Question

I know how to pipe one MySQL query into another:

SELECT user_name FROM users WHERE user_id=( SELECT user_id FROM entries WHERE header='foo' ); 

Out of pure intellectual curiosity, how I dynamically choose a column or a table?

Ex:

SELECT (      SELECT column_name FROM column_names WHERE id = 1  ) FROM (      SELECT table_name FROM table_names WHERE id = 1  );  

Answer 1

Use a prepared statement:

 mysql> SET @sql = CONCAT('SELECT ', (SELECT 'NOW()')); Query OK, 0 rows affected (0.00 sec)  mysql> SELECT @sql; +--------------+ | @sql         | +--------------+ | SELECT NOW() |  +--------------+ 1 row in set (0.00 sec)  mysql> PREPARE stmt FROM @sql; Query OK, 0 rows affected (0.00 sec) Statement prepared  mysql> EXECUTE stmt; +---------------------+ | NOW()               | +---------------------+ | 2009-04-06 23:08:31 |  +---------------------+ 1 row in set (0.00 sec)