4. Constraints on what can be edited

Changes to the database can fail for a number of reasons:

• When inserting or updating entries, some fields are effectively mandatory; normally these are the name and/or number for the entry. Not entering this information may mean that the changes are not made.
• Some columns or combinations of columns must be unique; for example there can normally be only one stanza 3 for Ragnarsdrápa. Attempting to add another stanza with the same number will give an error 'Duplicate entry' or similar. In many tables, the name of the text, poem, manuscript, etc. must be unique.
• Some columns must have a link to a row in another table for the row to be changed or inserted; for example, if you attempt to insert a stanza without a link to a poem you will get an error like 'a foreign key constraint fails'
• Some entries have links to them from other tables; in most cases, if you attempt to delete the entry you will get an error similar to the previous. Normally when you attempt the delete, the web interface will not give the confirmation button and instead list the entries that are linked to the one you are attempting to delete so that you can first delete the linked entries.

References