foreign key field access

database implementation method of "Reference integrity" as described in the basic database theory. Foreign key attributes can certainly be removed. If you don't want to use these constraints, it will certainly not affect programming, however, the "reference integrity" check is not performed for the input data. For example, there are two tables A (A, B): A is the primary

deleting a primary table record, the sub-Table record will be retrieved Based on the foreign key field, select/+ All_rows/Count (1) from "TEST ". "T2" where "T1_ID" =: 1, which means that if the foreign key does not have an index, this SQL statement performs a full table sc

the primary key: MySQL:Alter table Customer add primary key (SID ); Oracle:Alter table Customer add primary key (SID ); SQL Server:Alter table Customer add primary key (SID ); Note thatALTER TABLEBefore adding a primary key, we need to confirm that the column used as the pr

Label:1: What is a primary keyIn a table, the set of fields used to uniquely identify a record is called the primary key or primary key, or the primary key (or main code), which says "field set" because the primary key may be represented by a

The primary key of Table A acts as a foreign key in Table B. Who is the primary table and who is the subtable? A as the primary tableB is the slave table I personally think: A foreign key is a type of constraint. There is no master-slave relationship, and only the reference

delete cascade on update cascade; # prerequisites for creating a foreign key: the data type of the restricted field must be the same as that of the foreign key. The restricted field must be set to an index, and the

the foreign key to the curriculum (s) defining primary and foreign keys is primarily to maintain the integrity of the relational database, summarizing: 1. A primary key is a unique identifier that determines a record, for example, a record that includes an identity plus sign, name, and age. The ID number is the

curriculum.Defining primary and foreign keys is primarily to maintain the integrity of the relational database, summarizing:A primary key is a unique identifier that determines a record, for example, a record that includes an identity plus sign, name, and age. The ID number is the only person who can identify you, others may have duplicates, so the ID number is the primary key.A

Table (i)Student (student table ) Add PRIMARY KEY constraint number SnoTable (ii) Course (curriculum)Add a PRIMARY KEY constraint course number con, add foreign KEY constraint faculty number tonTable (iii) score (score table)Add two foreign

Oracle FOREIGN KEY constraints A FOREIGN key constraint is defined as having a record of another table to constrain itself. Another table here is the main table. When the record of the primary table is deleted, we can delete the record (on delete CASCADE) with the main table, or the corresponding

Tags: com CLR database optimization multiple date let person altPRIMARY KEY constraint A table typically has a column or set of columns that contains values that uniquely identify each row in the table. Such a column or columns is called the primary key (PK) of the table and is used to enforce the entity integrity of the table. Because primary KEY constraints gua

Tags: style blog http color using strong data forIf a field of one entity points to the primary key of another entity, it is called a foreign keyThe entity that is pointed to is called the primary entity (the primary table), also called the parent entity (the parent table).The entity that is responsible for pointing, called from the entity (from the table), also

Understanding of primary key and foreign key in database In relational database, the data structure has logical structure and physical structure. Physical structure refers to the structure of data files stored on physical media. The logical structure is the relationship, which is a sheet of two-dimensional tables. A column in a table is a

while the foreign key is being constructed.(4) When updating a primary key record, there is also a selection of cascading updates and rejected executions.3.2 SettingsSelect the table, right-click: Design, select the field that you want to set the foreign