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This chapter is from the book

This chapter is from the book

Foreign Keys and Referential Integrity

A foreign key relationship allows you to declare that an index in one table is related to an index in another. It also allows you to place constraints on what may be done to the tables in the relationship. The database enforces the rules of this relationship to maintain referential integrity. For example, the score table in the sampdb sample database contains a student_id column, which we use to relate score records to students in the student table. When we created these tables in Chapter 1, we set up some explicit relationships between them. One of these was that we declared score.student_id to be a foreign key for the student.student_id column. That prevents a record from being entered into the score table unless its student_id value exists in the student table. In other words, the foreign key prevents entry of scores for non-existent students.

Foreign keys are not useful just for record entry, but for deletes and updates as well. For example, we could set up a constraint such that if a student is deleted from the student table, all corresponding records for the student in the score table are deleted automatically as well. This is called "cascaded delete," because the effect of the delete cascades from one table to another. Cascaded update is possible as well. For example, with cascaded update, changing a student's student_id value in the student table also changes the value in the student's corresponding score table records.

Foreign keys help maintain the consistency of your data, and they provide a certain measure of convenience. Without foreign keys, you are responsible for keeping track of inter-table dependencies and maintaining their consistency from within your applications. In some cases, doing this might not be much more work than issuing a few extra DELETE statements to make sure that when you delete a record from one table, you also delete the corresponding records in any related tables. But it is extra work, and if the database engine will perform consistency checks for you, why not let it? Automatic checking capability becomes especially useful if your tables have particularly complex relationships. You likely will not want to be responsible for implementing these dependencies in your applications.

In MySQL, the InnoDB storage engine provides foreign key support. This section describes how to set up InnoDB tables to define foreign keys, and how foreign keys affect the way you use tables. First, it's necessary to define some terms:

  • The parent is the table that contains the original key values.

  • The child is the related table that refers to key values in the parent.

  • Parent table key values are used to associate the two tables. Specifically, an index in the child table refers to an index in the parent. The child index values must match those in the parent or else be set to NULL to indicate that there is no associated parent table record. The index in the child table is known as the "foreign key"—that is, the key that is foreign (external) to the parent table but contains values that point to the parent. A foreign key relationship can be set up to disallow NULL values, in which case all foreign key values must match a value in the parent table.

InnoDB enforces these rules to guarantee that the foreign key relationship stays intact with no mismatches. This is called "referential integrity."

The syntax for defining a foreign key in a child table is as follows, with optional parts shown in square brackets:

[CONSTRAINT constraint_name]
FOREIGN KEY [index_name] (index_columns)
 REFERENCES tbl_name (index_columns)
 [ON DELETE action]
 [ON UPDATE action]
 [MATCH FULL | MATCH PARTIAL | MATCH SIMPLE]

Note that although all parts of this syntax are parsed, InnoDB does not implement the semantics for all the clauses: The MATCH clause is not supported and is ignored if you specify it. Also, some action values are recognized but have no effect1. InnoDB pays attention to the following parts of the definition:

  • The CONSTRAINT clause, if given, supplies a name for the foreign key constraint. If you leave it out, InnoDB creates a name.

  • FOREIGN KEY indicates the indexed columns in the child table that must match index values in the parent table. index_name, if given, is ignored.

  • REFERENCES names the parent table and the index columns in that table to which the foreign key in the child table refers. The index_columns part of the REFERENCES clause must have the same number of columns as the index_columns that follows the FOREIGN KEY keywords.

  • ON DELETE enables you to specify what happens to the child table when parent table records are deleted. The default if no ON DELETE clause is present is to reject any attempt to delete rows in the parent table that have child rows pointing to them. To specify an action value explicitly, use one of the following clauses:

    • ON DELETE NO ACTION and ON DELETE RESTRICT are the same as omitting the ON DELETE clause. (Some database systems have deferred checks, and NO ACTION is a deferred check. In MySQL, foreign key constraints are checked immediately, so NO ACTION and RESTRICT are the same.)

    • ON DELETE CASCADE causes matching child records to be deleted when the corresponding parent record is deleted. In essence, the effect of the delete is cascaded from the parent to the child. This allows you to perform multiple-table deletes by deleting rows only from the parent table and letting InnoDB take care of deleting rows from the child table.

    • ON DELETE SET NULL causes index columns in matching child records to be set to NULL when the parent record is deleted. If you use this option, all the indexed child table columns named in the foreign key definition must be defined to allow NULL values. (One implication of using this action is that you cannot define the foreign key to be a PRIMARY KEY because primary keys do not allow NULL values.)

    • ON DELETE SET DEFAULT is recognized but unimplemented and InnoDB issues an error.

  • ON UPDATE enables you to specify what happens to the child table when parent table records are updated. The default if no ON UPDATE clause is present is to reject any inserts or updates in the child table that result in foreign key values that don't have any match in the parent table index, and to prevent updates to parent table index values to which child rows point. The possible action values are the same as for ON DELETE and have similar effects.

