properties of foreign key

The role of the primary and foreign keys of sql:Foreign key value rule: A null value or a reference primary key value.(1) When a non-null value is inserted, it cannot be inserted if the value is not in the primary key table.(2) When updating, you

First, what is primary key, foreign key:A record in a relational database has several properties, and if one of the attribute groups (note the group) uniquely identifies a record, the attribute group can be a primary key such as: Student form

Concept:A primary key (primary key) uniquely identifies a property or group of properties for a row in a table. A table can have only one primary key, but there may be multiple candidate indexes. Primary keys often form referential integrity

First, what is the primary key, foreign keyThere are several properties in a record in a relational database if one of the attribute groups (note thatGroup) can uniquely identify a record the attribute group can be a primary keyLike whatStudent

first, what is primary key, foreign key:A record in a relational database has several properties, and if one of the attribute groups (one or more) can uniquely identify a record, the attribute group can be a primary key Like what Student form

Transferred from: http://blog.csdn.net/duck_arrow/article/details/8264686Concept:A primary key (primary key) uniquely identifies a property or group of properties for a row in a table. A table can have only one primary key, but there may be multiple

The role of the primary and foreign keys of sql:Foreign key value rule: A null value or a reference primary key value.(1) When a non-null value is inserted, it cannot be inserted if the value is not in the primary key table.(2) When updating, you

This article mainly introduces the specific definition and usefulness of super-key, candidate key, primary key, main attribute, foreign key in MySQL database. You can also refer to the topic.alibabacloud.com MySQL video tutorial and MySQL

Both OnDelete and OnUpdate have the Restrict, NoAction, Cascade, and SetNull attributes. Now let's explain the meaning of their properties separately. ONDELETErestrict: when deleting the corresponding record in the parent table (that is, the source

First look at the definitions of the various keys: Super Key: An attribute set that uniquely identifies a tuple in a relationship is called a super key for a relational pattern candidate Keys (Candidate key): A key that does not contain extra