What is the difference between primary, unique and foreign key ...
2009年11月7日 · Primary/foreign : A "primary" key is a set of attributes whose values must form a combination that is unique in the entire table. There can be more than one such set (> 1 key), …
Is it fine to have foreign key as primary key? - Stack Overflow
2012年6月11日 · Foreign keys are almost always "Allow Duplicates," which would make them unsuitable as Primary Keys. Instead, find a field that uniquely identifies each record in the …
SQL - Unique Key, Primary Key & Foreign Key - Stack Overflow
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 …
How do I create a foreign key in SQL Server? - Stack Overflow
You first need to check if the primary-key exists for the column you want to set your foreign key to reference to. In this example, a foreign key on table T_ZO_SYS_Language_Forms is created, …
sql - can we have a foreign key which is not a primary key in any …
By definition a foreign key must reference a candidate key of some table. It doesn't necessarily have to be the primary key. As a matter of detail the constraint called a FOREIGN KEY in SQL …
How Should I Declare Foreign Key Relationships Using Code First …
2011年10月3日 · Now ParentId is foreign key property and defines required relation between child and existing parent. Saving the child without exsiting parent will throw exception. If your FK …
How to create a foreign key in phpmyadmin - Stack Overflow
2016年6月3日 · The key must be indexed to apply foreign key constraint. To do that follow the steps. Open table structure. (2nd tab) See the last column action where multiples action …
SQL: Having a primary key also be a foreign key - Stack Overflow
2019年2月7日 · A column can be a primary key as well foreign key. For example, refer to the following: A column can be both a primary key and a foreign key. For example: create table A ( …
JPA: Foreign key that is also a primary key mapping
2014年12月5日 · I know this is very old qs, but for completeness of your case you can just have (jpa 2.0) @Entity @Data public class Vehicle implements Serializable{ @Id @GeneratedValue …
How to create foreign key that is also a primary key in MySQL?
2016年1月11日 · Add FOREIGN KEY (sale_id) REFERENCES Sale(sale_id) to each foreign table:. CREATE TABLE Sale( sale_id CHAR(40), PRIMARY KEY(sale_id), discount DOUBLE, …