SELECT k.column_name
FROM information_schema.table_constraints t
JOIN information_schema.key_column_usage k
USING(constraint_name,table_schema,table_name)
WHERE t.constraint_type='PRIMARY KEY'
  AND t.table_schema='db'
  AND t.table_name='tbl';

http://lists.mysql.com/mysql/204944

Select the primary key for a table

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