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Eye 5, This works if you specify the auto-incrementing column first in the primary key definition. Maybe this changed, I just tested it in 5. I thus avoid using it. It is important that the Logical Model for every Entity have at least one set of "business attributes" which comprise a Key for the entity.
When we then build tables for these Entities their Candidate Keys become Natural Keys in those tables. It is only through those Natural Keys that users are able to uniquely identify rows in the tables; as surrogate keys should always be hidden from users.
This is because Surrogate Keys have no business meaning. However the Physical Model for our tables will in many instances be inefficient without a Surrogate Key. Recall that non-covered columns for a non-clustered index can only be found in general through a Key Lookup into the clustered index ignore tables implemented as heaps for a moment.
When our available Natural Key s are wide this 1 widens the width of our non-clustered leaf nodes, increasing storage requirements and read accesses for seeks and scans of that non-clustered index; and 2 reduces fan-out from our clustered index increasing index height and index size, again increasing reads and storage requirements for our clustered indexes; and 3 increases cache requirements for our clustered indexes.
When set as the clustering key, so as to be used for key lookups into the clustered index from non-clustered indexes and foreign key lookups from related tables, all these disadvantages disappear. Our clustered index fan-outs increase again to reduce clustered index height and size, reduce cache load for our clustered indexes, decrease reads when accessing data through any mechanism whether index scan, index seek, non-clustered key lookup or foreign key lookup and decrease storage requirements for both clustered and nonclustered indexes of our tables.
Note that these benefits only occur when the surrogate key is both small and the clustering key. If a GUID is used as the clustering key the situation will often be worse than if the smallest available Natural Key had been used.
If the table is organized as a heap then the 8-byte heap RowID will be used for key lookups, which is better than a byte GUID but less performant than a 4-byte integer.
If a GUID must be used due to business constraints than the search for a better clustering key is worthwhile. If for example a small site identifier and 4-byte "site-sequence-number" is feasible then that design might give better performance than a GUID as Surrogate Key.
If the consequences of a heap hash join perhaps make that the preferred storage then the costs of a wider clustering key need to be balanced into the trade-off analysis. It is worth noting that Anita possibly only wishes to add the LastName to this constraint in order to make that a covered field, which is unnecessary in a clustered index because ALL fields are covered by it.
The ability in SQL Server to designate a Primary Key as nonclustered is an unfortunate historical circumstance, due to a conflation of the meaning "preferred natural or candidate key" from the Logical Model with the meaning "lookup key in storage" from the Physical Model.The evidence is overwhelming.
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Can I have multiple primary keys in a single table? Primary care is the day-to-day healthcare given by a health care lausannecongress2018.comlly this provider acts as the first contact and principal point of continuing care for patients within a healthcare system, and coordinates other specialist care that the patient may need.
Patients commonly receive primary care from professionals such as a primary care physician (general practitioner or family.