How can I get the foreign keys of a table in mysql

问题

I am creating a class which, takes a table from a database, and displays it to a web-page, with as much functionality as possible. One of the things I would like to support, would be having the class detect which columns in the table have a foreign key constraint on them, so that it can then go to those tables, get all of their values and use them in a select-box which is called when you edit those fields, to avoid someone violating foreign key constraints,

The main problem is discovering which fields have a foreign key constraint on them, and which tables they are pointing to. Does anyone know how to do this???

Thanks,

Lemiant

回答1:

Simple way to get foreign keys for given table:

SELECT
    `column_name`, 
    `referenced_table_schema` AS foreign_db, 
    `referenced_table_name` AS foreign_table, 
    `referenced_column_name`  AS foreign_column 
FROM
    `information_schema`.`KEY_COLUMN_USAGE`
WHERE
    `constraint_schema` = SCHEMA()
AND
    `table_name` = 'your-table-name-here'
AND
    `referenced_column_name` IS NOT NULL
ORDER BY
    `column_name`;

回答2:

The INFORMATION_SCHEMA database contains details of the full schema of all other databases, including constraints:

http://dev.mysql.com/doc/refman/5.0/en/information-schema.html

You can also run a SHOW CREATE TABLE query to get the SQL to create a table, including its constraints.

回答3:

Much can be retrieved from MySQL's information_schema, foreign keys included, as pointed out by dev-null-dweller.

1

SELECT * FROM information_schema.table_constraints 
         WHERE table_schema = 'dbname' AND table_name='mytable';

Instead of dbname use the function SCHEMA() to set the name of the database in USE.

2

As pointed out by Dan Grossman, the command

SHOW CREATE TABLE `yourtablename`

can be used basically get an SQL dump of the create table statement.

~3

MySQL provides a SHOW KEYS command. As such you could theoretically get the FK if you know a lower cardinality threshold and have few other keys in the table.

SHOW KEYS FROM `address` WHERE Non_unique AND CARDINALITY > 10000

As the key's cardinality changes each time the internal database is changed, this is rather theoretical. See the cardinality change for instance with running ANALYZE TABLE.

It is useful to stick to a naming schema, such as foreigntablename_foreignfieldname. For example the field user_id in a table billing. Several ORMs of big Web Content Frameworks use this schema.

来源：https://stackoverflow.com/questions/4159998/how-can-i-get-the-foreign-keys-of-a-table-in-mysql

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