foreign key field access

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

, generally more than one field has a foreign key, so the number of scans is a series of growth! One of my program storage in 3 hours to finish, if you add foreign keys, it takes 28 hours!结论：1, in large-scale systems (performance requirements are not high, high security requirements), the use of

to which the entity belongs2, N-Many-to-manyDesign: Typically, using an intermediate table that represents the correspondence between entitiesForeign key foreign key concept: if one of the entities (student) is a field (class_id),A foreign

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 think that the establishment of

the same signed or not. Change to a consistent data class and whether the data class has a signed bit: Create table users ( Id smallint unsigned primary key AUTO_INCREMENT, Username VARCHAR (10) not null, Pid smallint unsigned, Foreign key (pid) REFERENCES provinces (id) ); When creating a data table, we know that the sub-table is users, the parent table is pro

is saved in other tables. You need to connect to the table to query the product name. The item ID field exists in the item table of document 1, and the item ID field exists in the item table of document 2. If you do not use a foreign key, when product ID = 3 is used in both document 1 and 2, if you delete the correspo

Mysql-InnoDB storage engine Foreign key constraint -- (ForeignKeyConstraint) Mysql foreign key SQL foreign key constraints The foreign key i

message: You need to ensure that the data type is consistent.Modified to be of the same data type, the default is signed bit:CREATE TABLE Users (ID SMALLINT UNSIGNED PRIMARY KEY auto_increment,Username VARCHAR (Ten) is not NULL,pid SMALLINT,FOREIGN KEY (PID) REFERENCES provinces (ID));There will still be error messages: This time is whether to keep the same unsi

Tags: database Add delete Modify index foreign key columnSummary of MySQL, SQL Server, Oracle Three database maintenance indexes, foreign keys, field syntax1. mysql database 1) Create an indexCREATE INDEX index_name on table_name (column_list)CREATE UNIQUE INDEX index_name on table_name (column_list)To modify a table b

ConstraintsThe foreign key constraint is used to maintain the integrity of the reference from the table and the primary table. Therefore, the foreign key constraint involves two tables.Foreign key: indicates the column in the child table at the table level.REFERENCES: colum

Oracle Table-Building, primary key, foreign key basic syntax -Create TABLE syntax:CREATE TABLE Table name (Field name 1 field type (length) is empty,Field Name 2 field type is NULL); -

', CONSTRAINT 'City _ ibfk_1 'eign KEY ('Country _ id') REFERENCES 'country' ('Country _ id ')) The problem above is that the country_id field cannot be changed because of association. Then I read the book again and found that my SQL statement does not have the innodb foreign key constraint (cascade, set null, no act

How to add foreign key constraints to Oracle databases The foreign key specifies that the values of one column (or a group of columns) must match the values of some rows in another table. We say this is to maintain the integrity of the associated table.On the foreign

changes)Set null (set null)SET DEFAULT)No action (no action, default)Add a foreign key to the added data table:Syntax: alter table name add constraint FK_ID foreign key (your foreign key fiel