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.

score 31 · Accepted Answer · answered May 21 '09 at 16:54

31

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.

answered May 21 '09 at 16:54

Frank Farmer

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14

Thanks, that is great. I ended up using this: SHOW KEYS FROM WHERE Key_name = 'PRIMARY' – joshwbrick May 21 '09 at 17:13
1

This returns the full row... if you want just the column name, see Jake Sully's answer below. – Andrew Feb 11 '16 at 17:53
As the guy who wrote this answer, I have to agree: Jake Sully's answer is the best one so far. @macinjosh I'd recommend un-accepting my answer, and accepting Jake's instead. – Frank Farmer Mar 10 '16 at 23:53

lukmdo · Answer 2 · 2016-03-10T00:44:50.577

20

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.

edited Mar 10 '16 at 00:44

answered Mar 24 '10 at 09:54

lukmdo

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Hi, I know this is an old post but I just wanted to ask why you said the first method is not advised. The accepted answer from [this duplicate question](https://stackoverflow.com/questions/2341278/php-get-primary-key-of-table) says you don't always have access to the information_schema so it suggests the first method. Plus you said yourself that the second method is a few times slower than the first solution. If that is the case, why is the second method solid? Thanks. – Cave Johnson Mar 09 '16 at 18:18
As pointed in http://mysql-0v34c10ck.blogspot.com/2011/05/better-way-to-get-primary-key-columns.html it can not be used as mysql variable or think view etc but if the first case works in your scenario go for it @Andrew – lukmdo Mar 10 '16 at 00:42

score 18 · Answer 3 · answered Jun 01 '11 at 13:26

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

score 0 · Answer 4 · answered Dec 29 '16 at 05:36

0

Also

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

Is equivalent to

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

answered Dec 29 '16 at 05:36

Hikmat Sijapati

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score 0 · Answer 5 · answered Feb 09 '17 at 22:23

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

MySQL: Determine Table's Primary Key Dynamically

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