DBMS offers a special service. We can undo a single or even multiple consecutive write and delete operations. To do so we use the command ROLLBACK. When modifying data, the DBMS writes in a first step all new, changed or deleted data to a temporary space. During this stage the modified data is not part of the 'regular' database. If we are sure the modifications shall apply, we use the COMMIT command. If we want to revert our changes, we use the ROLLBACK command. All changes up to the finally COMMIT or ROLLBACK are considered to be part of a so called transaction.
The syntax of COMMIT and ROLLBACK is very simple.
COMMIT WORK; -- commits all previous INSERT, UPDATE and DELETE commands, which -- occurred since last COMMIT or ROLLBACK ROLLBACK WORK; -- reverts all previous INSERT, UPDATE and DELETE commands, which -- occurred since last COMMIT or ROLLBACK
The keyword 'WORK' is optional.
The feature AUTOCOMMIT automatically performs a COMMIT after every write operation (INSERT, UPDATE or DELETE). This feature is not part of the SQL standard, but is implemented and activated by default in some implementations. If we want to use the ROLLBACK command, we must deactivate the AUTOCOMMIT. (After an - automatic or explicit - COMMIT command a ROLLBACK command is syntactically okay, but it does nothing as everything is already committed.) Often we can deactivate the AUTOCOMMIT with a separate command like 'SET autocommit = 0;' or 'SET autocommit off;' or by clicking an icon on a GUI.
To test the following statements it is necessary to work without AUTOCOMMIT.
Let us insert a new person into the database and test the COMMIT.
-- Store a new person with id 99. INSERT INTO person (id, firstname, lastname, date_of_birth, place_of_birth, ssn, weight) VALUES (99, 'Harriet', 'Flint', DATE'1970-10-19', 'Dallas', '078-05-1120', 65); -- Is the new person really in the database? The process who executes the write operation will see its results, -- even if they are actually not committed. (One hit expected.) SELECT * FROM person WHERE id = 99; -- Try COMMIT command COMMIT; -- Is she still in the database? (One hit expected.) SELECT * FROM person WHERE id = 99;
Now we remove the person from the database.
-- Remove the new person DELETE FROM person WHERE id = 99; -- Is the person really gone away? Again, the process who performs the write operation will see the changes, even -- if they are actually not committed. (No hit expected.) SELECT * FROM person WHERE id = 99; -- Try COMMIT command COMMIT; -- Is the person still in the database? (No hit expected.) SELECT * FROM person WHERE id = 99;
So far, so boring.
The exciting command is the ROLLBACK. It restores changes of previous INSERT, UPDATE or DELETE commands.
We delete and restore Mrs. Hamilton from our example database.
DELETE FROM person WHERE id = 3; -- Lisa Hamilton -- no hit expected SELECT * FROM person WHERE id = 3; -- ROLLBACK restores the deletion ROLLBACK; -- ONE hit expected !!! Else: check AUTOCOMMIT SELECT * FROM person WHERE id = 3;
The ROLLBACK is not restricted to one single row. It may affect several rows, several commands, different kind of commands and even several tables.
-- same as above DELETE FROM person WHERE id = 3; -- destroy all e-mail addresses UPDATE contact SET contact_value = 'unknown' WHERE contact_type = 'email'; -- verify modifications SELECT * FROM person; SELECT * FROM contact; -- A single ROLLBACK command restores the deletion in one table and the modifications in another table ROLLBACK; -- verify ROLLBACK SELECT * FROM person; SELECT * FROM contact;
Supose the hobby table contains 9 rows and the person table 10 rows. We execute the following operations:
add 3 hobbies
add 4 persons
add 5 hobbies
add 6 persons
How many rows are in the hobby table?
How many rows are in the person table?