TSQL – identity column, guid, sequence

–1. IDENTIY Column cannot be empty , Default value cannot be set , Cannot use after creation ALTER TABLE TableName ALTER COLUMN modify , Each table can only have one auto increment column –2. Check the current value ：SELECT IDENT_CURRENT(‘TableName’), — View incremental values ：SELECT IDENT_INCR(‘TableName’) — View the original seed value ：SELECT IDENT_SEED(‘TableName’), Starting value , TRUNCATE TABLE Initial value after . –3. allow Explicit Insert auto increment column ：SET IDENTITY_INSERT TableName ON; Set to ON after , Allow the current callback to specify a value when inserting the auto increment column , This setting only affects the current reply , And only one table can be modified at the same time in the same reply IDENTITY_INSERT attribute , When you set other tables again, you will be prompted ：” surface ‘XXX1’ Of IDENTITY_INSERT Have been to ON. Can't check the table ‘XXX2’ perform SET operation .“, After explicitly inserting values for auto incrementing columns , The current value of the auto increment column will be checked or modified to be the maximum value in the whole table . –4. IDENT_CURRENT Not limited by scope and session , And limited to the specified table . SCOPE_IDENTITY and @@IDENTITY Returns the last identity value generated in any table in the current session . however ,SCOPE_IDENTITY Returns only the values inserted into the current scope ;@@IDENTITY Not limited to a specific scope [email protected]@IDENTITY You can get the trigger triggered by the current statement , Built in stored procedures and other inverted self increment . – As for the watch T1 Insert trigger pair table T2 Also insert ,@@IDENTITY obtain T2 Since the appreciation , and SCOPE_IDENTITY Get the current scope T1 Since the appreciation .

–4. If the required value is unique in all tables , have access to UNIQUEIDENTIFIER, Use NEWID() To get the unique value . –5. To avoid GUID Page fragments and page splits as clustered indexes , have access to NEWSEQUENTIALID() To get self increasing GUID： — CREATE TABLE myTable (ColumnA uniqueidentifier DEFAULT NEWSEQUENTIALID()) — If you consider security and confidentiality , Recommendation not applicable NEWSEQUENTIALID() –6. NEWSEQUENTIALID() Only for tables DEFAULT To achieve . Use NEWSEQUENTIALID() Each of the generated GUID It is unique on this computer . Only if the source computer has a network card , Use NEWSEQUENTIALID() Generated –GUID It is the only one on multiple computers –7. Sequence ,2012 New characteristics , Database level objects , Increase automatically after each acquisition . –============================================= – Create sequence CREATE SEQUENCE [schema_name . ] sequence_name [ AS [ built_in_integer_type | user-defined_integer_type ] ] [ START WITH <constant> ] [ INCREMENT BY <constant> ] [ { MINVALUE [ <constant> ] } | { NO MINVALUE } ] [ { MAXVALUE [ <constant> ] } | { NO MAXVALUE } ] [ CYCLE | { NO CYCLE } ] [ { CACHE [ <constant> ] } | { NO CACHE } ] [ ; ] –============================================= – Get sequence NEXT VALUE FOR [ database_name . ] [ schema_name . ] sequence_name [ OVER (<over_order_by_clause>) ] –============================================= – Get the sequence value of the range sp_sequence_get_range [ @sequence_name = ] N'<sequence>’ , [ @range_size = ] range_size , [ @range_first_value = ] range_first_value OUTPUT [, [ @range_last_value = ] range_last_value OUTPUT ] [, [ @range_cycle_count = ] range_cycle_count OUTPUT ] [, [ @sequence_increment = ] sequence_increment OUTPUT ] [, [ @sequence_min_value = ] sequence_min_value OUTPUT ] [, [ @sequence_max_value = ] sequence_max_value OUTPUT ] [ ; ]

–============================================= – Create an initial value of 1 Increasing 1 Sequence CREATE SEQUENCE dbo.mySquence AS INT INCREMENT BY 1 START WITH 1

–============================================= – Get next value SELECT NEXT VALUE FOR dbo.mySquence –============================================= – Use the table to sort and get the auto increment sequence SELECT NEXT VALUE FOR dbo.mySquence OVER (ORDER BY C2) AS RID, C2 FROM [dbo].[TB4] –============================================= – It can be listed in DEFAULT Use sequences in ALTER TABLE Test.MyTable ADD DEFAULT N’AdvWorks_’ + CAST(NEXT VALUE FOR Test.CounterSeq AS NVARCHAR(20)) FOR IDColumn;

–============================================ – obtain 4 A continuous increasing sequence , And return the minimum DECLARE @range_first_value sql_variant , @range_first_value_output sql_variant ;

EXEC sp_sequence_get_range @sequence_name = N’dbo.mySquence’ , @range_size = 4 , @range_first_value = @range_first_value_output OUTPUT ;

SELECT @range_first_value_output AS FirstNumber ;

–===================================== – View the auto increment column in the current database SELECT * FROM sys.identity_columns

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