What is the difference between primary, unique and foreign key ...
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 …
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 …
sql - difference between secondary key, foreign key, and primary …
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 …
sql - Foreign Keys vs Joins - Stack Overflow
2010年6月1日 · FOREIGN KEYs and JOINs don't do the same thing! A FOREIGN KEY enforces data integrity, making sure the data conforms to some rules when it is added to the DB. A …
What is difference between foreign key and reference key?
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 …
MySQL: what's the difference between INDEX, UNIQUE, FOREIGN …
A FOREIGN KEY is a referential constraint between two tables. This column/index must have the same type and length as the referred column within the referred table. An example of a …
sql - What's the difference between alternate key and Foreign key …
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 …
What is the difference between an Index and a Foreign Key?
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 …
Differences between "foreign key" and "constraint foreign key"
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 …
Difference between Primary Key, Unique Key and candidate key
2010年9月1日 · Difference between Candidate Key vs Primary Key:- ) Both Primary and Candidate keys can uniquely identify records in a table on database. 2) Both Primary and …