Updating an identity column sql server

Computed columns can be used in select lists, WHERE clauses, ORDER BY clauses, or any other locations in which regular expressions can be used, with the following exceptions: Based on the expressions that are used, the nullability of computed columns is determined automatically by the Database Engine.The result of most expressions is considered nullable even if only nonnullable columns are present, because possible underflows or overflows also produce NULL results.There is no penalty from having a missing value in a row.The ‘missing’ value can be queried, updated, indexed, exactly as if the update during ALTER TABLE really occurred.Prior to SQL Server 2012 when you add a new non-NULLable column with default values to an existing table a size-of data operation occurs: every row in the table is updated to add the default value of the new column.For small tables this is insignificant, but for large tables this can be so problematic as to completely prohibit the operation.

Consider this update: Key Hash Value = (8194443284a0) Slot 1 Offset 0x60 Length 15 Record Type = PRIMARY_RECORD Record Attributes = NULL_BITMAP Record Size = 15 Memory Dump @0x000000000AB8A060 0000000000000000: 10000c00 02000000 83010000 020000†††††††††††††........ƒ......The ‘magic’ is that the table metadata has changed and it now contains a column with a ‘default’ value: table (the third row in the image above) has a default with value 42.This is how SQL Server 2012 knows how to show a value for Column 3 for this record, even though is physically missing on the page.Is an expression that defines the value of a computed column.A computed column is a virtual column that is not physically stored in the table, unless the column is marked PERSISTED.

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