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13.2.5.5 InnoDB and MySQL Replication »
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13.2.5.4. FOREIGN KEY Constraints

Starting from MySQL 3.23.44, InnoDB features foreign key constraints.

InnoDB supports foreign key constraints. The syntax for a foreign key constraint definition in InnoDB looks like this: 

[CONSTRAINT [symbol]] FOREIGN KEY
    [index_name] (index_col_name, ...)
    REFERENCES tbl_name (index_col_name,...)
    [ON DELETE reference_option]
    [ON UPDATE reference_option]

reference_option:
    RESTRICT | CASCADE | SET NULL | NO ACTION

index_name represents a foreign key ID. If given, this is ignored if an index for the foreign key is defined explicitly. Otherwise, if InnoDB creates an index for the foreign key, it uses index_name for the index name.

Foreign keys definitions are subject to the following conditions:

  • Both tables must be InnoDB tables and they must not be TEMPORARY tables.

  • Corresponding columns in the foreign key and the referenced key must have similar internal data types inside InnoDB so that they can be compared without a type conversion. 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.

  • InnoDB requires indexes on foreign keys and referenced keys so that foreign key checks can be fast and not require a table scan. 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 is in contrast to versions older than MySQL 4.1.2, in which indexes had to be created explicitly or the creation of foreign key constraints would fail.) index_name, if given, is used as described previously.

  • InnoDB allows a foreign key to reference any index column or group of columns. However, in the referenced table, there must be an index where the referenced columns are listed as the first columns in the same order.

  • Index prefixes on foreign key columns are not supported. One consequence of this is that BLOB and TEXT columns cannot be included in a foreign key because indexes on those columns must always include a prefix length.

  • If the CONSTRAINT symbol clause is given, the symbol value must be unique in the database. If the clause is not given, InnoDB creates the name automatically.

InnoDB rejects any INSERT or UPDATE 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. The action InnoDB takes for any UPDATE or DELETE operation that attempts to update or delete a candidate key value in the parent table that has some matching rows in the child table is dependent on the referential action specified using ON UPDATE and ON DELETE subclauses of the FOREIGN KEY clause. When the user attempts to delete or update a row from a parent table, and there are one or more matching rows in the child table, InnoDB supports five options regarding the action to be taken. If ON DELETE or ON UPDATE are not specified, the default action is RESTRICT.

  • CASCADE: Delete or update the row from the parent table and automatically delete or update the matching rows in the child table. ON DELETE CASCADE is supported starting from MySQL 3.23.50 and ON UPDATE CASCADE is supported starting from 4.0.8. Between two tables, you should not define several ON UPDATE CASCADE clauses that act on the same column in the parent table or in the child table.

  • SET NULL: Delete or update the row from the parent table and set the foreign key column or columns in the child table to NULL. This is valid only if the foreign key columns do not have the NOT NULL qualifier specified. ON DELETE SET NULL is available starting from MySQL 3.23.50 and ON UPDATE SET NULL is available starting from 4.0.8.

    If you specify a SET NULL action, make sure that you have not declared the columns in the child table as NOT NULL.

  • NO ACTION: In standard SQL, NO ACTION means no action in the sense that an attempt to delete or update a primary key value will not be allowed to proceed if there is a related foreign key value in the referenced table. Starting from 4.0.18 InnoDB rejects the delete or update operation for the parent table.

  • RESTRICT: Rejects the delete or update operation for the parent table. Specifying RESTRICT (or NO ACTION) is the same as omitting the ON DELETE or ON UPDATE 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 is the same as RESTRICT.)

  • SET DEFAULT: This action is recognized by the parser, but InnoDB rejects table definitions containing ON DELETE SET DEFAULT or ON UPDATE SET DEFAULT clauses.

InnoDB supports foreign key references within a table. In these cases, “child table records” really refers to dependent records within the same table.

