Sql 函数对象_ID的第二个参数

我在Itzik Ben Gan的《TSQL基础知识》一书中遇到了以下代码

我知道函数

OBJECT\u ID

用于检查特定对象是否存在。然而，我对它的第二个参数有点困惑

N'U'

我查了一下，它说第二个参数代表对象类型，但我仍然不明白

N'U'

代表什么

有人能给我解释一下它到底代表了什么吗

IF OBJECT_ID(N'dbo.Employees', N'U') IS NOT NULL
BEGIN
    IF OBJECTPROPERTY(OBJECT_ID(N'dbo.Employees', N'U'), N'TableTemporalType') = 2
        ALTER TABLE dbo.Employees SET(SYSTEM_VERSIONING = OFF);

---

如果您按照链接到的页面中的链接进行操作

类型描述 ---- ------------------------------------------------------------ TT型_表 FN SQL_标量函数 SN同义词 如果SQL\u内联\u表\u值\u函数 唯一约束 SQ服务队列 F外键约束 用户表 D默认_约束 主键约束 V视图 S系统表 这是一张桌子 P SQL_存储过程 TF SQL_表值函数

U = Table (user-defined)

**Complete list (as of 2017-01-16)**

Object type:

AF = Aggregate function (CLR)
C  = CHECK constraint
D  = DEFAULT (constraint or stand-alone)
F  = FOREIGN KEY constraint
FN = SQL scalar function
FS = Assembly (CLR) scalar-function
FT = Assembly (CLR) table-valued function
IF = SQL inline table-valued function
IT = Internal table
P  = SQL Stored Procedure
PC = Assembly (CLR) stored-procedure
PG = Plan guide
PK = PRIMARY KEY constraint
R  = Rule (old-style, stand-alone)
RF = Replication-filter-procedure
S  = System base table
SN = Synonym
SO = Sequence object

 Applies to: SQL Server 2012 through SQL Server 2016.

SQ = Service queue
TA = Assembly (CLR) DML trigger
TF = SQL table-valued-function
TR = SQL DML trigger
TT = Table type
U  = Table (user-defined)
UQ = UNIQUE constraint
V  = View
X  = Extended stored procedure

 Applies to: SQL Server 2016 through SQL Server 2016, Azure SQL
Database, Azure SQL Data Warehouse, Parallel Data Warehouse.

ET = External Table

SELECT DISTINCT type,type_desc FROM sys.objects

type type_desc ---- ------------------------------------------------------------ TT TYPE_TABLE FN SQL_SCALAR_FUNCTION SN SYNONYM IF SQL_INLINE_TABLE_VALUED_FUNCTION UQ UNIQUE_CONSTRAINT SQ SERVICE_QUEUE F FOREIGN_KEY_CONSTRAINT U USER_TABLE D DEFAULT_CONSTRAINT PK PRIMARY_KEY_CONSTRAINT V VIEW S SYSTEM_TABLE IT INTERNAL_TABLE P SQL_STORED_PROCEDURE TF SQL_TABLE_VALUED_FUNCTION