To set up a foreign key relationship, follow these guidelines:

  • The child table must have an index where the foreign key columns are listed as its first columns. The parent table must also have an index in which the columns in the REFERENCES clause are listed as its first columns. (In other words, the columns in the key must be indexed in the tables on both ends of the foreign key relationship.) You must create the parent table index explicitly before defining the foreign key relationship. Before MySQL 4.1.2, you must also create the child table index explicitly. As of 4.1.2, InnoDB automatically creates an index on foreign key columns (the referencing columns) in the child table if the CREATE TABLE statement does not include such an index. This makes it easier to write the CREATE TABLE statement in some cases. However, an automatically created index will be a non-unique index and will include only the foreign key columns. You should define the index in the child table explicitly if you want it to be a PRIMARY KEY or UNIQUE index, or if it should include other columns besides those in the foreign key.

  • Corresponding columns in the parent and child indexes must have compatible types. For example, you cannot match an INT column with a CHAR column. Corresponding character columns must be the same length. Corresponding integer columns must have the same size and must both be signed or both UNSIGNED.

  • You cannot index prefixes of string columns in foreign key relationships. (That is, for string columns, you must index the entire column, not just a leading prefix of it.)

In Chapter 1, we created tables for the grade-keeping project that have simple foreign key relationships. Now let's work through an example that is more complex. Begin by creating tables named parent and child, such that the child table contains a foreign key that references the par_id column in the parent table:

CREATE TABLE parent
(
  par_id   INT NOT NULL,
  PRIMARY KEY (par_id)
) ENGINE = INNODB;

CREATE TABLE child
(
  par_id   INT NOT NULL,
  child_id  INT NOT NULL,
  PRIMARY KEY (par_id, child_id),
  FOREIGN KEY (par_id) REFERENCES parent (par_id)
    ON DELETE CASCADE
    ON UPDATE CASCADE
) ENGINE = INNODB;

The foreign key in this case uses ON DELETE CASCADE to specify that when a record is deleted from the parent table, MySQL also should remove child records with a matching par_id value automatically. ON UPDATE CASCADE indicates that if a parent record par_id value is changed, MySQL also should change any matching par_id values in the child table to the new value.

Now insert a few records into the parent table, and then add some records to the child table that have related key values:

mysql> INSERT INTO parent (par_id) VALUES(1),(2),(3);
mysql> INSERT INTO child (par_id,child_id) VALUES(1,1),(1,2);
mysql> INSERT INTO child (par_id,child_id) VALUES(2,1),(2,2),(2,3);
mysql> INSERT INTO child (par_id,child_id) VALUES(3,1);

These statements result in the following table contents, where each par_id value in the child table matches a par_id value in the parent table:

mysql> SELECT * FROM parent;
+--------+
| par_id |
+--------+
|      1 |
|      2 |
|      3 |
+--------+
mysql> SELECT * FROM child;
+--------+----------+
| par_id | child_id |
+--------+----------+
|      1 |        1 |
|      1 |        2 |
|      2 |        1 |
|      2 |        2 |
|      2 |        3 |
|      3 |        1 |
+--------+----------+

To verify that InnoDB enforces the key relationship for insertion, try adding a record to the child table that has a par_id value not found in the parent table:

mysql> INSERT INTO child (par_id,child_id) VALUES(4,1);
ERROR 1216 (23000): Cannot add or update a child row:
a foreign key constraint fails

To test cascaded delete, see what happens when you delete a parent record:

mysql> DELETE FROM parent WHERE par_id = 1;

MySQL deletes the record from the parent table:

mysql> SELECT * FROM parent;
+--------+
| par_id |
+--------+
|      2 |
|      3 |
+--------+

In addition, it cascades the effect of the DELETE statement to the child table:

mysql> SELECT * FROM child;
+--------+----------+
| par_id | child_id |
+--------+----------+
|      2 |        1 |
|      2 |        2 |
|      2 |        3 |
|      3 |        1 |
+--------+----------+

To test cascaded update, see what happens when you update a parent record:

mysql> UPDATE parent SET par_id = 100 WHERE par_id =2;
mysql> SELECT * FROM parent;
+--------+
| par_id |
+--------+
|      3 |
|    100 |
+--------+
mysql> SELECT * FROM child;
+--------+----------+
| par_id | child_id |
+--------+----------+
|      3 |        1 |
|    100 |        1 |
|    100 |        2 |
|    100 |        3 |
+--------+----------+

The preceding example shows how to arrange for deletes or updates of a parent record to cause cascaded deletes or updates of any corresponding child records. The ON DELETE and ON UPDATE clauses allow for other actions. For example, one possibility is to let the child records remain in the table but have their foreign key columns set to NULL. To do this, it's necessary to make several changes to the definition of the child table:

  • Use ON DELETE SET NULL rather than ON DELETE CASCADE. This tells InnoDB to set the foreign key column (par_id) to NULL instead of deleting the records.

  • Use ON UPDATE SET NULL rather than ON UPDATE CASCADE. This tells InnoDB to set the foreign key column (par_id) to NULL when matching parent records are updated.

  • The original definition of child defines par_id as NOT NULL. That won't work with ON DELETE SET NULL or ON UPDATE SET NULL, so the column definition must be changed to allow NULL.