Here is a simple example that relates parent and child tables through a single-column foreign key: 

CREATE TABLE parent (id INT NOT NULL,
                     PRIMARY KEY (id)
) TYPE=INNODB;
CREATE TABLE child (id INT, parent_id INT,
                    INDEX par_ind (parent_id),
                    FOREIGN KEY (parent_id) REFERENCES parent(id)
                      ON DELETE CASCADE
) TYPE=INNODB;

A more complex example in which a product_order table has foreign keys for two other tables. One foreign key references a two-column index in the product table. The other references a single-column index in the customer table: 

CREATE TABLE product (category INT NOT NULL, id INT NOT NULL,
                      price DECIMAL,
                      PRIMARY KEY(category, id)) TYPE=INNODB;
CREATE TABLE customer (id INT NOT NULL,
                       PRIMARY KEY (id)) TYPE=INNODB;
CREATE TABLE product_order (no INT NOT NULL AUTO_INCREMENT,
                            product_category INT NOT NULL,
                            product_id INT NOT NULL,
                            customer_id INT NOT NULL,
                            PRIMARY KEY(no),
                            INDEX (product_category, product_id),
                            FOREIGN KEY (product_category, product_id)
                              REFERENCES product(category, id)
                              ON UPDATE CASCADE ON DELETE RESTRICT,
                            INDEX (customer_id),
                            FOREIGN KEY (customer_id)
                              REFERENCES customer(id)) TYPE=INNODB;

Starting from MySQL 3.23.50, InnoDB allows you to add a new foreign key constraint to a table by using ALTER TABLE: 

ALTER TABLE tbl_name
    ADD [CONSTRAINT [symbol]] FOREIGN KEY
    [index_name] (index_col_name, ...)
    REFERENCES tbl_name (index_col_name,...)
    [ON DELETE reference_option]
    [ON UPDATE reference_option]

The foreign key can be self referential (referring to the same table). When you add a foreign key constraint to a table using ALTER TABLE, remember to create the required indexes first.

Starting from MySQL 4.0.13, InnoDB supports the use of ALTER TABLE to drop foreign keys: 

ALTER TABLE tbl_name DROP FOREIGN KEY fk_symbol;

If the FOREIGN KEY clause included a CONSTRAINT name when you created the foreign key, you can refer to that name to drop the foreign key. (A constraint name can be given as of MySQL 4.0.18.) Otherwise, the fk_symbol value is internally generated by InnoDB when the foreign key is created. To find out the symbol when you want to drop a foreign key, use the SHOW CREATE TABLE statement. For example: 

mysql> SHOW CREATE TABLE ibtest11c\G
*************************** 1. row ***************************
       Table: ibtest11c
Create Table: CREATE TABLE `ibtest11c` (
  `A` int(11) NOT NULL auto_increment,
  `D` int(11) NOT NULL default '0',
  `B` varchar(200) NOT NULL default '',
  `C` varchar(175) default NULL,
  PRIMARY KEY  (`A`,`D`,`B`),
  KEY `B` (`B`,`C`),
  KEY `C` (`C`),
  CONSTRAINT `0_38775` FOREIGN KEY (`A`, `D`)
REFERENCES `ibtest11a` (`A`, `D`)
ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `0_38776` FOREIGN KEY (`B`, `C`)
REFERENCES `ibtest11a` (`B`, `C`)
ON DELETE CASCADE ON UPDATE CASCADE
) TYPE=InnoDB CHARSET=latin1
1 row in set (0.01 sec)

mysql> ALTER TABLE ibtest11c DROP FOREIGN KEY `0_38775`;

You cannot add a foreign key and drop a foreign key in separate clauses of a single ALTER TABLE statement. Separate statements are required.

If ALTER TABLE for an InnoDB table results in changes to column values (for example, because a column is truncated), InnoDB's FOREIGN KEY constraint checks do not notice possible violations caused by changing the values.

