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SQL | Transactions exercise | Populate a column, then (maybe) roll back the transaction
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First (if you haven't already done so) run the script shown above to generate the Doctor Who training database.
Type in and run the following script to generate a new column called NumberEnemies in the tblEpisode table:
ALTER TABLE tblEpisode
ADD NumberEnemies int
Write an update query which will set this column to equal the number of enemies for each episode (but within a transaction), then:
- Roll back this transaction if more than 100 rows are affected (displaying a suitable message, as shown below); or
- Commit it otherwise and show a list of all of the episodes, including the newly populated field.
Use @@RowCount to see how many rows are affected, but don't forget to store this in a variable immediately after running the update so that its value doesn't get overwritten.
Note that the syntax of the update query is nasty:
-- set the number of enemies for each episode
NumberEnemies = (
tblEpisodeEnemy AS ee
ee.EpisodeId = e.EpisodeId
tblEpisode AS e
When you run this query, you should see something like this:
Because 117 rows were updated, your query should roll back the transaction.
Now change the threshold in the query from 100 to 120 and re-run it. This time the query should update the column, and display the results:
You have to wait until episode id 15 to get more than one enemy (at which point 3 come along at once).
When you've got this working, optionally save your query as Counting your enemies.sql, then close it down.