MySQL变量（使用声明）

Question

I have set MySQL variable:

SET @test = 'my_db';
select @test; 

Then

use  @test
ERROR 1049 (42000): Unknown database '@test'

OR

mysql> SET @test = 'my_db';
Query OK, 0 rows affected (0.00 sec)

mysql> select @test;
+-------+
| @test |
+-------+
| my_db   |
+-------+
1 row in set (0.00 sec)

mysql> use @test
ERROR 1049 (42000): Unknown database '@test'

How I can use variable value with use USE STATEMENT in MySQL?

Answer 1

Variables cannot be used as identifiers in MySQL. You're looking for macro expansion, which MySQL doesn't support.

User variables are intended to provide data values. They cannot be used directly in an SQL statement as an identifier or as part of an identifier, such as in contexts where a table or database name is expected, or as a reserved word such as SELECT

From http://dev.mysql.com/doc/refman/5.6/en/user-variables.html

It is possible to work around this for some statements such as SELECT by using prepared statements, but you cannot prepare a USE statement, so it's not an option for you in this case.

You'll have to select the database in your application code by using mysqli_select_db($test) or by executing a query such as mysqli_query("USE $test");