Question

I have 3 tables, images, icons, and banners, each with a unique primary key that is also auto_incremented named image_id, icon_id, and banner_id, respectively.

I’m looping through the above tables and I’m wondering if there’s a way I can select the id column without specifying it’s specific name.

Something like

SELECT PRIMARY_KEY
FROM {$table}

Where I don’t have to change my table structure or use * as there would be much data to return and would slow down my application.

Answer 1

I’m not a LAMP guy, but it looks to me like you want the INFORMATION_SCHEMA tables.

A query something like :

SELECT     pk.table_name, column_name as 'primary_key'
FROM       information_schema.table_constraints pk 
INNER JOIN information_schema.key_column_usage C 
                                    on c.table_name = pk.table_name  and
                                       c.constraint_name = pk.constraint_name
where   constraint_type = 'primary key'
           -- and pk.table_name  LIKE '%whatever%'

This above query (filtered to whatever relevant set of tables you need) will give you bit a list of table names and associated Primary Keys. What that information on hand you could query something like :

SELECT {$PK_ColumnName}
FROM {$table}

Note, you might needs a more complicated syntax and string builder if you have composite primary keys (i.e. more than one field per key). Also, the information schema can be relatively expensive to query, so you’ll either want to cache the result set up, or query it infrequently.