Our journey will take us through the practical aspects of primary keys, including how to create them in SQL. You’ll learn about the syntax of the primary key constraint and how to define a primary key when creating a table. You will understand, by the end of this post, not just the ...
Note:Although naming a constraint using[CONSTRAINT constraint_name]is optional, doing so makes it easier to make changes to and delete the constraint. Primary Key Error In SQL, we will get an error If we try to insertNULLor duplicate values in the primary key column. NOT NULL Constraint Err...
SQL PRIMARY KEY 约束 PRIMARY KEY 约束唯一标识数据库表中的每条记录 SQL PRIMARY KEY Constraint on CREATE TABLE SQL PRIMARY KEY Constraint on ALTER TABLE 主键必选包含唯一的值 主键列不能包含NULL值 每个表都应该有一个主键,并且每个表只能有一个主键 在“Persons”表创建时在"Id_P"列创建PRIMARY KEY约束...
The Syntax to create the Alter Table in SQL is: Alter table table_name Add constraint constraint_name Primary key (column1, column2,….column_n) Table names are the ones that add a primary key. Constraint name is the name of the primary key. The columns are the ones that make up the...
PRIMARYKEY(ID) ); SQL Server / Oracle / MS Access: CREATETABLEPersons ( ID intNOTNULLPRIMARYKEY, LastName varchar(255)NOTNULL, FirstName varchar(255), Age int ); To allow naming of aPRIMARY KEYconstraint, and for defining aPRIMARY KEYconstraint on multiple columns, use the following SQL...
constraint_name The name of the primary key. pk_col1, pk_col2, ... pk_col_n The columns that make up the primary key. Example Let's look at an example of how to create a primary key using the CREATE TABLE statement in SQL. We will start with a very simple one where our primar...
SQL>Constraint>Primary Key A primary key is used to uniquely identify each row in a table. It can either be part of the actual record itself , or it can be an artificial field (one that has no meaning other than being an identifier of the record). How to set primary keys is an imp...
Sname CHAR(20) CONSTRAINT C2 NOT NULL, Sage NUMERIC(3) CONSTRAINT C3 CHECK (Sage < 30), Ssex CHAR(2) CONSTRAINT C4 CHECK (Ssex IN ( '男','女')), CONSTRAINT StudentKey PRIMARY KEY(Sno) );在Student表上建立了5个约束条件,包括主码约束(命名为StudentKey)以及C1、C2、C3、C4四个列级约束...
CONSTRAINT PK_Person PRIMARY KEY (ID,LastName) ); 注意:在上面的示例中,只有一个主键(PK_Person)。但是,主键的值由两列组成(ID+LastName)。已存在的表中添加 PRIMARY KEY 约束 如果在表已存在的情况下为 "ID" 列创建 PRIMARY KEY 约束,请使用下面的 SQL:MySQL...
SQL PRIMARY KEY Constraint SQL 主键约束 主键包含的都是Unique 不重复,Not NULL 不为空的数据。 The PRIMARY KEY constraint uniquely identifies each record in a table. 一个表格只能有一个主键,逐渐可以包含一个或者多个列。 SQL PRIMARY KEY on CREATE TABLE ...