Starting from MySQL 3.23.50, the InnoDB parser allows table and column identifiers in a FOREIGN KEY ... REFERENCES ... clause to be quoted within backticks. (Alternatively, double quotes can be used if the ANSI_QUOTES SQL mode is enabled.) The InnoDB parser also takes into account the setting of the lower_case_table_names system variable.

Before MySQL 3.23.50, ALTER TABLE or CREATE INDEX should not be used in connection with tables that have foreign key constraints or that are referenced in foreign key constraints: Any ALTER TABLE removes all foreign key constraints defined for the table. You should not use ALTER TABLE with the referenced table, either. Instead, use DROP TABLE and CREATE TABLE to modify the schema. When MySQL does an ALTER TABLE it may internally use RENAME TABLE, and that confuses the foreign key constraints that refer to the table. In MySQL, a CREATE INDEX statement is processed as an ALTER TABLE, so the same considerations apply.

Starting from MySQL 3.23.50, InnoDB returns the foreign key definitions of a table as part of the output of the SHOW CREATE TABLE statement: 

SHOW CREATE TABLE tbl_name;

mysqldump also produces correct definitions of tables in the dump file, and does not forget about the foreign keys.

You can also display the foreign key constraints for a table like this: 

SHOW TABLE STATUS FROM db_name LIKE 'tbl_name';

The foreign key constraints are listed in the Comment column of the output.

When performing foreign key checks, InnoDB sets shared row-level locks on child or parent records it has to look at. InnoDB checks foreign key constraints immediately; the check is not deferred to transaction commit.

To make it easier to reload dump files for tables that have foreign key relationships, mysqldump automatically includes a statement in the dump output to set foreign_key_checks to 0 as of MySQL 4.1.1. This avoids problems with tables having to be reloaded in a particular order when the dump is reloaded. For earlier versions, you can disable the variable manually within mysql when loading the dump file like this: 

mysql> SET foreign_key_checks = 0;
mysql> SOURCE dump_file_name;
mysql> SET foreign_key_checks = 1;

This allows you to import the tables in any order if the dump file contains tables that are not correctly ordered for foreign keys. It also speeds up the import operation. Setting foreign_key_checks to 0 can also be useful for ignoring foreign key constraints during LOAD DATA and ALTER TABLE operations. However, even if foreign_key_checks = 0, InnoDB does not allow the creation of a foreign key constraint where a column references a nonmatching column type. Also, if an InnoDB table has foreign key constraints, ALTER TABLE cannot be used to change the table to use another storage engine. To alter the storage engine, you must drop any foreign key constraints first. foreign_key_checks is available starting from MySQL 3.23.52 and 4.0.3.

InnoDB does not allow you to drop a table that is referenced by a FOREIGN KEY constraint, unless you do SET foreign_key_checks = 0. When you drop a table, the constraints that were defined in its create statement 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 right column names and types, and it must have indexes on the referenced keys, as stated earlier. If these are not satisfied, MySQL returns error number 1005 and refers to error 150 in the error message.

If MySQL reports an error number 1005 from a CREATE TABLE statement, and the error message refers to error 150, table creation failed because a foreign key constraint was not correctly formed. Similarly, if an ALTER TABLE fails and it refers to error 150, that means a foreign key definition would be incorrectly formed for the altered table. Starting from MySQL 4.0.13, you can use SHOW INNODB STATUS to display a detailed explanation of the latest InnoDB foreign key error in the server.

Important

For users familiar with the ANSI/ISO SQL Standard, please note that no storage engine, including InnoDB, recognizes or enforces the MATCH clause used in referential-integrity constraint definitions. 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. Starting from MySQL 3.23.50, InnoDB does not check foreign key constraints on those foreign key or referenced key values that contain a NULL column. InnoDB essentially implements the semantics defined by MATCH SIMPLE, which allow a foreign key to be all or partially NULL. In that case, the (child table) row containing such a foreign key is allowed to be inserted, and does not match any row in the referenced (parent) table.

