MySQL: Determine Table's Primary Key Dynamically

Question

I'm, generating a SQL query like this in PHP:

$sql = sprintf("UPDATE %s SET %s = %s WHERE %s = %s", ...);

Since almost every part of this query is dynamic I need a way to determine the table's primary key dynamically, so that I'd have a query like this:

$sql = sprintf("UPDATE %s SET %s=%s WHERE PRIMARY_KEY = %s", ...);

Is there a MySQL keyword for a table's primary key, or a way to get it?

I've used the information_schema DB before to find information like this, but it'd be nice if I didn't have to resort to that.

Answer 1

SHOW INDEX FROM <tablename>

You want the row where Key_name = PRIMARY

http://dev.mysql.com/doc/refman/5.0/en/show-index.html

You'll probably want to cache the results -- it takes a while to run SHOW statements on all the tables you might need to work with.

Answer 2

It might be not advised but works just fine:

SHOW INDEX FROM <table_name> WHERE Key_name = 'PRIMARY';

The solid way is to use information_schema:

SELECT k.COLUMN_NAME
FROM information_schema.table_constraints t
LEFT JOIN information_schema.key_column_usage k
USING(constraint_name,table_schema,table_name)
WHERE t.constraint_type='PRIMARY KEY'
    AND t.table_schema=DATABASE()
    AND t.table_name='owalog';

As presented on the mysql-list. However its a few times slower from the first solution.

Answer 3

A better way to get Primary Key columns:

SELECT `COLUMN_NAME`
FROM `information_schema`.`COLUMNS`
WHERE (`TABLE_SCHEMA` = 'dbName')
  AND (`TABLE_NAME` = 'tableName')
  AND (`COLUMN_KEY` = 'PRI');

From http://mysql-0v34c10ck.blogspot.com/2011/05/better-way-to-get-primary-key-columns.html

Answer 4

Also

SHOW INDEX FROM <table_name> WHERE Key_name = 'PRIMARY';

Is equivalent to

SHOW KEYS FROM <table_name> WHERE Key_name = 'PRIMARY';

Answer 5

Based on @jake-sully and @lukmdo answers, making a merge of their code, I finished with the following snippet:

SELECT `COLUMN_NAME`
FROM `information_schema`.`COLUMNS`
WHERE (`TABLE_SCHEMA` = DATABASE())
AND (`TABLE_NAME` = '<tablename>')
AND (`COLUMN_KEY` = 'PRI');

Hope it could help someone