you can have a number of options for the identity column.

• START WITH initial_value controls the initial value to use for the identity column. The default initial value is 1.
• INCREMENT BY internval_value defines the interval between generated values. By default, the interval value is 1.
• CACHE defines a number of values that Oracle should generate beforehand to improve the performance. You use this option for the column that has a high number of inserts.

Oracle identity column examples

Let’s take some examples of using the Oracle identity columns.

A) GENERATED ALWAYS example

The following statement creates a table named identity_demo that consists of an identity column:

The following statement inserts a new row into the identity_demo table:

Because we did not specify a value for the id column, Oracle automatically generated a sequential value starting from 1.

The following statement attempts to insert a value into the id identity column:

Oracle issued an error:

Because the id column was defined as GENERATED ALWAYS, it could not accept any provided value.

B) GENERATED BY DEFAULT example

Let’s change the id column to GENERATED BY DEFAULT:

The following statement inserts a new row into the identity_demo table:

It worked as expected.

The following statement inserts a new row into the identity_demo table with a provided value for the id column:

In this example, Oracle used the provided value and inserted it to the table.

The following example attempts to insert a null value into the id column:

Oracle issued an error:

C) GENERATED BY DEFAULT ON NULL example

First, change the id column of the identity_demo table to GENERATED BY DEFAULT ON NULL:

The following statement provides no value for the id column, Oracle will automatically generate a value for insert:

The following statement inserts a NULL value into the id column because the id column has been defined as GENERATED BY DEFAULT ON NULL, Oracle generates a sequential value and uses it for insert:

D) START WITH option example

First, recreates the identity_demo table whose the id column is defined as identity column with the initial value starts from 100:

Second, insert a row into to the identity_demo table:

Third, query data from the identity_demo table:

As you can see, the initial value of the id column is 100 as specified in identity clause.

E) INCREMENT BY option example

First, change the id column of the identity_demo table that includes both START WITH and INCREMENT BY options.

Second, insert two rows into the identity_demo table:

Third, query data from the table to verify the inserts:

As you can see, the first row has the id value 10. The second row has the id value 20. This is what we defined for the id column that should start with 10 and increase by 10 for the new row.

Oracle identity column restrictions

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• Each table has one and only one identity column.
• The data type of the identity column must be a numeric data type. the user-defined data type is not allowed to use with the identity clause.
• The identity column is not inherited by the CREATE TABLE AS SELECTstatement.
• The identity column cannot have another DEFAULT constraint.
• The encryption algorithm for encrypted identity columns can be inferred therefore you should use a strong encryption algorithm.
• The inline constraint of the identity column must not conflict with the NOT NULL and NOT DEFERRABLE constraint stated by the identity clause.

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In this tutorial, you have learned how to use the Oracle identity column that allows you easily define an automatic generated numeric column for a table.