Additionally, MySQL and InnoDB require that the referenced columns be indexed for performance. 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 UPDATE or DELETE CASCADE. You are advised to use foreign keys that reference only UNIQUE and NOT NULL keys.

Furthermore, InnoDB does not recognize or support “inline REFERENCES specifications” (as defined in the SQL standard) where the references are defined as part of the column specification. InnoDB accepts REFERENCES clauses only when specified as part of a separate FOREIGN KEY specification. For other storage engines, MySQL Server parses and ignores foreign key specifications.

Deviation from SQL standards: If there are several rows in the parent table that have the same referenced key value, InnoDB acts in foreign key checks as if the other parent rows with the same key value do not exist. For example, if you have defined a RESTRICT type constraint, and there is a child row with several parent rows, InnoDB does not allow the deletion of any of those parent rows.

InnoDB performs cascading operations through a depth-first algorithm, based on records in the indexes corresponding to the foreign key constraints.

Deviation from SQL standards: A FOREIGN KEY constraint that references a non-UNIQUE key is not standard SQL. It is an InnoDB extension to standard SQL.

Deviation from SQL standards: If ON UPDATE CASCADE or ON UPDATE SET NULL recurses to update the same table it has previously updated during the cascade, it acts like RESTRICT. This means that you cannot use self-referential ON UPDATE CASCADE or ON UPDATE SET NULL operations. This is to prevent infinite loops resulting from cascaded updates. A self-referential ON DELETE SET NULL, on the other hand, is possible from 4.0.13. A self-referential ON DELETE CASCADE has been possible since ON DELETE was implemented. Since 4.0.21, cascading operations may not be nested more than 15 levels.

Deviation from SQL standards: Like MySQL in general, in an SQL statement that inserts, deletes, or updates many rows, InnoDB checks UNIQUE and FOREIGN KEY constraints row-by-row. According to the SQL standard, the default behavior should be deferred checking. That is, constraints are only checked after the whole SQL statement has been processed. Until InnoDB implements deferred constraint checking, some things will be impossible, such as deleting a record that refers to itself via a foreign key.


User Comments

Posted by Dennis Haney on July 15 2003 10:26am[Delete] [Edit]

For those encountering the problem " ERROR 1216: Cannot add or update a child row: a foreign key constraint fails", it actually means what it says! Some row in the child does not comply with the constraint, correct the problem.
You find the rows like this:
select child.id from child left join parent on (child.parent_id=parent.id) where child.id is not null and parent.id is null;

Posted by Kai Baku on October 10 2003 11:09pm[Delete] [Edit]

There may be rare cases where circular dependencies would make sense. In the case of employees and store, you may have a circular dependency; in which all employees must be stationed at a store, so the employees table will have storeID and EmployeeID attached as a concatonated primary key (Presuming that an Employee can only have one store stationed) or even just a simple non-dependent foriegn key. Then each store must have a top general manager in charge which is stationed there, so the store will have an EmployeeID Foreign Key to the store table to represent that the store has that employee as the manager.

In this case, you have StoreID as an attribute of Employee, and EmployeeID (the Manager) as an attribute of Store.

While this works, it may not be the best method. There are likely better ways to handle such cases, but if your business rules requires such circular dependencies, then it happens.

