ADD COLUMN to sqlite db IF NOT EXISTS - flex/air s

问题:

I've got a flex/air app I've been working on, it uses a local sqlite database that is created on the initial application start.

I've added some features to the application and in the process I had to add a new field to one of the database tables. My questions is how to I go about getting the application to create one new field that is located in a table that already exists?

this is a the line that creates the table

stmt.text = "CREATE TABLE IF NOT EXISTS tbl_status ("+"status_id INTEGER PRIMARY KEY AUTOINCREMENT,"+" status_status TEXT)";

And now I'd like to add a status_default field.

thanks!

Thanks - MPelletier

I've add the code you provided and it does add the field, but now the next time I restart my app I get an error - 'status_default' already exists'.

So how can I go about adding some sort of a IF NOT EXISTS statement to the line you provided?

回答1:

ALTER TABLE tbl_status ADD COLUMN status_default TEXT;

http://www.sqlite.org/lang_altertable.html

That being said, adding columns in SQLite is limited. You cannot add a column anywhere but after the last column in your table.

As for checking if the column already exists, PRAGMA table_info(tbl_status); will return a table listing the various columns of your table.

ADD ON:

I've been using a strategy in database design that allows me to distinguish which modifications are required. For this, you will need a new table (call it DBInfo), with one field (Integer, call it SchemaVersion). Alternately, there is also an internal value in SQLite called user_version, which can be set with a PRAGMA command. Your code can, on program startup, check for schema version number and apply changes accordingly, one version at a time.

Suppose a function named UpdateDBSchema(). This function will check for your database schema version, handle DBInfo not being there, and determine that the database is in version 0. The rest of this function could be just a large switch with different versions, nested in a loop (or other structure available to your platform of choice).

So for this first version, have an UpgradeDBVersion0To1() function, which will create this new table (DBInfo), add your status_default field, and set SchemaVersion to 1. In your code, add a constant that indicates the latest schema version, say LATEST_DB_VERSION, and set it to 1. In that way, your code and your database have a schema version, and you know you need to synch them if they are not equal.

When you need to make another change to your schema, set the LATEST_DB_VERSION constant to 2 and make a new UpgradeDBVersion1To2() function that will perform the required changes.

That way, your program can be ported easily, can connect to and upgrade an old database, etc.

回答2:

I know this is an old question... however.

I've hit this precise problem in the SQLite implementation in Adobe AIR. I thought it would be possible to use the PRAGMA command to resolve, but since adobe air's implementation does not support the PRAGMA command, we need an alternative.

What I did, that I thought would be worth while sharing here, is this:

var sql:SQLStatement = new SQLStatement();
sql.sqlConnection = pp_db.dbConn;
sql.text = "SELECT NewField FROM TheTable";
sql.addEventListener(SQLEvent.RESULT, function(evt:SQLEvent):void {
});

sql.addEventListener(SQLErrorEvent.ERROR, function(err:SQLErrorEvent):void {
    var sql:SQLStatement = new SQLStatement();
    sql.sqlConnection = pp_db.dbConn;
    sql.text = "ALTER TABLE TheTable ADD COLUMN NewField NUMERIC;";
    sql.execute();
    sql.addEventListener(SQLEvent.RESULT, function (evt:SQLEvent):void {
    });
});
sql.execute();

Hope it helps someone.

回答3:

I solved a similar problem using the answer from this question: ALTER TABLE ADD COLUMN IF NOT EXISTS in SQLite

Use built in user_version parameter to keep track of your updates. You set it using:

PRAGMA user_version = 1

and you retrieve it using

PRAGMA user_version

So basically retrieve user_version (it's 0 by default), check if it's 0. If yes, perform your updates and set it to 1. If you have more updates in the future, check if it's 1, perform updates and set it to 0. And so on...

回答4:

In some cases I execute the command and get the exception for "duplicate column". Just a quick solution, not the perfect.