从 SQL Server SEQUENCE 获取当前值

Question

我想从序列中获取当前值 - 与序列属性窗口 SQL Server Management Studio 中显示的值相同

我的序列是使用以下语句创建的：

CREATE SEQUENCE [OrderNumberSequence]
    as int
    START WITH 4000
    INCREMENT BY 1
    MINVALUE 0
    NO MAXVALUE
    NO CACHE;
GO

我已经尝试过这个 来自 MSDN 的 SQL – 但结果是每次运行查询时我的数字都会增加 5

DECLARE  
  @FirstSeqNum sql_variant
, @LastSeqNum sql_variant
, @CycleCount int
, @SeqIncr sql_variant
, @SeqMinVal sql_variant
, @SeqMaxVal sql_variant ;

EXEC sys.sp_sequence_get_range
@sequence_name = N'[OrderNumberSequence]'
, @range_size = 5
, @range_first_value = @FirstSeqNum OUTPUT 
, @range_last_value = @LastSeqNum OUTPUT 
, @range_cycle_count = @CycleCount OUTPUT
, @sequence_increment = @SeqIncr OUTPUT
, @sequence_min_value = @SeqMinVal OUTPUT
, @sequence_max_value = @SeqMaxVal OUTPUT ;

-- The following statement returns the output values
SELECT
  @FirstSeqNum AS FirstVal
, @LastSeqNum AS LastVal
, @CycleCount AS CycleCount
, @SeqIncr AS SeqIncrement
, @SeqMinVal AS MinSeq
, @SeqMaxVal AS MaxSeq ;

有没有一种方法可以在不改变数字的情况下获取值？

Answer 1

您可以从

current_value

中选择

sys.sequences

：

SELECT current_value FROM sys.sequences WHERE name = 'OrderNumberSequence' ;

演示

Answer 2

如果你使用 PostgreSQL，你可以使用这个：

SELECT last_value FROM <sequence_name>;

Answer 3

这是一个很老的问题，但是如果您想知道如何获取序列的正确值，即使没有从中获取任何值，您也应该考虑列

last_value_used

。
第一次创建序列时，简单语句

SELECT current_value FROM sys.sequences WHERE name = 'OrderNumberSequence' ;

将返回序列在第一个

next value from ..

之后返回的第一个值。然后，在第一个

next value from

之后，该语句将返回相同的结果！

相反，当从未从序列中获取任何值时，最好使用

NULL

，例如当序列刚刚创建或重新启动时。

表

sys.sequences

有一个

last_value_used

列，当未从序列中获取值或序列刚刚重新启动时，该列为

NULL

。
换句话说，虽然

current_value

列报告将使用第一个“

next value from

”上的序列的第一个值，并且从那时起将报告“当前实际值”，而

last_value_used

列将是

NULL

直到您要求第一个值。
从那时起，

last_value_used

将始终等于

current_value

。

您可以使用此处的示例片段自行测试。

drop sequence if exists dbo.MySequence
go

-- 1. create sequence without touching it
CREATE SEQUENCE [dbo].[MySequence] 
 AS [int]
 START WITH 1
 INCREMENT BY 1
 MINVALUE 1
 MAXVALUE 2147483647
 CYCLE 
 CACHE 
GO

-- 2. observe current_value and last_used_value columns (should be current_value = 1, last_used_value = null)
SELECT 'Observe initial state of sequence after creation' msg, current_value, last_used_value FROM sys.sequences WHERE name = 'MySequence'

-- 3. get current value of sequence, based on last_used_value nullable state
declare @CurrentValue sql_variant
SELECT @CurrentValue = CASE WHEN last_used_value IS NULL THEN NULL ELSE current_value END
FROM sys.sequences WHERE name = 'MySequence'

select 'Current actual value' msg, @CurrentValue current_value -- should be null

-- 4. get next value from sequence
select 'Get first value from sequence' msg, next value for dbo.MySequence value -- should be 1

-- 5. observe current_value and last_used_value columns (should be current_value = 1, last_used_value = 1)
SELECT 'Observe state of sequence after getting first value' msg, current_value, last_used_value FROM sys.sequences WHERE name = 'MySequence'

-- 6. get current value of sequence, based on last_used_value nullable state
SELECT @CurrentValue = CASE WHEN last_used_value IS NULL THEN NULL ELSE current_value END
FROM sys.sequences WHERE name = 'MySequence'

select 'Current actual value' msg, @CurrentValue current_value -- should be 1

-- 7. get another value from seqeunce
select 'Get second value from sequence' msg, next value for dbo.MySequence value -- should be 2

-- 8. observe current_value and last_used_value columns (should be current_value = 2, last_used_value = 2)
SELECT 'Observe state of sequence after getting second value' msg, current_value, last_used_value FROM sys.sequences WHERE name = 'MySequence'

-- 9. get current value of sequence, based on last_used_value nullable state
SELECT @CurrentValue = CASE WHEN last_used_value IS NULL THEN NULL ELSE current_value END
FROM sys.sequences WHERE name = 'MySequence'

select 'Current actual value' msg, @CurrentValue current_value -- should be 2