Also, it is interesting to note that while this query works (Note the PRIMARY KEY line):

CREATE TABLE `ffxi_characterJob` (
`serverID` int(11) NOT NULL,
`userid` int(10)unsigned NOT NULL,
`characterName` varchar(255) NOT NULL,
`jobAbbr` char(4) NOT NULL,
`jobLevel` int(11) default '0',
PRIMARY KEY (`serverID`,`userid`,`characterName`,`jobAbbr`),
INDEX (`jobAbbr`),
CONSTRAINT FOREIGN KEY (`serverID`,`userid`,`characterName`) REFERENCES `ffxi_characters` (`serverID`,`userid`,`characterName`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT FOREIGN KEY (`jobAbbr`) REFERENCES `ffxi_jobType` (`jobAbbr`) ON DELETE CASCADE ON UPDATE CASCADE
) TYPE=InnoDB;

This query will give you an error 1005 and errno 150:

CREATE TABLE `ffxi_characterJob` (
`serverID` int(11) NOT NULL,
`userid` int(10)unsigned NOT NULL,
`characterName` varchar(255) NOT NULL,
`jobAbbr` char(4) NOT NULL,
`jobLevel` int(11) default '0',
PRIMARY KEY (`jobAbbr`,`serverID`,`userid`,`characterName`),
INDEX (`jobAbbr`),
CONSTRAINT FOREIGN KEY (`serverID`,`userid`,`characterName`) REFERENCES `ffxi_characters` (`serverID`,`userid`,`characterName`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT FOREIGN KEY (`jobAbbr`) REFERENCES `ffxi_jobType` (`jobAbbr`) ON DELETE CASCADE ON UPDATE CASCADE
) TYPE=InnoDB;

In order to make the second one work, you have to add:
INDEX (`serverID`,`userid`,`characterName`)
before the the foreign key is made.

Posted by Turadg Aleahmad on January 7 2004 12:55pm[Delete] [Edit]

In a previous comment Dennis Haney provided an SQL snippet for finding rows that violate intended foreign key constraints. I had a lot of data to check so I write a little shell script to save me time:

--

#!/bin/sh
# find-fk-conflicts.sh
# (c) 2004 Turadg Aleahmad, licensed under GPL
# USAGE: find-fk-conflict.sh child_table child_key parent_table parent_key

# NOTE: set this
db="TARGET DATABASE"

child_table=$1
child_key=$2
parent_table=$3
parent_key=$4

query="SELECT $child_table.$child_key FROM $child_table LEFT JOIN $parent_table
ON ( $child_table.$child_key = $parent_table.$parent_key)
WHERE $child_table.$child_key IS NOT NULL AND $parent_table.$parent_key IS NULL;
"

mysql --verbose -u root -e "$query" $db

Posted by Andrew Penry on July 7 2004 2:35pm[Delete] [Edit]

If you have a join on part of a primary key, foriegn key constraints may behave in an unexpected way.

CREATE TABLE doc (
docID INTEGER NOT NULL AUTO_INCREMENT,
langCode CHAR(2) NOT NULL,
title VARCHAR(32),
PRIMARY KEY (docID, langCode)
) Type=InnoDB;

CREATE TABLE author (
authorID INTEGER NOT NULL AUTO_INCREMENT,
docID CHAR(2) NOT NULL,
name VARCHAR(32),
PRIMARY KEY (authorID),
FOREIGN KEY (docID) REFERENCES doc(docID) ON DELETE CASCADE ON UPDATE CASCADE
) Type=InnoDB;

In this case you have documents in several languages. The primary key of the document is the docID and the langCode for that translation. The author of the document is only dependant on the docID, not the language of a particular translation. Therefore the FOREIGN KEY is only on docID.

Although this makes sense, the restraint acts a little funny. Say you have the following data:
doc table
docID langCode title
1 hu A Szamitogep
1 en The Computer

author table
authorID docID name
7 1 Kaposzta Csaba

Deleteing any version of the document will delete the entry in the author table. For example:
DELETE FROM doc WHERE docid=1 AND langCode=en;

now the tables look like:

doc table
docID langCode title
1 hu A Szamitogep

author table
authorID docID name

As you can see, deleting just the translation has deleted the author.

I am unsure about whether this is correct behavior. I've tried this using MS Access to compare, and it won't let me buid relationships on partial primary keys. My feeling is that InnoDB should probably not allow me either, because docID is clearly not a unique index.

