PRIMARY KEYs(主键) 和 UNIQUE KEYs(唯一键约束) 是类似的, PRIMARY KEY通常是一列,也有可能多列,通常由他来决定一行数据(row)。 一张表只能有一个 PRIMARY KEY,但可以有很多 UNIQUE KEY。 当给一列设置为 UNIQUE KEY 之后,不能有两行在该列上有相同的数据。 PRIMARY KEY 不允许有 NULL值,但是 UNIQUE KE...
id_unique1 INT NULL UNIQUE KEY, id_unique2 INT NULL UNIQUE KEY); ERROR 1068 (42000): Multiple primary key defined mysql> CREATE TABLE uni_pri (id_primary1 INT PRIMARY KEY, id_unique1 INT NULL UNIQUE KEY, id_unique2 INT NULL UNIQUE KEY); Query OK, 0 rows affected (...
I answered Primary Key won't allow nulls and Unique Key allows one null and I got it wrong - but it's showing correct in the answer? Andre Ranieri SSCrazy Points: 2819 More actions January 30, 2015 at 11:19 am #1774432 +1 great question. Thanks!
此示例演示索引的PrimaryKey和唯一属性。 该代码创建一个包含两列的新表。PrimaryKey和Unique属性用于将一列用作不允许重复值的主键。 复制 ' BeginPrimaryKeyVB Sub Main() On Error GoTo PrimaryKeyXError Dim catNorthwind As New ADOX.Catalog Dim tblNew As New ADOX.Table Dim idxNew As New ADOX.Index...
Uniqueness.A primary key must have a unique value, which implies that no other row in the table has the same value in the column. Simplicity.When creating a primary key, basic attributes such as whole numbers or short character strings should be used. Avoid special characters, spaces and dif...
UniqueConstraint.cs 取得數值,指示條件約束是否在主索引鍵上。 C# publicboolIsPrimaryKey {get; } 屬性值 Boolean 如果條件約束在主索引鍵上,則為true,否則為false。 範例 下列範例會檢查IsPrimaryKey屬性。 VB PrivateSubGetPrimaryKey()DimdataRelationAsDataRelationDimuniqueConstraintAsUniqueConstraint' Get a Da...
PrimaryKey 属性 (ADOX) RelatedColumn 属性 (ADOX) RelatedTable 属性 (ADOX) SortOrder 属性 (ADOX) 类型属性(列)(ADOX) 类型属性(键)(ADOX) Type 属性(表)(ADOX) 唯一 属性 (ADOX) UpdateRule 属性 (ADOX) ADOX 方法 ADOX 枚举常量 ADOX 代码示例 已弃用:RDS API 参考 RDS 对象 RDS 属性 RDS 方法 ...
create table d(id int primary key auto_increment , name varchar(32) unique) d表中name就是唯一索引,唯一索引可以有多个null,不能是重复的内容 相比主键索引,主键字段不能为null,也不能重复 2. 查询索引 show indexes from table_name; show keys from table_name; ...
However, you can change the order of columns after the primary key is created. For more information, see How to: Modify Primary Keys. See Also Tasks How to: Delete Primary Keys How to: Change Relationship Properties How to: Modify Primary Keys Reference Primary or Unique Key Properties ...
public bool IsPrimaryKey { get; set; } 属性值类型:System.Boolean.NET Framework 安全性对直接调用方的完全信任。此成员不能由部分信任的代码使用。有关更多信息,请参见通过部分受信任的代码使用库。请参见参考UniqueConstraint 类Microsoft.Data.Schema.ScriptDom.Sql 命名空间中文...