MySQL: “Foreign Keys” Used in an Example

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This means that you cannot use self-referential ON UPDATE CASCADE or ON UPDATE SET NULL operations. Our enrollment table with all valid entries we created looks like the below. The handling of foreign key references to nonunique keys or keys that contain NULL values is not well defined for operations such as or DELETE CASCADE.

Such an index is created on the referencing table automatically if it does not exist. However, in the referenced table, there must be an index where the referenced columns are listed as the first columns in the same order.

MySQL: “Foreign Keys” Used in an Example

The FOREIGN KEY clause is specified in the child table. The parent and child tables must use the same storage engine. They must not be TEMPORARY tables. The size and sign of integer types must be the same. The length of string types need not be the same. For nonbinary character string columns, the character set and collation must be the same. The workaround is described in. In the referencing table, there must be an index where the foreign key columns are listed as the first columns in the same order. Such an index is created on the referencing table automatically if it does not exist. This index might be silently dropped later, if you create another index that can be used to enforce the foreign key constraint. However, in the referenced table, there must be an index where the referenced columns are listed as the first columns in the same order. NDB requires an explicit unique key or primary key on any column referenced as a foreign key. One consequence of this is that and columns cannot be included in a foreign key because indexes on those columns must always include a prefix length. If the clause is not given, or a symbol is not included following the CONSTRAINT keyword, a name for the constraint is created automatically. This includes both parent and child tables. This restriction does not apply for tables that are partitioned by KEY or LINEAR KEY the only user partitioning types supported by the NDB storage engine ; these may have foreign key references or be the targets of such references. Additional aspects of FOREIGN KEY constraint usage are described under the following topics in this section: Referential Actions This section describes how foreign keys help guarantee. For storage engines supporting foreign keys, MySQL rejects any or operation that attempts to create a foreign key value in a child table if there is no a matching candidate key value in the parent table. When an or operation affects a key value in the parent table that has matching rows in the child table, the result depends on the referential action specified using ON UPDATE and ON DELETE subclauses of the FOREIGN KEY clause. Both ON DELETE CASCADE and ON UPDATE CASCADE are supported. Between two tables, do not define several ON UPDATE CASCADE clauses that act on the same column in the parent table or in the child table. Note Cascaded foreign key actions do not activate triggers. Both ON DELETE SET NULL and ON UPDATE SET NULL clauses are supported. If you specify a SET NULL action, make sure that you have not declared the columns in the child table as NOT NULL. Specifying RESTRICT or NO ACTION is the same as omitting the ON DELETE or ON UPDATE clause. In MySQL, equivalent to RESTRICT. The MySQL Server rejects the delete or update operation for the parent table if there is a related foreign key value in the referenced table. Some database systems have deferred checks, and NO ACTION is a deferred check. In MySQL, foreign key constraints are checked immediately, so NO ACTION is the same as RESTRICT. For an ON DELETE or ON UPDATE that is not specified, the default action is always RESTRICT. MySQL supports foreign key references between one column and another within a table. A column cannot have a foreign key reference to itself. One foreign key references a two-column index in the product table. When you add a foreign key constraint to a table using , remember to create the required indexes first. The server prohibits changes to foreign key columns with the potential to cause loss of referential integrity. A workaround is to use before changing the column definition and afterward. Foreign Keys and Other MySQL Statements Table and column identifiers in a FOREIGN KEY... The setting of the system variable is also taken into account. To make it easier to reload dump files for tables that have foreign key relationships, automatically includes a statement in the dump output to set to 0. This avoids problems with tables having to be reloaded in a particular order when the dump is reloaded. It also speeds up the import operation. Setting to 0 can also be useful for ignoring foreign key constraints during and operations. However, even if , MySQL does not permit the creation of a foreign key constraint where a column references a nonmatching column type. Also, if a table has foreign key constraints, cannot be used to alter the table to use another storage engine. To change the storage engine, you must drop any foreign key constraints first. When you drop a table, any constraints that were defined in the statement used to create that table are also dropped. If you re-create a table that was dropped, it must have a definition that conforms to the foreign key constraints referencing it. It must have the correct column names and types, and it must have indexes on the referenced keys, as stated earlier. If these are not satisfied, MySQL returns Error 1005 and refers to Error 150 in the error message, which means that a foreign key constraint was not correctly formed. Similarly, if an fails due to Error 150, this means that a foreign key definition would be incorrectly formed for the altered table. For InnoDB tables, you can obtain a detailed explanation of the most recent InnoDB foreign key error in the MySQL Server, by checking the output of. Use of an explicit MATCH clause will not have the specified effect, and also causes ON DELETE and ON UPDATE clauses to be ignored. For these reasons, specifying MATCH should be avoided. The MATCH clause in the SQL standard controls how NULL values in a composite multiple-column foreign key are handled when comparing to a primary key. MySQL essentially implements the semantics defined by MATCH SIMPLE, which permit a foreign key to be all or partially NULL. In that case, the child table row containing such a foreign key is permitted to be inserted, and does not match any row in the referenced parent table. It is possible to implement other semantics using triggers. Additionally, MySQL requires that the referenced columns be indexed for performance reasons. However, the system does not enforce a requirement that the referenced columns be UNIQUE or be declared NOT NULL. The handling of foreign key references to nonunique keys or keys that contain NULL values is not well defined for operations such as or DELETE CASCADE. You are advised to use foreign keys that reference only UNIQUE including PRIMARY and NOT NULL keys. MySQL accepts REFERENCES clauses only when specified as part of a separate FOREIGN KEY specification. For storage engines that do not support foreign keys such as , MySQL Server parses and ignores foreign key specifications.

While using this site, you agree to have read and accepted our. I made an application that can create new entries in mysql table but I have a problem. Additionally, MySQL requires that the referenced columns be indexed for performance reasons. You can omit the ON UPDATE clause to let MySQL reject any updates to the rows in the child table when the rows in the parent table are updated. Sometimes, it is very useful to disable foreign key checks e. That may be a bad habit to get into. For an ON DELETE or ON UPDATE that is not specified, the default action is always RESTRICT. The server prohibits changes to foreign key columns with the potential to cause loss of referential integrity. The parent and child tables must use the same storage engine. In MySQL, foreign key constraints are checked immediately, so NO ACTION is the same as RESTRICT. If the clause is not given, or a symbol is not included following the CONSTRAINT keyword, a mysql set foreign key for the constraint is created automatically.