Posted by Frank Vanderhallen on August 4 2004 8:10am[Delete] [Edit]

I've too much tables to execute the foreign key dependency checking script by hand. This little script does it all:

#!/bin/sh

# check_constraints.sh
# --------------------
# Check foreign key contraints on MySQL database.
#
# Written by Frank Vanderhallen, licensed under GPL.

if [ -z "$1" ]
then
echo "\nUsage:\n\t./`uname $0` <database> [-h <host>] [-u user] [-p <passwd>]\n"
exit
fi

CONSTRAINTS=`mysqldump $* | grep "CREATE\|CONSTRAINT" | sed 's/ /+/g'`

for c in $CONSTRAINTS
do
if [ "`echo $c | cut -d '+' -f 3`" = "CONSTRAINT" ]
then
CONSTRAINT=`echo $c | cut -d '+' -f 4 | tr -d '\`'`
CHILD_KEY=`echo $c | cut -d '+' -f 7 | tr -d '()\`,'`
PARENT_TABLE=`echo $c | cut -d '+' -f 9 | tr -d '\`'`
PARENT_KEY=`echo $c | cut -d '+' -f 10 | tr -d '()\`,'`
QUERY="select c.$CHILD_KEY from $CHILD_TABLE as c left join $PARENT_TABLE as p on p.$PARENT_KEY=c.$CHILD_KEY where c.$CHILD_KEY is not null and p.$PARENT_KEY is null;"
echo "Checking table '$CHILD_TABLE' constraint '$CONSTRAINT'"
mysql -verbose $* -e "$QUERY"
else
CHILD_TABLE=`echo $c | cut -d '+' -f 3`
fi
done

Posted by Fabio Venuti on September 20 2004 1:32pm[Delete] [Edit]

The fact that "NO ACTION" and "RESTRICT" should be treated equally means that there is no way to delete a parent row without deleting the child row unless you disable the foreign key check. This is normally what we want, but there might be exceptions where it makes sense to keep an orphan row, e.g., when you have a "history" table that maintains some information about records that can be safely deleted.

Posted by matt emmons on September 4 2006 7:56pm[Delete] [Edit]

In 4.1.18 this;

CREATE TABLE foo (
a int(11) NOT NULL default '0',
b int(11) NOT NULL default '0',
PRIMARY KEY (a,b),
KEY b (b),
CONSTRAINT FOREIGN KEY (a) REFERENCES other_table1.a,
CONSTRAINT FOREIGN KEY (b) REFERENCES other_table2.b
) ENGINE=InnoDB;

provokes errno 150 / 1005. Show InnoDB status' latest foreign key error reports, "cannot resolve table name close to...."

All tables (foo, other_table1 & other_table2) are InnoDB. other_table1.a and other_table2.b are single attribute primary keys (thus satisfying the "first column" index requirement).

This, on the other hand, works fine:

CREATE TABLE foo (
a int(11) NOT NULL default '0',
b int(11) NOT NULL default '0',
PRIMARY KEY (a,b),
KEY b (b),
CONSTRAINT FOREIGN KEY (a) REFERENCES other_table1(a),
CONSTRAINT FOREIGN KEY (b) REFERENCES other_table2(b)
) ENGINE=InnoDB;

The only difference is how the referenced field is specified - table.field v. table(field).

I wonder if the indexes, in either the referring or referenced tables, being named the same as their respective fields isn't a problem.

