SQL viewId 比较好看的 Id

更新 : 2019-06-29 

identity column 的局限是, 它只可以在一个 table 使用，而且它只能随着 insert 产生

另一个方法是用 SEQUENCE， sql server 2012 的功能

https://docs.microsoft.com/en-us/sql/t-sql/statements/create-sequence-transact-sql?view=sql-server-2017

https://docs.microsoft.com/en-us/sql/t-sql/statements/alter-sequence-transact-sql?view=sql-server-2017

https://docs.microsoft.com/en-us/ef/core/modeling/relational/sequences

https://www.talkingdotnet.com/use-sql-server-sequence-in-entity-framework-core-primary-key/

https://blog.csdn.net/m0_38002798/article/details/78904102

它的好处是可以跨 table, 而且不需要 insert 也可能产生. 但是也有不足的地方，就是不支持 transaction , 无法 rollback 

如果你的 running number 允许跳号, 那么 SEQUENCE 是最佳选择. 

如果以上 2 个无法满足需求，那么就得自己通过表，锁表的方式去实现了. 

有时候我们希望 Id 要好看一些，比如 Id=1 -> Id=T000001

refer : 

http://www.kodyaz.com/t-sql/custom-sequence-string-as-sql-identity-column.aspx

http://www.sqlteam.com/article/custom-auto-generated-sequences-with-sql-server

方法有很多，我目前选择的方式是使用触发器在插入之后更新 

我们把这个好看一点的Id称为 viewId 吧。

先做一个方法来格式化Id

调用 viewId = dbo.getViewId('T',CAST(inserted.Id as nvarchar(50)),6,'0'); 

drop function dbo.getViewId;
create function [dbo].[getViewId] (
 @Prefix nvarchar(10),
 @Id int,
 @Length int,
 @PaddingChar char(1) = '0'
)
returns nvarchar(MAX)
as
begin

return (
 select @Prefix + RIGHT(REPLICATE(@PaddingChar, @Length) + CAST(@Id as nvarchar(10)), @Length)
)

end

针对某个表写入触发器逻辑

drop trigger OrdersAfterInsert;
create trigger OrdersAfterInsert on [dbo].[Orders] 
for insert
as 
    update Orders set viewId = dbo.getViewId('O', CAST(inserted.Id as nvarchar(50)),6,'0') 
        from Orders inner join inserted on Orders.Id= inserted.Id;     
go

如果表已经存在数据的话，要更新哦

update Orders set viewId = dbo.getViewId('O', CAST(Id as nvarchar(50)),6,'0');

创建column and UNIQUE

alter table Orders add viewId nvarchar(50) null;
create unique nonclustered index [UNIQUE_Orders_viewId]
    on [dbo].[Orders]([viewId] asc) where ([viewId] IS NOT NULL AND [viewId] IS NOT NULL);

使用 Entity Framewrok的话可以把这个数据标签为 Computed.

public class Order
{
    [Key]
    public int Id { get; set; }
    [DatabaseGenerated(DatabaseGeneratedOption.Computed)]
    public string viewId { get; set; }
} 

以上.