Description
Destroying and Altering Relations
DROP TABLE
ALTER TABLE
SELECT
Exercise
Inserting more data into previous tables
Deleting rows
Single-table queries
Multiple-table queries
The Command DROP TABLE destroys the table and deletes all records on that relation.
Usage: DROP TABLE TableName
The command ALTER TABLE allows us to make several modifications to a table we have created before.
We can add/drop columns and constraints, rename table name, columns and do much more (Check the PostgreSQL manual)
Adding a column to an already created table
ALTER TABLE TableName
ADD COLUMN ColumnName ColumnType;
We can also add a column with an additional integrity constraint.
ALTER TABLE TableName
ADD COLUMN ColumnName ColumnType
CHECK ( Constraint );
Open the query tool. By using ALTER TABLE as described in previous slide, do the following
Add Country column to Artist table (say, with the type VARCHAR(20))
Add a Rating column to the Customer table, with the following check constraint: the rating value has to be BETWEEN 1 AND 10.
The simple SELECT clause that we have seen in the previous lab can be extended by adding more clauses.
GROUP BY: Groups all resulting rows of our query in terms of one or more attributes with this clause.
HAVING: Group qualification is specified here. Groups which satisfy this qualification will be displayed.
ORDER BY: We can sort the data based on one or more attributes with this clause.
SELECT [ DISTINCT ] select-list
FROM from-list
WHERE record-qualification
GROUP BY grouping-list
HAVING group-qualification
You will insert more data into the tables we created last week.
You will delete rows from some of those tables.
Then, you’ll code queries involving single and multiple tables.
Insert the following into the Artist table.
(‘Leonardo’, ‘Florence’, ‘Renaissance’, ’04-15-1452′, ‘Italy’)
(‘Michelangelo’, ‘Arezzo’, ‘Renaissance’, ’03-06-1475′,
‘Italy’)
(‘Josefa’, ‘Seville’, ‘Baroque’, ’09-09-1630′, ‘Spain’)
(‘Hans Hofmann’, ‘Weisenburg’, ‘Modern’, ’02-17-1966′, ‘Germany’)
(‘John’, ‘San Francisco’, ‘Modern’, ’02-17-1920′, ‘USA’)
Insert the following into the Artwork table,
(‘Waves’, 2000, null, 4000.00, ‘John’)
(‘Three Musicians’, 1921, ‘Modern’, 11000.00, ‘Picasso’)
Insert the following into Customer table,
(1, ‘Emre’, ‘Preston’, 20000.00, 5)
(2, ‘Saeid’, null, 40000.00, 6)
Insert the following into LikeArtist table,
(1, ‘Picasso’)
(2, ‘Picasso’) and (2, ‘Leonardo’)
We can delete certain rows satisfying a condition from a table with the DELETE command.
Condition has the same format as that in the WHERE clause of a SELECT query.
If you omit the WHERE clause, all records will be permanently deleted.
Syntax: 0DELETE FROM0 0TableName0 0WHERE0 0Condition
Suppose the artist ‘Smith’ moved to another gallery, and we have to remove him from our database.
Write a DELETE query to remove him from the DB.
Note that Artwork table has a foreign key to the Artist table.
Two ways of doing this:
Manual: We remove all records in all tables related to the ‘Smith’ record in Artist
Automated: We remove ‘Smith’ from Artist and all related information is removed by the DBMS.
To try them both, we need to backup and restore the database.
Backup: A snapshot of the database (including data and structure) at any point in time.
Generates a data file *.backup that you save on disk.
Restore: Uses a previously generated backup file to bring the database to a certain state in time.
Before restore, we need to:
Either remove all tables (DROP TABLE)
Or remove the table data (DELETE FROM…)
The manual way: (perform a backup first)
If no backup before deleting Smith, then every erased record cannot be recovered later on. They have to be manually generated again.
Delete all art works related to Smith.
Then delete Smith from the artist list.
The automatic way
Remove all tables with DROP TABLE statement.
Perform restore using the backup file.
The ‘Smith’ author should be there again.
Select Properties on the artwork table
Remove the existing foreign key constraint
Create a new foreign key constraint but now selecting the ‘Cascade’ option for UPDATE and DELETE operations.a
Delete ‘Smith’ from the author list.
All Smith’s artworks are automatically deleted.
List the names and birthplaces of all Artists
List the title and price of all Artworks that were painted after 1600.
List the title and type of all Artworks that was either painted in 2000 or was painted by Picasso.
List the names and birthplaces of all Artists who were born between 1880 and 1930. (HINT: EXTRACT(YEAR FROM Dateofbirth) gives you the year from a DATE attribute)
List the names and country of birth of all Artists whose painting style are Modern, Baroque or Renaissance. (HINT: Use the IN keyword).
List all details of the Artworks in the database, ordered by Title.
Note that the following two queries involve more than one table.
List the names and customer ids of all customers who like Picasso.
List the names of all customers who like Artists from the Renaissance style and having an amount larger than 30000.
If the time was not enough, please complete today’s lab before next lab, since we might use the data that we have created in previous labs.
