Generate Surrogate Key In Teradata
Notes on Data Vault and Teradata. Posted on 2012/09/20; by Dan Linstedt; in Data Vault, DV Standards; in this entry i will explore the use of implementation of data vault on teradata. However, this entry is applicable to everyone in the data vault community, as it covers definitions and descriptions of primary keys, indexing, surrogate keys, natural keys, and performance and tuning. Nov 19, 2019 Teradata Identity Columns The advantage of an IDENTITY column is that a new surrogate key is generated by the system as soon as a row is inserted into the key table. However, it is important to define the IDENTITY column so that no duplicates are created. For this, the IDENTITY column must be defined with GENERATED. ➠ How to generate surrogate key in Teradata By using analytical functions By using CSUM analytical function, Syntax/Example 1: SELECT (SELECT ZEROIFNULL(MAX(empno)) FROM employee) + CSUM(1,1) AS surrogatekey1newempno,empno,empname from employee; Single AMP (usually vproc 0). Surrogate keys are typically arbitrary system-generated sequential integers. See “CREATE TABLE (Column Definition Clause)” in SQL Data Definition Language Detailed Topics for information about how to generate surrogate keys in Teradata Database. Rules for Primary Keys There are several rules that define the bounds of primary keys.
This article demonstrates how to “roll your own” surrogate keys and sequences in a platform-independent way, using standard SQL.
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CSUM and ROWNUMBER comparison in Teradata - Example script with statistics and syntax - Surrogate key generation in Teradata We keep getting questions about which one to use for sequence number generation in Teradata. Many people use CSUM(1,1) as its easier and other databases support that. But, in Teradata. Now in teradata, I can't seem to get it to work. There isn't a column that I can sort on and have unique values (lots of duplication) unless I use 3 columns together. The old way would be something like, create table temp1 as select rownum as insertnum, col1, col2, col3 from tables a join b on a.id=b.id.
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.
Download microsoft office professional plus 2010 product key generator. A surrogate key uniquely identifies each entity in the dimension table regardless of its source key. A separate field can be used to contain the key used in the source system.Systems developed independently in company divisions may not use the same keys, or they may use keys that conflict with data in the systems of other divisions.
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:
How To Generate Surrogate Key In Teradata
There’s no good primary key in this data. I will have to add a surrogate key. Passport photo studio 1.5.1. 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:
Generate Surrogate Key In Teradata Data
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.