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3.6.6 Using Foreign Keys

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In MySQL, InnoDB tables support checking of foreign key constraints. See Chapter 15, The InnoDB Storage Engine, and Section 1.8.2.3, “Foreign Key Differences”.

A foreign key constraint is not required merely to join two tables. For storage engines other than InnoDB, it is possible when defining a column to use a REFERENCES tbl_name(col_name) clause, which has no actual effect, and serves only as a memo or comment to you that the column which you are currently defining is intended to refer to a column in another table. It is extremely important to realize when using this syntax that:

  • MySQL does not perform any sort of check to make sure that col_name actually exists in tbl_name (or even that tbl_name itself exists).

  • MySQL does not perform any sort of action on tbl_name such as deleting rows in response to actions taken on rows in the table which you are defining; in other words, this syntax induces no ON DELETE or ON UPDATE behavior whatsoever. (Although you can write an ON DELETE or ON UPDATE clause as part of the REFERENCES clause, it is also ignored.)

  • This syntax creates a column; it does not create any sort of index or key.

You can use a column so created as a join column, as shown here:

  1. CREATE TABLE person (
  2.     id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,
  3.     name CHAR(60) NOT NULL,
  4.     PRIMARY KEY (id)
  5. );
  6.  
  7. CREATE TABLE shirt (
  8.     id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,
  9.     style ENUM('t-shirt', 'polo', 'dress') NOT NULL,
  10.     color ENUM('red', 'blue', 'orange', 'white', 'black') NOT NULL,
  11.     owner SMALLINT UNSIGNED NOT NULL REFERENCES person(id),
  12.     PRIMARY KEY (id)
  13. );
  14.  
  15. INSERT INTO person VALUES (NULL, 'Antonio Paz');
  16.  
  17. SELECT @last := LAST_INSERT_ID();
  18.  
  19. INSERT INTO shirt VALUES
  20. (NULL, 'polo', 'blue', @last),
  21. (NULL, 'dress', 'white', @last),
  22. (NULL, 't-shirt', 'blue', @last);
  23.  
  24. INSERT INTO person VALUES (NULL, 'Lilliana Angelovska');
  25.  
  26. SELECT @last := LAST_INSERT_ID();
  27.  
  28. INSERT INTO shirt VALUES
  29. (NULL, 'dress', 'orange', @last),
  30. (NULL, 'polo', 'red', @last),
  31. (NULL, 'dress', 'blue', @last),
  32. (NULL, 't-shirt', 'white', @last);
  33.  
  34. SELECT * FROM person;
  35. +----+---------------------+
  36. | id | name                |
  37. +----+---------------------+
  38. |  1 | Antonio Paz         |
  39. |  2 | Lilliana Angelovska |
  40. +----+---------------------+
  41.  
  42. SELECT * FROM shirt;
  43. +----+---------+--------+-------+
  44. | id | style   | color  | owner |
  45. +----+---------+--------+-------+
  46. |  1 | polo    | blue   |     1 |
  47. |  2 | dress   | white  |     1 |
  48. |  3 | t-shirt | blue   |     1 |
  49. |  4 | dress   | orange |     2 |
  50. |  5 | polo    | red    |     2 |
  51. |  6 | dress   | blue   |     2 |
  52. |  7 | t-shirt | white  |     2 |
  53. +----+---------+--------+-------+
  54.  
  55.  
  56. SELECT s.* FROM person p INNER JOIN shirt s
  57.    ON s.owner = p.id
  58.  WHERE p.name LIKE 'Lilliana%'
  59.    AND s.color <> 'white';
  60.  
  61. +----+-------+--------+-------+
  62. | id | style | color  | owner |
  63. +----+-------+--------+-------+
  64. |  4 | dress | orange |     2 |
  65. |  5 | polo  | red    |     2 |
  66. |  6 | dress | blue   |     2 |
  67. +----+-------+--------+-------+

When used in this fashion, the REFERENCES clause is not displayed in the output of SHOW CREATE TABLE or DESCRIBE:

  1. SHOW CREATE TABLE shirt\G
  2. *************************** 1. row ***************************
  3. Table: shirt
  4. Create Table: CREATE TABLE `shirt` (
  5. `id` smallint(5) unsigned NOT NULL auto_increment,
  6. `style` enum('t-shirt','polo','dress') NOT NULL,
  7. `color` enum('red','blue','orange','white','black') NOT NULL,
  8. `owner` smallint(5) unsigned NOT NULL,
  9. PRIMARY KEY  (`id`)
  10. ) ENGINE=MyISAM DEFAULT CHARSET=utf8mb4

The use of REFERENCES in this way as a comment or “reminder” in a column definition works with MyISAM tables.


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Document created the 26/06/2006, last modified the 26/10/2018
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