mysql foreign key example

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 whose ID is 1 ~ Let's talk about the foreign

MySQL has two common engine types: MyISAM and InnoDB. Currently, only the InnoDB engine type supports foreign key constraints. InnoDB The syntax for a FOREIGN KEY constraint definition is as follows: [CONSTRAINT [Symbol]] FOREIGN

tables use category_id. The categories table has four records and the articles table has three records. However, for some reason, we may have deleted the records whose category_id is 4 in the categories table, but the articles table still has a record whose category_id is 4. Obviously, this record with category_id = 4 should not exist in the articles table, which can easily cause data disorder. On the contrary, the Foreign

Failed to add MySQL foreign key ERROR1452 BitsCN.com An ERROR 1452 ERROR occurred while adding foreign keys to MySQL. today, when I learned how to add foreign keys to the database, I first created two tables orders and order_ite

MySQL foreign key settings and functions Foreign key: Data consistency and integrity are maintained to control data stored in the foreign key table. Associate two tables. the

table i_student, but the actual student table i_student often not only two columns of information. Through a simple sentence can not be simpler, all the database must know the statement to find out the entire student table i_student Name column name:Select Id,name from I_studentThe query results are as follows:3, set the first step of the query results for T1, the second step of the query result is T2, the T1 and T2 two tables connected together, only the retention ID and student_id correspondi

Tags: index FOREIGN key ESC nbsp Combination des SQL composed keyIf key is empty, then the column value can be repeated, indicating that the column has no index, or is a non-leading column of a non-unique composite index2. If key is a PRI, then the column is part of the primary key3. If

This article mainly explains the foreign key constraints in MySQL. A constraint overview The purpose of creating a constraint is to ensure the integrity and consistency of the data. Constraints are divided into table-level constraints and column-level based on how many fields are constrained by the constraintConstraints.If the constraint is divided by function, t

People who have developed database-driven small web applications with MySQL know that creating, retrieving, updating, and deleting tables for relational databases is a simpler process. In theory, as long as you have the most common use of SQL statements and familiarize yourself with the server-side scripting language you choose to use, you'll be able to handle the various operations you need for the MySQL t

When you create a foreign key in Mysql, you will often encounter problems and failures. this is because Mysql still has many details to pay attention to. I have summarized and reviewed the relevant information and listed the following common causes. 1. the type or size of the two fields are not strictly matched. For

MySQL version: 5.5.28 System Platform: RHEL 5.8 32-bit (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 t

MySQL common index, unique index, primary index, foreign key index, composite index, full-text index usage, mysql full-text index1. Common Index The only task of a common INDEX (INDEX defined by the KEY or INDEX keyword) is to speed up data access. Therefore, you should crea

In this article, we will learn how to use a basic abstract class in PHP 5 to update data in two InnoDB tables using foreign key constraints. The following example shows how to use the foreign key constraint in the script language on the server. I. Update

Tags: query sort no--strong blog cannot str mys body DefinedPrimary Key-uniquely identifies a record, cannot be duplicated, is not allowed to be emptyForeign key--the foreign key of the table is the primary key of another table, the

common SQL statements and are familiar with the server-side scripting language you have chosen, it is enough to cope with the various operations required for the MySQL table, especially when you use the quick MyISAM database engine. However, even in the simplest circumstances, things are much more complicated than we think. The following is a typical example. Assume that you are running a blog website, whi

supported currently );2. The foreign key column must have an index. MySQL 4.1.2 and later versions will automatically create an index when creating the foreign key. However, if the foreign ke