在MySQL中动态选择列 [英] Dynamically choosing a column in MySQL

问题描述

我知道如何将一个MySQL查询传递到另一个:

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" );

出于纯粹的求知欲，我如何动态选择列或表?

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

例如:

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

推荐答案

使用准备好的语句:

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)

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