数据库插入语句的重复数据问题

在数据库插入语句中，如何保证表中有该数据时不再插入新数据？有什么关键字可以用到吗？

将不能重复的列设置为主键，或者添加唯一索引

Create Table #TempTable
(
    StudentName nvarchar(20) Not Null,
    SubjectName nvarchar(20) Not Null,
    Score int  null
)

Declare @StudentName As nvarchar(20)
Declare @SubjectName As nvarchar(20)
Declare @Score As int
Select @StudentName = N'小明'
Select @SubjectName = N'语文'
Select @Score = 88

Merge Into  #TempTable T1
Using (Select * From (Values(@StudentName,@SubjectName,@Score)) TempTable(StudentName,SubjectName,Score)) T2 On T2.StudentName=T1.StudentName And T2.SubjectName=T1.SubjectName
When Matched Then
    Update Set T1.Score = T2.Score
When Not Matched Then
    Insert (StudentName,SubjectName,Score)
        Values (T2.StudentName,T2.SubjectName,T2.Score);