mysql foreign key example

Label:the role of foreign keys The main role of foreign keys is to maintain data consistency and integrity. About foreign keys, I have some of my own understanding, but I do not know whether it is correct, for example to express my view: If you need to build a table, what kind of table it? A student's personal informat

specific principle here is not clear, the above is only speculation, hereby recordedIf you find a reason to update!Update:Https://stackoverflow.com/questions/22013511/mysql-error-1215-hy000-cannot-add-foreign-key-constraintThat is to say, my above guess is correct!Take a look at the unique test:Transferred from: http://www.cnblogs.com/Andy963/p/7634537.htmlThe l

duplicate values. For example, because a person might have the same name, it may occur two or more times in the same employee profile data table. If you can determine that a data column will contain only values that are different from each other, you should define it as a unique index with the keyword unique when creating an index for that data column. The advantage of this is that it simplifies MySQL's management of the index, which makes it more ef

MySQL creates a primary key, a foreign key, and a composite primary key. For more information, see. MySQL creates a primary key, a foreign

Mysql -- foreign key (froeign key), mysql -- froeign If a field of an object points to the primary key of another object, it is called a foreign key.The object to be pointed to is calle

child table. Set null:Delete or update rows from the parent table, and set the foreign key column in the child table to NULL.. If the foreign key column does NOT specify the not null qualifier, this is unique and valid. The on delete set null and on update set null clauses are supported. No action: in ANSI SQL-92

operations can not be nested more than 15 layers deep.divergence from the SQL standard : Similar to general MySQL, within an SQL statement that inserts, deletes, or updates many rows, InnoDB checks the unique and foreign key constraints line by row. According to the SQL standard, the default behavior should be deferred check, that is, the constraint is checked o

comparison operations. You should use 0, a special value, or an empty string instead of a null value. Time to build the index:In general, the columns that appear in the where and join need to be indexed, but not entirely, because MySQL uses the index only for FOREIGN key:is one or more columns that are used to establish and strengthen links between two table data.

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 l

MYSQL Tutorial: MYSQL foreign key application Foreign key: Data consistency and integrity are maintained to control data stored in the foreign