Posted by jim kraai on January 4 2007 8:16pm[Delete] [Edit]

Dynamic Stored Procedure to identify conflicts prior to adding a FOREIGN KEY constraint

Here's a stored proc inspired by by Turadg Aleahmad's shell script named find-fk-conflicts.sh seen in the refman/5.0 page comments:

<?
DELIMITER |

DROP PROCEDURE IF EXISTS sp_find_fk_conflict |

CREATE PROCEDURE         sp_find_fk_conflict(
  IN dbname CHAR(64),    -- database name
  IN ctn CHAR(64),       -- child table name
  IN ckn CHAR(64),       -- child key name
  IN ptn CHAR(64),       -- parent table name
  IN pkn CHAR(64)        -- parent table name
)

COMMENT
"""
  sp_find_fk_conflict

  Created:  20060913
  By jim kraai (jim NO at SPAM kraai.org)

  Inspired by Turadg Aleahmad's find-fk-conflicts shell script

  Inputs:
    dbname
    child table name
    child key name
    parent table name
    parent key name
  Outputs
    rows that would conflict if adding a foreign key constrant

  USAGE:  call sp_find_fk_conflict('some_db','some_childTable','some_childTable_key','some_parentTable','some_parentTable_key');

"""

BEGIN

  DECLARE s TEXT;

  SET @s = CONCAT(
    'SELECT ',    dbname,'.',ctn,'.',ckn,' ',
    'FROM ',      dbname,'.',ctn,' ',
    'LEFT JOIN ', dbname,'.',ptn,
     ' ON ',
        '( ',     dbname,'.',ctn,'.',ckn,
                  ' = ',
                  dbname,'.',ptn,'.',pkn,
        ') ',
    'WHERE ',
                  dbname,'.',ctn,'.',ckn,' IS NOT NULL ',
     ' AND ',
                  dbname,'.',ptn,'.',pkn,' IS NULL;'
  );

  PREPARE stmt FROM @s;
  EXECUTE stmt;
  DEALLOCATE PREPARE stmt;

END; |

DELIMITER ;
?>

(Ignore the PHP <? ?> around the SP, it's there to preserve formatting)

Posted by Dennis Nikolaenko on June 5 2007 12:29pm[Delete] [Edit]

Note that as of version 5.0.38, InnoDB allows two or more foreign keys on the column, they may reference diffrent tables/columns. It even allows foreign keys with the same definition, but different constraint name.

Posted by Lupus Arctos on July 19 2007 8:34pm[Delete] [Edit]

Modified foreign key dependency checker script posted by Frank Vanderhallen.
This script supports composite keys.

#!/bin/sh

# check_constraints.sh
# --------------------
# Check foreign key contraints on MySQL database.
#
# Written by Frank Vanderhallen and modified by Lupus Arctos, licensed under GPL.

if [ -z "$1" ]
then
echo "\nUsage:\n\t./`uname $0` <database> [-h <host>] [-u user] [-p <passwd>]\n"
exit
fi

CONSTRAINTS=`mysqldump $* | grep "CREATE\|CONSTRAINT" | sed 's/, /,/g' | sed 's/ /+/g'`

for c in $CONSTRAINTS
do
if [ "`echo $c | cut -d '+' -f 3`" = "CONSTRAINT" ]
then

CONSTRAINT=`echo $c | cut -d '+' -f 4 | tr -d '\`'`
CHILD_KEY=`echo $c | cut -d '+' -f 7 | tr -d '()\`'`
PARENT_TABLE=`echo $c | cut -d '+' -f 9 | tr -d '\`'`
PARENT_KEY=`echo $c | cut -d '+' -f 10 | tr -d '()\`'`

declare -a PARENT_KEYS=($(echo $PARENT_KEY|sed 's/,/ /g'))
declare -a CHILD_KEYS=($(echo $CHILD_KEY|sed 's/,/ /g'))

let PARENT_KEYS_LASTIDX=${#PARENT_KEYS[@]}-1
let CHILD_KEYS_LASTIDX=${#CHILD_KEYS[@]}-1

JOINON=
CHILD_TABLE_KEY=
for k in `seq 0 $PARENT_KEYS_LASTIDX`; do
JOINON=`echo $JOINON p.${PARENT_KEYS[k]}=c.${CHILD_KEYS[k]}`
CHILD_TABLE_KEY=`echo $CHILD_TABLE_KEY c.${CHILD_KEYS[k]}`
if [ $k != $PARENT_KEYS_LASTIDX ]; then
JOINON=`echo $JOINON and`
CHILD_TABLE_KEY=`echo $CHILD_TABLE_KEY,`
fi
if [ $k == 0 ]; then
CHILD_WHEN=`echo p.${PARENT_KEYS[k]} is not null`
PARENT_WHEN=`echo c.${CHILD_KEYS[k]} is null`
fi
done

