foreign key field access

http://www.shangxueba.com/jingyan/122163.html primary key:1. PRIMARY KEY constraint: A table can have only one primary KEY constraint. The primary key can be a single field, or it can be multiple fields. In either case, all of its fields are not NULL.2.Unique constraints: A

In other words, I have a forum with two tables, one with the primary thread and the other with the reply. The primary key is the only field in the table that recognizes records, it is generally the post ID, which is reflected in the access time, for example, thread. PHP? Id = 1 indicates that I want to access a post wh

varchar (255), City varchar (255), CONSTRAINT pk_personid PRIMARY KEY (id_p, LastName) ) 　　 To create a PRIMARY KEY constraint for the id_p column if the table already exists, use the following SQL Mysql/sql Server/oracle/ms Access ALTER TABLE Persons ADD PRIMARY KEY (id_p) 　　 To name the PRIMARY

( 10 ) Not Null ) Tablespace Cici; -- Add Field Alter Table Person Add Depid Integer ; -- Add a foreign key constraint person --> Department -- Deptid --> deptid Alter Table Person Add Constraint Fk_person_dept Foreign Key (Depid) References Departmen

foreign key of the curriculum, 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 a social security number, name, and age. The ID number is the only

What is the foreign key in MySQL? what is the role of the foreign key in Mysql? This article participates in the blog contest. if you are satisfied, click here to vote for me. Click here to view the original article.I recently learned the MySQL database by myself. I had a question, but I had never been able to solve

First, create a tabular format:Create Tabletable name (Whether the Field field type can be empty, ...); Comment on columnTable/Field isAn example of a comment:Create TableCompany (com_idINTEGER not NULL, Com_nameVARCHAR2( +)NULL, Com_addressVARCHAR2( -)NULL); Second, create a primary key/

Tags: SQL primary key foreign keyprimary KEY (Primary key) each of the data in the table is a unique value in the form. In other words, it is a unique way to identify each row of data in a table. The primary key can be a field in

table schema: 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); Please note that. Before using the ALTER TABLE statement to join the primary key, we need to confirm that the

Label:First, SQL constraints Constraints are used to restrict the type of data being added to the tag 　　You can specify the constraint (via the CREATE TABLE statement) when creating the table, or after the table is created (via the ALTER table statement) The following constraints are mainly discussed: 1) Not NULL 2) UNIQUE 3) PRIMARY KEY 4) FOREIGN KEY 5) CHECK 6

should also increase constrained= "true"to make the current primary key add a FOREIGN key constraint on the other end of the--class= "Manager" constrained= "true" >class>Manager.hbm.xml Package= "Com.cqupt.dayday" > className= "Manager" table= "manager" > native: Using Database Local mode-class= "Native"/> class= "Department" >class>2: Based on the

(1) usage of Foreign keys: The foreign key has two main functions:One is to allow the database to ensure data integrity and consistency through foreign keys.One is to increase the readability of the erimage.Some people think that the establishment of foreign keys will cause

Tags: des style http Java color using MySQL base day03_ storage engine and foreign key-mysql 5.6Conditions for foreign keys:1, the table's storage engine is the InnoDB storage engine2, the type of the foreign key field in the

, frequently inserted data, if the table has 3 foreign keys, then each time you insert a bar, you must have 3 foreign keys corresponding to the 3 table to do the corresponding lookup to determine if there is no corresponding data, if the 3 tables are also large, The judgment time of the 3 tables is very often, although the foreign

generating null.Constantgenerator is used to produce a fixed value of the data, the generator only one attribute constant, given the value to insert the database;Datetimegenerator is used to produce datetime data, which includes four properties, namely, Startdate,enddate,returnedtype and Nulls, StartDate describes the start time, EndDate describes the expiration time, in the form "Yyyy-mm-dd Hh24:MM:ss", and Returnedtype describes the type of data generated, which can be date, time, or timestam

Use' Wfc_database '; # Main Table (also known as: Referenced table, referencedTable, outtable)ALTER TABLE' App ' ENGINE=innodb;# from a table (also known as: Reference table, appearance, referencingTable)ALTER TABLE' App_version ' ENGINE=innodb;# an "app" can have multiple "app versions" # So apps and App_version are1: N Relationship (one app_id corresponds to multiple av_app_id) # app_id is the primary key of the app table, av_app_id is the index of

1, the creation of the primary keyMethod One: Declare the field PRIMARY KEY constraint directly in the SQL statementCREATE TABLE table_name (ID type[length] constraint pk_name primary key,name tyoe[length],age type[length],class_id);Method two: Alter change table ADD constraintALTER TABLE TABLE_NAME ADD constraint Pk_name primary

: cascade: foreign key field values are updated, or the columns in which they are located are deleted. restrict: restrict is also equivalent to no action, that is, no action is made. That is, deny parent table Update FOREIGN Key Association column, delete record.

abstract class To demonstrate how to use the MySQL class cascade to update the data table in the previous article, we need to redefine the two tables so that they can only perform these update operations. Here are their definitions. These two tables will serve as the data layer of our sample blog application: Code highlighting produced by actipro codehighlighter (freeware)Drop table if exists 'test'. 'blogs '; Create Table 'test'. 'blogs '( 'Id' int (10) unsigned auto_increment, 'Title' text,

ConceptKeyword: foreign key, also called FOREIGN KEY constraint!If a field of entity A is just pointing to the primary key of another entity B, then the field of entity A is called a