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

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).

As previously defined (see “Surrogate key” under “Definitions” on page 78), a surrogate key is an artificial, simple key used to identify individual entities when there is no natural key or when the situation demands a simple key, but no natural simple key exists. Surrogate keys do not identify individual entities in a meaningful way: they are simply an arbitrary method to distinguish among those entities (see Hall. A surrogate key is an artificial simple key used to identify individual rows uniquely when there is no natural key or when the situation demands a simple key, but no natural non‑composite key exists. Surrogate keys do not identify individual rows in a meaningful way: they are simply an arbitrary method to distinguish among them. Surrogate keys are typically arbitrary system‑generated sequential integers. You can generate surrogate keys in Teradata Database using the identity column. You can generate surrogate keys in Teradata Database using the identity column feature (see Identity Columns). To use an identity column to generate surrogate key values, specify the GENERATED ALWAYS and NO CYCLE options and ensure that the data type for the column is either NUMERIC(18,0) or DECIMAL(18,0). Jun 12, 2009 Putting together a few steps to generate surrogate key: Most of you might've dealt with it already. But, sending it as it might be a quick reference incase of future use. Generating it as such isn’t a big deal, it might get a little tricky when you are trying to insert new values in continuation of already existing surrogate key.

It’s sometimes desirable not Generate key file from certificate. 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. /free-random-steam-key-generator.html. 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:

Database Surrogate Key

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

Surrogate Key Definition

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.