To generate a surrogate key, Microsoft Access uses a(n) data type. AutoNumber If a table has multiple candidate keys and one of those candidate keys is a composite key, the table can have based on this composite candidate key even when the primary key chosen is a single attribute. Aug 14, 2009 Hi folks, I'm loading a slowly changing dimension table and I need to be able to create a surrogate key. Is there a function or a statement I can call to create this surrogate key? Do I have to create a procedure? Thanks, jnanasakti The easiest way is to add a new integer identity column. ALTER TABLE yourTable ADD NewColumn INT IDENTITY(1,1) NOT.
-->Applies To: Microsoft Dynamics AX 2012 R3, Microsoft Dynamics AX 2012 R2, Microsoft Dynamics AX 2012 Feature Pack, Microsoft Dynamics AX 2012
This topic describes several terms and concepts of keys on data tables, as they apply to Microsoft Dynamics AX.
All keys are unique keys, meaning they disallow duplicate values and null values.
Terminology for Major Concepts of Keys
This section describes the terminology for keys that appear in property names in the AOT Properties window.
Primary Key
A primary key is one type of key. The other type of key is an alternate key. There is a maximum of one primary key per table, whereas a table can have several alternate keys. The primary key is usually the type of key that other tables, called child tables, refer to when a foreign key field in those other tables need a relational identifier.
Starting in Microsoft Dynamics AX 2012 the primary key for every new table is always enforced by an index that has exactly one field. The one field is usually an incremented number or a completely meaningless number that is generated by the system. For new tables the default is a primary key based on the RecId field. This is represented as the surrogate key in the user interface.
The following table describes the PrimaryIndex property and other major properties that are related to keys.
Property | Description |
|---|---|
PrimaryIndex | The drop-down list contains the surrogate key plus every index on the table that has its AlternateKey property set to Yes. |
CreateRecIdIndex | This property controls whether the system creates a unique index on the RecId field. The default value is Yes. This is the basis of the surrogate key. No other field is added to this index, not even DataAreaId. |
ReplacementKey | The drop-down list contains every index that has its AlternateKey property set to Yes. You might change the default blank value to an index whose field values within each record provide a name or other moniker that is meaningful to people. If a ReplacementKey is chosen, its fields can appear on forms to helpfully identify each record. The ReplacementKey should be a set of fields that represent the natural key. |
ClusterIndex | The ClusterIndex value is given to the underlying Microsoft SQL Server database system as a performance tuning choice. This choice generally controls the physical sequence in which the records are stored in the underlying database. |
The following AOT image highlights the table properties that are related to keys.
Properties of the AtomicElement demonstration table
Alternate Key
A table can have several alternate keys. Any one alternate key can switch to being the primary key, if the alternate key is comprised of only one field.
A table can reference the alternate key of another table. However, it is more common for a table to reference the primary key of another table. As an option, an alternate key can be chosen as the ReplacementKey of a table.
In practice each alternate key relies on a unique index for its implementation and enforcement. However, a unique index alone does not make an alternate key. The AlternateKey property must be set to Yes to make a unique index be an alternate key.
The following table describes properties on the AOT node for an index.
Property | Description |
|---|---|
AllowDuplicates | No means that the combined fields of the index must together make a value in each record which no other record has. |
AlternateKey Adobe photoshop cs2 keygen download. | Yes means that other tables can create foreign key relations that reference this key, as an alternative to referencing the primary key. Indexes with two or more fields cannot have their AlternateKey property value set to Yes. |
ValidTimeStateKey | A key that is marked as a valid time state key is not a candidate key for child tables to reference in their foreign key relations. Instead, this key is meant for managing date effective data in its own table. The default is No. This field can be Yes only if the ValidTimeStateFieldType property is Yes on the table. Yes means this key contains the ValidFrom and ValidTo fields. The ValidTimeStateKey property cannot be set to Yes when the AlternateKey property is set to No. |
The following image shows that the SymIdx index is an alternate key. Its AlternateKey property is set to Yes.
The properties of the SymIdx index
Relation
In Microsoft Dynamics AX a relation represents a foreign key. The following image shows that the AtomStIdx alternate key of the AtomicState parent table is referenced by this foreign key of the AtomicElement child table. The foreign key is comprised of the AtomicStateName field.
The properties for the AtomStFkyRel relation
The following image displays the AtomStIdx alternate key on the AtomicState table. The previous AtomStFkyRel relation references this alternate key.
The properties of the AtomStIdx alternate key and index
For more information about the properties of table relations, see Table Relation Properties.
ReplacementKey
Surrogate Key Example
A replacement key is an alternate key that the system can display on forms instead of a meaningless numeric primary key value. Each table can have a maximum of one replacement key.
The replacement key is chosen by setting the ReplacementKey property on the table. The drop-down list offers every alternate key as an available value. In the previous image of the AtomicElement table properties, the ReplacementKey property is SymIdx.
Other Terminology for Keys
In Microsoft Dynamics AX, there are other terms that are used to describe table keys. These terms do not appear as property names in Microsoft Dynamics AX. These terms are described in the following table.
Term | Description |
|---|---|
foreign key | In Microsoft Dynamics AX, an AOT node under MyTable > Relations represents a foreign key. For more information, see the previous Relations section in this topic. |
natural key | A key whose value has meaning to people. Most replacement keys are natural keys. |
surrogate key | A key whose value has no meaning to people. A large number generated by the system, such as RecId, could be a surrogate key. |
unique key | A broad term that applies to primary keys and to alternate keys. It does not apply to foreign keys. This term emphasizes that all values for a given key must be unique within one table. All fields in a unique key must be not-nullable. |
See also
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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).
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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:
To Generate A Surrogate Key Microsoft Access Uses A Word
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.