ALTER TABLE时发生错误-消息102，级别15，状态1

Question

I'm using SQL Server and trying to change an existing table:

ALTER TABLE [Users] 
(
    [User_ID] INT IDENTITY(1,1) PRIMARY KEY,
    [UserName] [NVARCHAR](30) NOT NULL UNIQUE, -- the only change is "UNIQUE"
    [UserEmail] [NVARCHAR](30) NOT NULL UNIQUE,  -- the only change is "UNIQUE"
    [Password] [NVARCHAR](30) NOT NULL,
)

And get this error:

Msg 102, Level 15, State 1, Line 10
Incorrect syntax near '('.

I'm changing the syntax each time but still get this error

Msg 102, Level 15, State 1

What is the problem ?

Answer 1

You just need to add a UNIQUE constraint as

ALTER TABLE [Users] 
ADD CONSTRAINT U_UserName UNIQUE(UserName),
    CONSTRAINT U_UserEmail UNIQUE(UserEmail);

Answer 2

Your syntax for ALTER TABLE is completely wrong - if you just want to add UNIQUE to the two columns in question - use this:

ALTER TABLE [Users] 
ADD CONSTRAINT UQ_Users_UserName UNIQUE (UserName);

ALTER TABLE [Users] 
ADD CONSTRAINT UQ_Users_UserEmail UNIQUE (UserEmail);

Read all about the ALTER TABLE command on Docs@Microsoft

Answer 3

Alter are different with Create Statement..

Definition :

ALTER TABLE statement is used to add, delete, or modify columns in an existing table also used to add and drop various constraints on an existing table

You can check here for demo it is works : DEMO

Answer 4

Cannot add Unique Key constraint at column level. Unique Key Constraint are defined at table level.

You may alter table to add Unique key in below ways.

ALTER TABLE table_name
ADD CONSTRAINT constraint_name UNIQUE (column1, column2, ... column_n);

for eg

ALTER TABLE Users
ADD CONSTRAINT Users_Unique UNIQUE (UserName, UserEmail);