2009年11月7日 · A foreign key is a column (or set of columns) in a second table whose value is required to match the value of the primary key in the original table. Usually a foreign key is in a table that is different from the table whose primary key is required to match. A table can have multiple foreign keys. The primary key cannot accept null values.

2015年4月25日 · Foreign Key. Foreign key is a field in the table that is primary key in another table. Foreign key can accept multiple null value. Foreign key do not automatically create an index, clustered or non-clustered. You can manually create an index on foreign key. We can have more than one foreign key in a table.

2011年12月22日 · CREATE TABLE child ( id int PRIMARY KEY, parent_id int, date date, FOREIGN KEY (parent_id, date) REFERENCES parent(id, date) ); The other form is a column constraint, meaning it can only apply to the single column it is defined with. It cannot be used to reference a table with a multi-column primary key.

2018年6月1日 · Primary Key: Is a single field chosen by the designer to uniquely identify a record in a table (relation), cannot be null (empty/unassigned). Foreign Key: Is the Primary Key one table appearing (cross-referenced) in another table. Secondary (or Alternative) Key: Is any field in the table that isn't selected to be any of the two types above.

2016年4月21日 · A surrogate key is typically a numeric value. Within SQL Server, Microsoft allows you to define a column with an identity property to help generate surrogate key values. The PRIMARY KEY constraint uniquely identifies each record in a database table. Primary keys must contain UNIQUE values. A primary key column cannot contain NULL values.

2020年3月21日 · From SQL FOREIGN KEY Constraint. To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax. CREATE TABLE Orders ( O_Id int NOT NULL, OrderNo int NOT NULL, P_Id int, PRIMARY KEY (O_Id), CONSTRAINT fk_PerOrders FOREIGN KEY (P_Id) REFERENCES Persons(P_Id) )

2019年12月13日 · Alternate key is unique key reminiscent of primary key that can be a column or the column's group of the table. Further, alternate key is unique all time in the table. Unlike the primary key, unique key is non-clustered index type as well as foreign key is to create a relationship with another table and it's used for data integrity.

2014年1月29日 · The primary key will probably be implemented with a primary key constraint; the candidate keys will probably be implemented with unique constraints. A foreign key is an instance of one entity's candidate key in another entity, representing a relationship between the …

2012年10月10日 · Primary key -> Any column or set of columns that can uniquely identify a record in the table is a primary key. (There can be only one Primary key in the table) and the candidate key-> the same as Primary key but the Primary Key chosen by DB administrator's prospective for example(the primary key the least candidate key in size)

2017年1月18日 · The primary key for the table is a unique index, and usually only has one column. A foreign key is a value in a table that references a unique index in another table. It is used as a way to relate to tables together. For example, a child table can look up the one parent row via its column that is a unique index in the parent table.