I'm using SQLite, which doesn't support adding a constraint to an existing table.
So I can't do something like this (just as an example):
ALTER TABLE [Customer]
ADD CONSTRAINT specify_either_phone_or_email
CHECK (([Phone] IS NOT NULL) OR ([Email] IS NOT NULL));
Are there any workarounds for this scenario?
I know:
Ideas
To make a copy of a table with some schema changes, you have to do the creation and the copying manually:
BEGIN;
CREATE TABLE Customer_new (
[...],
CHECK ([...])
);
INSERT INTO Customer_new SELECT * FROM Customer;
DROP TABLE Customer;
ALTER TABLE Customer_new RENAME TO Customer;
COMMIT;
To read the schema, execute .schema Customer
in the sqlite3
command-line shell.
This gives you the CREATE TABLE statement, which you can edit and execute.
To change the table in place, you can use a backdoor.
First, read the actual table definition (this is the same as what you would get from .schema
):
SELECT sql FROM sqlite_master WHERE type = 'table' AND name = 'Customer';
Add your CHECK constraint to that string, then enable write access to sqlite_master
with PRAGMA writable_schema=1; and write your new table definition into it:
UPDATE sqlite_master SET sql='...' WHERE type='table' AND name='Customer';
Then reopen the database.
WARNING: This works only for changes that do not change the on-disk format of the table. If you do make any change that changes the record format (such as adding/removing fields, or modifying the rowid, or adding a constraint that needs an internal index), your database will blow up horribly.