#Mysql foreign key constraint update#
#Mysql foreign key constraint code#

Thus, a Foreign Key 'Constraint' would be a piece of code that gets called (a trigger, essentially), that looks like such: WHEN INSERT child_column IF (NOT IN parent_table) DO NOT ALLOW INSERT.Īnd then you have your Cascades and Updates and Delete rules (constraints), wth their various if / then conditions and operations, etc. A constraint is like an email filter: a piece of code that gets called on a certain action, such as WHEN (new email) IF (From.

A constraint is essentially a database trigger.

The Binary Search Tree (BST) then exists in computer memory and takes up physical disk space, but allows for O(log n) JOIN access time (almost instantly) from the child table to the parent table.Ĭreating a Foreign Key Constraint is creating a rule, which is a piece of code that gets called when you process (INSERT, SELECT, etc.) on the foreign key column. We just say Foreign Key, but not saying 'Index' is the cause of much confusion.Ĭreating a Foreign Key (a Foreign Key Index) creates a Binary Search Tree (also called a dictionary, because the tree is used to look up values). The Foreign Key Relationship is the requirement for a value in a child table to exist in its parent table, thus ensuring data integrity in a database.īecause Key means Index, we don't say 'Foreign Key Index'. The Index and the Constraint together make the 'Foreign Key Relationship'. 'Constraint' is short for Foreign Key Constraint. 'Foreign Key' is short for Foreign Key Index. Likewise, a Foreign Key and a Foreign Key Constraint are not the same thing, but they work together to create the idea of a 'Foreign Key Relationship'. Together, the engine and the crank-shaft make the car go. The engine converts gasoline explosions into straight line motion (the pistons), and the crankshaft converts that straight-line motion into turning motion, which then turns the wheels of the car. But if I am right, I think this will help.Ī Foreign Key and its associated Foreign Key Constraint are not the same thing, in the way a car engine and a crank-shaft are not the same thing. I am going to throw my hat in the ring here, although I don't actually know if my answer is accurate, so if you know the internal guts of database engineering, please correct me. Hope this explains and helps resolve the confusion. So ALTER TABLE my_table DROP FOREIGN KEY fk1_my_table works, but leaves behind the namesake INDEX.Īnd, note that ALTER TABLE my_table DROP INDEX fk1_my_table won’t work initially (when the FK is not yet dropped), with an error message that it is being used in a FK! If the DROP FK command has been executed successfully, only then the DROP INDEX works. In this format, MySQL is still creating INDEX-es with the names uk1_my_table and fk1_my_table automatically, but the FK object name is not something different – it’s fk1_my_table as mentioned in the DDL. Here’s an alternative DDL format wrt the constarints (Ref : ) :- CREATE TABLE my_dbschema.my_table (ĬONSTRAINT uk1_my_table UNIQUE KEY (account_nbr, account_name),ĬONSTRAINT fk1_my_table FOREIGN KEY (vendor_nbr) REFERENCES vendor(vendor_nbr) So ALTER TABLE my_table DROP FOREIGN KEY fk1_my_table won’t work (and hence frustrating and raising alarms), as there’s no FK db object by that name. In this format, MySQL is creating INDEX-es with the names uk1_my_table and fk1_my_table automatically but the FK object name is something different - my_table_ibfk_1 (ie.

UNIQUE KEY uk1_my_table(account_nbr, account_name),įOREIGN KEY fk1_my_table(vendor_nbr) REFERENCES vendor(vendor_nbr) Last_upd_usr_id VARCHAR(10) NOT NULL DEFAULT 'DFLTUSR',

#Mysql foreign key constraint update#

Last_upd_ts TIMESTAMP NOT NULL DEFAULT current_timestamp ON UPDATE current_timestamp, Id INT unsigned auto_increment PRIMARY KEY,Ĭreate_ts TIMESTAMP NOT NULL DEFAULT current_timestamp,Ĭreate_usr_id VARCHAR(10) NOT NULL DEFAULT 'DFLTUSR', As of now, our CREATE TABLE DDLs are of this format - notice the UNIQUE KEY and FOREIGN KEY definition syntax we have used.