Save and delete question

Question

How to find out if there are other related records in another table.
If there are other related record then delete is not allowed.
For eample, Order has many items in it. 2 tables -Order Table and OrderItem table - one to many relationship.
Then Can't delete a record in the order table if there are related record in the OrderItem table.
I have 3 text fields. Enter data in it and then press the save button.
It will save it to the database.
how to force people to enter the data.
That is, if they press save and if any of the 3 text fields is empty then
show a message box saying need to fill in all text fields.
How to find out if the text field is empty?
How to save to multiple tables.
That is, how to save the master table first then the detail tabes.
For example, there is a order table and a orderItem table.
When save button is clicked. must save to Order table first then to
orderItem table.
How to find out if the user is saving the data and that data already
exists in database. So this is wrong. Can't have duplicate data.
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Answer 1

For ur first question:
use the transction to delete.if u found any of the conditions u mention rollback.seee the Connection class in jdbc API.normally its AtuoCommint true.so before starting transction setAutoCommit fasle.
second question:
in the saveButton action performed first check the textField.getText().equals("") for all the 3 textfields.if the condtions doesnt met popup optionpane.showdialog();
if met just save it.
3 rd question answer:
this also same like first question.try to save in transaction.if anything happens wrong everything will be rollbacked.
4th question:
use unique in sql .u can set the 1 r more columns in table as unique key.
for all these to work ur database shud support transactions....