  • The original definition of child also defines par_id to be part of a PRIMARY KEY. However, a PRIMARY KEY cannot contain NULL values. Changing par_id to allow NULL therefore also requires that the PRIMARY KEY be changed to a UNIQUE index. UNIQUE indexes enforce uniqueness except for NULL values, which can occur multiple times in the index.

To see the effect of these changes, re-create the parent table using the original definition and load the same initial records into it. Then create the child table using the new definition shown here:

CREATE TABLE child
(
  par_id   INT NULL,
  child_id  INT NOT NULL,
  UNIQUE (par_id, child_id),
  FOREIGN KEY (par_id) REFERENCES parent (par_id)
    ON DELETE SET NULL
    ON UPDATE SET NULL
) ENGINE = INNODB;

With respect to inserting new records, the child table behaves the same. That is, it allows insertion of records with par_id values found in the parent table, but disallows entry of values that aren't listed there2:

mysql> INSERT INTO child (par_id,child_id) VALUES(1,1),(1,2);
mysql> INSERT INTO child (par_id,child_id) VALUES(2,1),(2,2),(2,3);
mysql> INSERT INTO child (par_id,child_id) VALUES(3,1);
mysql> INSERT INTO child (par_id,child_id) VALUES(4,1);
ERROR 1216 (23000): Cannot add or update a child row:
a foreign key constraint fails

A difference in behavior occurs when you delete a parent record. Try removing a parent record and then check the contents of the child table to see what happens:

mysql> DELETE FROM parent WHERE par_id = 1;
mysql> SELECT * FROM child;
+--------+----------+
| par_id | child_id |
+--------+----------+
|   NULL |        1 |
|   NULL |        2 |
|      2 |        1 |
|      2 |        2 |
|      2 |        3 |
|      3 |        1 |
+--------+----------+

In this case, the child records that had 1 in the par_id column are not deleted. Instead, the par_id column is set to NULL, as specified by the ON DELETE SET NULL constraint.

Updating a parent record has a similar effect:

mysql> UPDATE parent SET par_id = 100 WHERE par_id = 2;
mysql> SELECT * FROM child;
+--------+----------+
| par_id | child_id |
+--------+----------+
|   NULL |        1 |
|   NULL |        1 |
|   NULL |        2 |
|   NULL |        2 |
|   NULL |        3 |
|      3 |        1 |
+--------+----------+

To see what foreign key relationships an InnoDB table has, use the SHOW CREATE TABLE or SHOW TABLE STATUS statement.

If an error occurs when you attempt to create a table that has a foreign key, use the SHOW INNODB STATUS statement to get the full error message.

Living Without Foreign Keys

If your MySQL server doesn't have InnoDB support, or you are using another storage engine because you need features that InnoDB does not support (such as FULLTEXT indexes or spatial data types), you cannot take advantage of foreign keys. What should you do to maintain the integrity of relationships between your tables?

The constraints that foreign keys enforce often are not difficult to implement through application logic. Sometimes, it's simply a matter of how you approach the data entry process. Consider the student and score tables from the grade-keeping project. These are related by a foreign key relationship through the student_id values in each table. Suppose that we had created these as MyISAM tables rather than as InnoDB tables. MyISAM does not support foreign keys, so in this case the relationship between the tables would be implicit rather than explicit. When you administer a test or quiz and have a new set of scores to add to the database, you'd have to make sure that you don't add score records with student_id values that are not listed in the student table.

In some respects, this is simply a matter of taking the proper approach to data entry. To avoid inserting scores for non-existent students, the way you'd enter a set of scores probably would be to use an application that begins with a list of students from the student table. For each one, it would take the score and use the student's ID number to generate a new score table record. With this procedure, you would never enter a record for a student that doesn't exist. Nevertheless, it would still be possible to enter a bad record, for example, if you issued an INSERT statement manually. (With InnoDB tables and foreign keys, no such possibility exists.)

What about the case where you delete a student record? Suppose that you want to delete student number 13. This also implies you want to delete any score records for that student. With a foreign key relationship in place that specifies cascading delete, you'd simply delete the student table record with the following statement and let MySQL take care of removing the corresponding score table records automatically:

DELETE FROM student WHERE student_id = 13;

Without foreign key support, you must explicitly delete records for all relevant tables to achieve the same effect as cascading on DELETE:

DELETE FROM student WHERE student_id = 13;
DELETE FROM score WHERE student_id = 13;

Another way to do this is to use a multiple-table delete that achieves the same effect as a cascaded delete with a single query. But watch out for a subtle trap. The following statement appears to do the trick, but it's actually not quite correct:

DELETE student, score FROM student, score
WHERE student.student_id = 13 AND student.student_id = score.student_id;

The problem with this statement is that it will fail in the case where the student doesn't have any scores. The WHERE clause will find no matches and thus will not delete anything from the student table. In this case, a LEFT JOIN is more appropriate, because it will identify which student table record to delete even in the absence of any matching score table records:

DELETE student, score FROM student LEFT JOIN score USING (student_id)
WHERE student.student_id = 13;
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