QUERY="select $CHILD_TABLE_KEY from $CHILD_TABLE as c left join $PARENT_TABLE as p on $JOINON where $CHILD_WHEN and $PARENT_WHEN;"
echo "Checking table '$CHILD_TABLE' constraint '$CONSTRAINT'"
#mysql -v $* -e "$QUERY"
mysql $* -e "$QUERY"
else
CHILD_TABLE=`echo $c | cut -d '+' -f 3`
fi
done

Posted by Alex Baeza on March 24 2008 7:15pm[Delete] [Edit]

It is often difficult to determine which tables have children.

One nice new feature of MySQL 5.02 and above is the information_schema. You can use the information_schema to determine dependencies using a query such as:

SELECT
ke.referenced_table_name parent,
ke.table_name child,
ke.constraint_name
FROM
information_schema.KEY_COLUMN_USAGE ke
WHERE
ke.referenced_table_name IS NOT NULL
ORDER BY
ke.referenced_table_name;

This will show all the parent tables that have children in your current database. This example can also be modified to show all parent child relationships across multiple databases.

Posted by Altemir Soares on November 28 2008 1:06am[Delete] [Edit]

Hi,

This way you can do foreign key checks just from any sql client tool.

SET @child_table='EMPLOYEES';
SET @child_key='DEPARTMENT_ID';
SET @parent_table='DEPARTMENTS';
SET @parent_key='DEPARTMENT_ID';

select CONCAT('SELECT ',@child_table ,'.',@child_key,' FROM ',@child_table,' LEFT JOIN ', @parent_table,
CONCAT('\n ON (',@child_table,'.',@child_key,' = ',@parent_table,'.',@parent_key,')'),
CONCAT('\n ','WHERE ',@child_table,'.',@child_key,' IS NOT NULL AND ',@parent_table,'.',@parent_key,' IS NULL'));

Thanks

Posted by Ran Wei on March 10 2009 7:55pm[Delete] [Edit]

RE: Kai Baku
I believe the reason why your second example does not work is the following line from the Manual:
"In the referencing table, there must be an index where the foreign key columns are listed as the first columns in the same order"

In your first example, your primary keys are specified in the order, (`serverID`,`userid`,`characterName`,`jobAbbr`) which is the same as your foreign key specification.

In the second example, your keys are specified in the order, (`jobAbbr`,`serverID`,`userid`,`characterName`), but your foreign key specification places 'jobAbbr' last.

AFAIK, specifying INDEX (`jobAbbr`) will create a new index for jobAbbr but this one will be at the end, rather than the beginning, which again makes your index creation and foreign key creation match up.

Posted by Claude Schlesser on April 3 2009 2:40pm[Delete] [Edit]

I have recently written a (german) article on how to import InnoDB tables and prevent the error "Cannot delete or update a parent row: a foreign key constraint fails".

You can find it here:
http://www.lunar.lu/cannot-delete-or-update-a-parent-row-a-foreign-key-constraint-fails/

Regards,

Posted by Toby Thain on May 22 2009 5:21am[Delete] [Edit]

In our projects, we:
1) maintain separate SQL file per table (nicer version control histories)
2) need automated builds
3) use FK constraints extensively

Instead of disabling checks, which disables meaningful validity checks on the constraints, we use make to find a valid creation order. This also detects circular references. Code here: http://www.telegraphics.com.au/svn/fk/trunk/

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