mysql中数据类型不一样用IF查会出错

Microsoft Windows XP [版本 5.1.2600] (C) 版权所有 1985-2001 Microsoft Corp. C:\Documents and Settings\Administrator>mysql -u root -p Enter password: Welcome to the MySQL monitor.  Commands end with ; or \g. Your MySQL connection id is 3 Server version: 5.5.36 MySQL Community Server (GPL) Copyright (c) 2000, 2014, Oracle and/or its affiliates. All rights reserved. Oracle is a registered trademark of Oracle Corporation and/or its affiliates. Other names may be trademarks of their respective owners. Type 'help;' or '\h' for help. Type '\c' to clear the current input statement. mysql> use sondome; ERROR 1049 (42000): Unknown database 'sondome' mysql> use sondemo; Database changed mysql> show tables; +--------------------+ | Tables_in_sondemo  | +--------------------+ | info_user          | | info_user_consumer | +--------------------+ 2 rows in set (0.00 sec) mysql> alter info_user add C_id int; ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'info_user add C_id int' at line 1 mysql> alter table info_user add column C_id int not null; Query OK, 3 rows affected (0.08 sec) Records: 3  Duplicates: 0  Warnings: 0 mysql> desc info_user; +--------+--------------+------+-----+---------+----------------+ | Field  | Type         | Null | Key | Default | Extra          | +--------+--------------+------+-----+---------+----------------+ | U_id   | int(11)      | NO   | PRI | NULL    | auto_increment | | U_name | varchar(100) | NO   |     | NULL    |                | | C_id   | int(11)      | NO   |     | NULL    |                | +--------+--------------+------+-----+---------+----------------+ 3 rows in set (0.00 sec) mysql> select if(C_id,C_id,U_id) as chuan,C_id from info_user; +-------+------+ | chuan | C_id | +-------+------+ |     1 |    0 | |     2 |    0 | |     3 |    0 | +-------+------+ 3 rows in set (0.00 sec) mysql>