1 Sub-queries and Views

2 A Complex Query We would like to create a table containing 3 columns: –Sailor id –Sailor age –Age of the oldest Sailor How can this be done?

3 Attempt 1 SELECT S.sid, S.age, MAX(S.age) FROM Sailors S; Why is this wrong?

4 Attempt 2 SELECT S.sid, S.age, MAX(S.age) FROM Sailors S GROUP BY S.name, S.age; Why is this wrong?

5 Solution 1: Sub-query in FROM SELECT S.sid, S.age, M.mxage FROM Sailors S,(SELECT MAX(S2.age) as mxage FROM Sailors S2) M; We can put a query in the FROM clause instead of a table The query in the FROM clause must be renamed with a range variable (M in this case).

6 Solution 1: Sub-query in SELECT SELECT S.sid, S.age, (SELECT MAX(S2.age) FROM Sailors S2) FROM Sailors S; A query in the SELECT clause must return at most one value for each row returned by the outer query.

7 Another Example of a Sub-query in SELECT SELECT S.sid, S.age, (SELECT MAX(S2.age) FROM Sailors S2 WHERE S2.age<S.age) FROM Sailors S; This query returns for each sailor the age of the oldest sailor that is younger than him. Note the use of S (defined in the outer query) within the inner query.

8 Another Example of a Sub-query in FROM?? SELECT S.sid, S.age, M.mxage FROM Sailors S, (SELECT MAX(S2.age) as mxage FROM Sailors S2 WHERE S2.age<S.age); Why is this wrong?

9 Solution 3: Views A View is a query that looks like a table and can be used as a table. CREATE OR REPLACE VIEW MaxAge as SELECT MAX(S.age) as mxage FROM Sailors S; SELECT S.sid, S.age, M.mxage FROM Sailors S, MaxAge M;

10 Another Example of a View CREATE OR REPLACE VIEW MaxAges as SELECT S1.sid, S2.age as mxage FROM Sailors S1, Sailors S2 WHERE S2.age = (SELECT MAX(S3.age) FROM Sailors S3 WHERE S3.age < S1.age); SELECT S.sid, S.age, M.mxage FROM Sailors S, MaxAges M WHERE S.sid = M.sid;

11 Views For Restricting Access Suppose that we have a table: Grades(Login, Exercise, Grade) We would like a user to only be able to see his own grades. We create the following view and grant privileges to query the view (not the underlying table) CREATE OR REPLACE VIEW UserGrades as SELECT * FROM Grades WHERE Login = User; Pseudo-column which is equal to the user name.

12 Delete and Update

13 Deleting Tuples The basic form of a delete statement is: DELETE FROM TableName WHERE Condition;

14 Examples DELETE FROM Sailors WHERE rating < 3; DELETE FROM Sailors S1 WHERE S1.rating = (SELECT MIN(S2.rating) FROM Sailors S2) Delete Sailors with rating less than 3: Delete Sailors with the minimum rating:

15 Updating Tuples The basic form of an update statement is: UPDATE TableName SET Column1 = Value1, … ColumnN = ValueN WHERE Condition;

16 Example UPDATE Boats SET color = ‘red’, bname = ‘Voyager’ WHERE bid = 13; Update boat 13: color to red and name to voyager

17 Another Example UPDATE Sailors SET rating = (SELECT MAX(rating) FROM Sailors) WHERE sname = ‘Rusty’; Update rating of Rusty to be the maximum rating of any sailor Note: When updating with a subquery, the subquery must return one value only!

18 Connecting to a Database with JDBC

19 Why Access a Database with Java? There are queries that can not be computed in Sql: –Given a table Bus(Source, Destination) find all pairs of places that it is possible to travel (paths of any length Java allows for a convenient user interface to the database

20 Learning JDBC On the Tirgul page of the Online Resources of the course homepage I will put: –A link to a JDBC tutorial –A link to the JDBC API –Several Example Programs

21 Packages to Import In order to connect to the Oracle database from java, import the following packages: –java.sql.*; –oracle.jdbc.driver.*;

22 Connecting to the Database DriverManager.registerDriver(new oracle.jdbc.driver.OracleDriver()); String driver = "jdbc:oracle:thin:"; String user = "sam"; String password = "samIam"; String conStr = driver + user + “/” + password + Connection con = DriverManager.getConnection(conStr);

23 Accessing the Database The database is accessed using the Connection object. 3 Types of statements are possible: –Statement –PreparedStatement –CallableStatement (not discussed today) Note that CallableStatement inherits from PreparedStatement which inherits from Statement

24 Querying with Statement String queryStr = "SELECT * FROM Member " + "WHERE Lower(Name) = 'harry potter'"; Statement stmt = con.createStatement(); ResultSet rs = stmt.executeQuery(queryStr); Statements are used for queries that are only used once. The executeQuery method returns an object representing the query result.

25 Changing DB with Statement String deleteStr = “DELETE FROM Member " + "WHERE Lower(Name) = ‘lord voldemort’"; Statement stmt = con.createStatement(); int delnum = stmt.executeUpdate(deleteStr); executeUpdate is used for data manipulation: insert, delete, update, create table, etc. executeUpdate returns the number of rows modified.

26 Querying with PreparedStatement String queryStr = "SELECT * FROM Program " + "WHERE Name = ? and Cost < ?”; PreparedStatement pstmt = con.prepareStatement(queryStr); pstmt.setString(1, “Unfogging the Future”); pstmt.setInt(2, 1000); ResultSet rs = pstmt.executeQuery();

27 About Prepared Statements Prepared Statements are used for queries that are executed many times. They are parsed only once. Using setString(i, value) (setInt(i, value), etc.) the i-th question mark is set to the given value.

28 Changing DB with PreparedStatement String deleteStr = “DELETE FROM Program " + "WHERE Name = ? and Cost < ?”; PreparedStatement pstmt = con.prepareStatement(deleteStr); pstmt.setString(1, “Unfogging the Future”); pstmt.setInt(2, 1000); int delnum = pstmt.executeUpdate();

29 Printing Query Output: Result Set (1) ResultSetMetaData rsmd = rs.getMetaData(); int numcols = rsmd.getColumnCount(); for (int i = 1 ; i <= numcols; i++) { if (i > 1) System.out.print(","); System.out.print(rsmd.getColumnLabel(i)); } Print Column Headers:

30 Printing Query Output: Result Set (2) while (rs.next()) { for (int i = 1 ; i <= numcols; i++) { if (i > 1) System.out.print(","); System.out.print(rs.getString(i)); } System.out.println(""); } To get the data in the i-th column: rs.getString(i) To get the data in column Url: rs.getString(“Url”)

31 Cleaning Up After Yourself Remember to close the Connections, Statements and ResultSets con.close(); stmt.close(); rs.close()

32 Dealing With Exceptions A exception can have more exceptions in it. catch (SQLException e) { while (e != null) { System.out.println(e.getSQLState()); System.out.println(e.getMessage()); System.out.println(e.getErrorCode()); e = e.getNextException(); }