Updating foreign key
Summary: in this tutorial, you will learn about My SQL foreign key and how to create, add, and drop foreign key constraints in My SQL.A foreign key is a field in a table that matches another field of another table.If you omit it, My SQL generates a constraint name for you.To obtain the generated constraint name of a table, you use the CREATE TABLE products ( prd_id int(11) NOT NULL AUTO_INCREMENT, prd_name varchar(355) NOT NULL, prd_price decimal(10,0) DEFAULT NULL, cat_id int(11) NOT NULL, vdr_id int(11) NOT NULL, PRIMARY KEY (prd_id), KEY fk_cat (cat_id), KEY fk_vendor(vdr_id), CONSTRAINT products_ibfk_2 FOREIGN KEY (vdr_id) REFERENCES vendors (vdr_id) ON DELETE NO ACTION ON UPDATE CASCADE, CONSTRAINT products_ibfk_1 FOREIGN KEY (cat_id) REFERENCES categories (cat_id) ON UPDATE CASCADE ) ENGINE=Inno DB; Sometimes, it is very useful to disable foreign key checks e.g., when you import data from a CSV file into a table.If ever in doubt, it's often safer to create indexes on ALL foreign keys, despite the possible overhead of maintaining unneeded indexes.The landmark book "Advanced Oracle SQL Tuning The Definitive Reference" is filled with valuable information on Oracle SQL Tuning.A foreign key places constraints on data in the related tables, which enables My SQL to maintain referential integrity. The columns in the child table often refer to the primary key columns in the parent table.Let’s take a look at the following database diagram in the sample database. A table may have more than one foreign key, and each foreign key in the child table may refer to a different parent table.
This saves you time and efforts of using multiple DELETE statements or a DELETE JOIN statement.Question: I understand that enforcing referential integrity is important in Oracle and foreign key constraints are critical to maintaining parent child relationships.However, I see discussion where they indicate that the rules for indexing foreign keys of a child table have changed over the releases of Oracle.Obviously, if you are using "" and you do not create an index on the child parent key the deletion of a parent row would require a full-table scan of the child table, to find and delete the child rows.Likewise, and "on delete restrict" without an index would force Oracle to perform a full-table scan against the child table to see if any children exist.
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Without a foreign key index, this SQL could not perform a When you create a foreign key constraint, you can choose the "on delete restrict" option to ensure that a parent rows cannot be deleted while a child row still exists.