android sqlite新添唯一键constraint错误的解决方法

　　今天在做项目，需要在先前的表上面新增一个constraint，然后就用了“alter table mag_content_list add constraint unique(id);”这么一条语句。

　　结果，在升级的时候出错了。查询了一下。查到这么一句话“The ALTER TABLE command in SQLite allows the user to rename a table or to add a new column to an existing table. It is not possible to rename a column, remove a column, or add or remove constraints from a table.”　

　　意思就是说sqlite不支持重命名，删除字段或者添加、删除约束。

　　解决的办法就是：

　　　 １、新建一个临时表，当前表数据插入

２、新建一个新表，把临时表数据插入

３、删除临时表和旧的表

以上信息参照了网上的一个文章：http://rimland.blog.163.com/blog/static/883254620129220925454/

内容如下：

Altering SQLite column type and adding PK constraint

How to change the type of a column in a SQLite table?

I've got:

    CREATE TABLE table(        id INTEGER,        salt TEXT NOT NULL UNIQUE,        step INT,        insert_date TIMESTAMP    );

I'd like to change salt's type to just TEXT and id's type to INTEGER PRIMARY KEY.

Answer:

Below is an excerpt from the SQLite manual discussing the ALTER TABLE command (see URL: SQLite Alter Table):

SQLite supports a limited subset of ALTER TABLE. The ALTER TABLE command in SQLite allows the user to rename a table or to add a new column to an existing table. It is not possible to rename a colum, remove a column, or add or remove constraints from a table.

As the manual states, it is not possible to modify a column's type or constraints, such as converting NULL to NOT NULL. However, there is a work around by

1. copying the old table to a temporary table,
2. creating a new table defined as desired, and
3. copying the data from the temporary table to the new table.

To give credit where credit is due, I learned this from the discussion on Issue #1 of hakanw's django-email-usernames project on bitbucket.org.

CREATE TABLE test_table(    id INTEGER,    salt TEXT NOT NULL UNIQUE,    step INT,    insert_date TIMESTAMP);ALTER TABLE test_table RENAME TO test_table_temp;CREATE TABLE test_table(    id INTEGER PRIMARY KEY,    salt TEXT,    step INT,    insert_date TIMESTAMP);INSERT INTO test_table SELECT * FROM test_table_temp;DROP TABLE test_table_temp;

Notes

1. I used the table name test_table since SQLite will generate an error if you try to name a table astable.
2. The INSERT INTO command will fail if your data does not conform to the new table constraints. For instance, if the original test_table contains two id fields with the same integer, you will receive an "SQL error: PRIMARY KEY must be unique" when you execute the "INSERT INTO test_table SELECT * FROM test_table_temp;" command.
3. For all testing, I used SQLite version 3.4.0 as included as part of Python 2.6.2 running on my 13" Unibody MacBook with Mac OS X 10.5.7.