You can t define a PRIMARY KEY constraint on a nullable column in table sc

If it's a sqlserver database.

Select Table SC right-click, Design.

See if the column you want to create a key constraint on isnull has a check mark on it, and if it does, it definitely can't be created.

If the column is allowed to be null value, the primary foreign key.

None of them can be created.

Null values cause a lack of uniqueness, which is a reason why they cannot be created.

The primary key column can't be empty, you can just change it to not null.

Remove the null item from the primary key column.

Modify the corresponding column to not null.

Of course, the primary key can't be empty.

The primary key can't be repeated.,You're empty.,How to identify the primary key.。

Summer Nanting Huaixin Da (Meng Haoran).

primary key

Constraints are not allowed to be empty. If you have a table with data, you can only delete the empty fields and define the corresponding constraints.

If there is data: delete from tablename where field is null;

alter table tablename add constraint pri_1 primary key(field)

This error indicates that you already have data in your table and you want to add a primary key.

Columns constrained by SQL must have null values, while SQL specifies that columns that are constrained as primary keys cannot have null values.

Workaround: Delete the tuple to which you want to add the primary key constraint column null.

That's it); If you are just doing a test and the data is not important, you can delete all the data in the table; Use delete * from for the table name.

Then run your name to add the primary key.

There is already data in the table, and there are null columns, primary keys.

Yes, it can't be null

Each backing to take the constraint will have a key-value name corresponding, if you don't specify the system will automatically generate a unique full library not duplicated.

Defining a primary key constraint for multiple columns means defining a federated primary key, for example: primary key(id,name).

Execute first. alter table name: alter column column name: int not null

Execute again. alter table name add constraint pk primary key (lie).

The first sentence must be executed only if there is no null value in the lie column in the table.

First, set the field to no null values:

alter table [table] alter column [lie] varchar(50) not null default(n'')

The above is just an example, please modify it according to your original field type.

Then execute the SQL you wrote.

First, set the lie to be non-nullable.

Then perform the primary key setting.

First of all, the lie column must meet the conditions of the primary key column (not duplicated, not empty), alert tablenameadd constraint pk tablename, lie primary key (lie)go

Under Construction indicates that a primary key is created.

create table tablenamelie int identity(1,1) primary key,..

Defining multiple columns as primary keys is similar to defining a single column as a primary key. You can think of a multi-column union primary key as a single column.

The steps are as follows:

1. First of all, we right-click with your mouse on this studentno column, and then select the modification inside, click on it, and pop up.

Interface. 2. Then there will be a window content as shown in the figure below, here select the studentno inside, and then right-click to select the primary key.

3. After the settings are completed, select CTRL + S to save.

4. After saving, when you check the primary key of the setting, it does not show that the setting is successful, here you need to right-click on the table and select Brush.

New. 5. After refreshing, you can see the primary key you set, and you can also see that the setting has been successfully set in the primary key list.

oracle:

sql> create table test_tab (

2 id int,3 name varchar(10),4 age int,5 val varchar(10)

table created.

sql> alter table test_tab

2 add constraint pk_test_tab primary key(id);

table altered.

SQL Server is a bit of a hassle.

1> create table test_tab (

2> id int,3> name varchar(10),4> age int,5> val varchar(10)

7> go

1> alter table test_tab

2> add constraint pk_test_tab primary key(id);

3> go

Msg 8111, Level 16, State 1, Server Home-Bed592453c SqlExpress, Line 1.

Unable to be in the table'test_tab'The primary key constraint can be defined on an empty column.

Msg 1750, Level 16, State 1, Server home-bed592453c sqlexpress, line 1.

Constraints cannot be created. See the previous error message.

First, add a not null constraint to the id column, and then set it as the primary key.

1> alter table test_tab

2> alter column id int not null;

3> go

1> alter table test_tab

2> add constraint pk_test_tab primary key(id);

3> go

mysqlmysql> create table test_tab (

id int,> name varchar(10),>age int,> val varchar(10)

query ok, 0 rows affected ( sec)

mysql> alter table test_tab

add constraint pk_test_tab primary key(id);

query ok, 0 rows affected ( sec)

records: 0 duplicates: 0 warnings: 0

Create a table: Specify the primary key independently after defining the columns

create table stu(

sid char(6),sname varchar(20),age int,gender varchar(10),primary key(sid)

Specify the primary key when modifying a table:

alter table stu

add primary key(sid);

Delete the primary key (just remove the primary key constraint, not the primary key column):

alter table stu drop primary key;

For example, if you want to set the T ID column of the teacher table as the primary key, if the T ID column does not exist, then enter a new column like the following to create a new column, alter table teacher add column T id varchar(20).

If the T ID column exists, you can directly enter the following to alter table teacher add primary key(T id).

I tested it in the database so that I could set a primary key to a table, provided that the table didn't have a primary key.

For example, in Table A1 2, add a primary key to the id.

alter table [a1_2] add constraint [pk_a1_2] primary key clustered

id] asc)

Do you use one of the primary keys of or ? This definitely won't work, redesign the database and add an auto-increment column as the primary key column.

Database design refers to the construction of the optimal database schema for a given application environment, and the establishment of the database and its application system so that it can effectively store data and meet the application needs (information requirements and processing requirements) of various users. In the field of databases, the various types of systems that use databases are often referred to collectively as database application systems.

The primary key constraint is violated'pk_sc'。can not be in the object'', insert a repeat key. The statement terminated is caused by a setup error, and the workaround is:

1. Because the content of the two objects is equal, but because the new keyword is used, the memory addresses are not equal, and all can be added to the identityhashmap, and the key can be repeated.

2. In normal operation, in order to prevent the key from being duplicated, you need to use the set interface to accept and filter.

3. The example illustrates the identityhashmap, you can put a duplicate key, and put two keys as "Zhang San" objects.

4. IdentityHashMap inherits the AbstractMap class and implements the Serializable interface and the Cloneable interface.

"Defining a primary key constraint for multiple columns" refers to multiple columns joining together to form a single primary key.

For example, if there are fields such as class, student number, and name, and the student number of each class is arranged independently, then the class and student number can be combined as a primary key. For example, number 11 in class 3 so that a student can be uniquely identified.

However, it is not used much in practice.

If you look closely, there are multiple columns, that is, multiple columns that form the primary key, and there is no duplicate combination for each row of these columns.