- A primary key can’t be created on a null able column wherein a unique key can be created on a null able column. - There can be only one primary key on a table wherein a table can have multiple unique key defined on it. Let’s create a table with a primary key and a unique...
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SQL Server contains the NEWID() function. This function creates a unique value of type uniqueidentifier. We can use this function in several ways to generate unique numbers to suit our requirements : NEWID() generate alphanumeric value of 36 char. ...
id int not null auto_increment primary key, name varchar(16) not null )default charset=utf8; create table boy_girl( id int not null auto_increment primary key, boy_id int not null, girl_id int not null, constraint fk_boy foreign key boy_girl(boy_id) references boy(id), constraint ...
Represents the unique or primary key constraint. This class translates 80 syntax into 90 syntax, this should be reflected in the AST, if a differentiator is to be written.C# 複製 [System.Serializable] public class UniqueConstraintDefinition : Microsoft.SqlServer.TransactSql.ScriptDom...
A uniqueifier (or uniquifier as reported by SQL Server internal tools) is a 4-byte value used to make each key unique in a clustered index that allow duplicate key values. This uniqueness is required because each clustering key must point to exactly one record, without the uniqueifier a ...
例如CREATE UNIQUE INDEX 索引的名字 ON tablename (列的列表); (2)修改表,例如ALTER TABLE tablename ADD UNIQUE 索引的名字 (列的列表); (3)创建表的时候指定索引,例如CREATE TABLE tablename ( [...], UNIQUE 索引的名字 (列的列 表) );3、主键主键是一种唯一性索引,但它必须指定为“PRIMARY KEY”...
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Constraints are rules that the SQL Server Database Engine enforces for you. For example, you can useUNIQUEconstraints to make sure that no duplicate values are entered in specific columns that don't participate in a primary key. Although both aUNIQUEconstraint and aPRIMARY KEYconstraint enforce ...
http://download.oracle.com/docs/cd/E11882_01/server.112/e16508/datainte.htm#CNCPT1642 uniquekey constraint requires that every value in a column or set of columns beunique. No rows of a table may have duplicate values in a column (the uniquekey) or set of columns (the...