Primary and foreign key in SQL?

Question

I am not sure about this , but do I need to create foreign key explicitly in the SQL command?

This guy did this:

CREATE TABLE languages (
lang_id TINYINT UNSIGNED NOT NULL AUTO_INCREMENT,
lang VARCHAR(60) NOT NULL,
lang_eng VARCHAR(20) NOT NULL,
PRIMARY KEY (lang_id),
UNIQUE (lang)
);


CREATE TABLE threads (
thread_id INT UNSIGNED NOT NULL AUTO_INCREMENT,
lang_id TINYINT(3) UNSIGNED NOT NULL,
user_id INT UNSIGNED NOT NULL,
subject VARCHAR(150) NOT NULL,
PRIMARY KEY  (thread_id),
INDEX (lang_id),
INDEX (user_id)
);

In this case, does it mean that INDEX(lang_id) becomes FOREIGN KEY automatically? I know INDEX makes search go faster, but I don't understand the part about foreign key

I would really appreciate any answer

Answer 1

No. An index is just that... an index on a field. A foreign key tells MySQL that "this particular field MUST have a matching record in that table over there".

MySQL's internal design requires that all fields used as foreign keys be indexed, but modern versions will automatically create that index for you.

The converse is not true, whoever. Adding an index to a field does not turn it into a foreign key - a foreign key definition must also include what the foreign table/field is, and a simple index declaration has none of that information.

For your sample table, you'd need to have

...
INDEX (lang_id),
FOREIGN KEY (lang_id) REFERENCES languages (lang_id),
...

to produce a foreign key.

Answer 2

A foreign key means that the value(s) must exist in the referenced column(s). It is not automatic - you need to write it explicitly.

FOREIGN KEY lang_id REFERENCES languages (lang_id)

Answer 3

No a foreign key has to be explicitly declared

CREATE TABLE threads (
thread_id INT UNSIGNED NOT NULL AUTO_INCREMENT,
lang_id TINYINT(3) UNSIGNED NOT NULL FOREIGN KEY FK_1 REFERENCES languages(lang_id),
user_id INT UNSIGNED NOT NULL,
subject VARCHAR(150) NOT NULL,
PRIMARY KEY  (thread_id),
INDEX (lang_id),
INDEX (user_id)
);

Answer 4

What you now have are two tables with primary keys and indexes on those primary key values.

You could stop here if you want but you won't have declared referential integrity enforcing that relationship between laguage and threads tables.

To do that you would explicitly create a foreign key relationship as explained here - http://dev.mysql.com/doc/refman/5.5/en/innodb-foreign-key-constraints.html

Answer 5

The differences are as follows :-

• The primary key identifies a record uniquely in a table with multiple rows.

• An index is a generic term, where by you can create more than one index for a table, in this case the database creates indexes based on the columns that you specified, so that when you query the appropriate index will kick in and give you results faster.

• A foreign key on the other hand says that this column in table b, is the primary column in table A, so that whenever you enter rows into table B the databse will check that the specified column/data exists in table A otherwise it will throw an error.