1. 1 Database Application Development Chapter 6

2. 2 Overview  SQL in application code  Embedded SQL  Cursors  Dynamic SQL  JDBC  Stored procedures

3. 3 SQL in Application Code  SQL commands can be called from within a host language (e.g., C++ or Java ) program.  SQL statements can refer to host variables (including special variables used to return status).  Must include statement to connect to right database.  Two main integration approaches:  Embed SQL in the host language (e.g., Pro*C, Embedded SQL, SQLJ)  Create special API (Call Level Interface) to call SQL commands (eg: JDBC, ODBC, PHP …)

4. 4 SQL in Application Code (Contd.) Impedance mismatch issues:  Type mismatch Data type casting (declare variables)  Set-oriented SQL relations are (multi-) sets of records, with no a priori bound on the number of records. Usually no such data structure exists in procedural programming languages such as C++. SQL supports a mechanism called a cursor to handle this.

5. 5 Embedded SQL

6. 6  Approach: Embed SQL in host language.  Given host language with embedded SQL  A preprocessor converts SQL statements into special function calls.  Then regular compiler used to compile the host language+function class into executable.  Final executable works for one DBMS only (not portable)

7. 7 Embedded SQL: Main Constructs  Connect to DB EXEC SQL CONNECT  Declare variables that can be used by both SQL and host language EXEC SQL BEGIN DECLARE SECTION … EXEC SQL END DECLARE SECTION  Executing SQL statements EXEC SQL …

8. 8 Embedding SQL in C: Oracle #include EXEC SQL BEGIN DECLARE SECTION; VARCHAR userid[20]; VARCHAR passwd[20]; int value; EXEC SQL END DECLARE SECTION; void sql_error (char *msg) { printf (“%s”, msg); exit (1); }

9. 9 Embedding SQL in C: Oracle int main () { strcpy (userid.arr, “me”); userid.len = strlen (userid.arr); strcpy (passwd.arr, “no-me”); passwd.len = strlen (passwd.arr); EXEC SQL WHENEVER SQLERROR DO sql_error (“Oracle Error\n”); EXEC SQL CONNECT :userid IDENTIFIED BY :passwd; EXEC SQL CREATE TABLE Test (a int); EXEC SQL INSERT INTO Test VALUES (1); EXEC SQL SELECT MAX (a) INTO :value from R; printf (“Max value=%d\n”,value); }

10. 10 Embedded SQL: Variables  Two special “error” variables :  SQLCODE (long, is negative if an error has occurred)  SQLSTATE (char[6], predefined codes for errors)

11. 11 Embedded SQL: Variables EXEC SQL BEGIN DECLARE SECTION char c_sname[20]; long c_sid; short c_rating; float c_age; EXEC SQL END DECLARE SECTION  Two special “error” variables :  SQLCODE (long, is negative if an error has occurred)  SQLSTATE (char[6], predefined codes for errors)

12. 12 Cursors  Can declare a cursor on a query statement which generates a relation.  Can open a cursor, repeatedly fetch a tuple, move the cursor, until all tuples have been retrieved.  Control order: ORDER BY, in queries that are accessed through a cursor  Can also modify/delete tuple pointed to by cursor.  Must close cursor at end.

13. 13 Cursor that gets names of sailors who’ve reserved a red boat, in alphabetical order EXEC SQL DECLARE sinfo CURSOR FOR SELECT S.sname FROM Sailors S, Boats B, Reserves R WHERE S.sid=R.sid AND R.bid=B.bid AND B.color=‘red’ ORDER BY S.sname

14. 14 Cursors EXEC SQL DECLARE myCursor CURSOR FOR SELECT bid from Reservations; EXEC SQL OPEN myCursor; EXEC SQL WHENEVER NOT FOUND DO break; while (1) { EXEC SQL FETCH myCursor INTO :num; … } EXEC SQL CLOSE myCursor;

15. 15 Embedding SQL in C: An Example char SQLSTATE[6]; EXEC SQL BEGIN DECLARE SECTION char c_sname[20]; short c_minrating; float c_age; EXEC SQL END DECLARE SECTION c_minrating = random(); EXEC SQL DECLARE sinfo CURSOR FOR SELECT S.sname, S.ageFROM Sailors S WHERE S.rating > :c_minrating ORDER BY S.sname; do { EXEC SQL FETCH sinfo INTO :c_sname, :c_age; printf(“%s is %d years old\n”, c_sname, c_age); } while (SQLSTATE != ‘02000’); -- empty cursor EXEC SQL CLOSE sinfo;

16. 16 Compiling  Create files with extension.pc, such as test.pc  Preprocessor = proc  Available at $ORACLE_HOME/bin  SET your library path as  setenv LD_LIBRARY_PATH ${LD_LIBRARY_PATH}:${ORACLE_HOME}/lib  Make them as make -f $ORACLE_HOME/precomp/demo/proc/demo_proc.mk build EXE=test OBJS=test.o (or) $ORACLE_HOME/bin/proc iname=test.pc code=cpp parse=none g++ -I$ORACLE_HOME/precomp/public test.c -lclntsh -lm

17. 17 Dynamic SQL  SQL queries are not always known at compile time  Example: spreadsheet, graphical DBMS frontend, web access.  Allow construction of SQL statements (query strings) on-the-fly

18. 18 Dynamic SQL Example char c_sqlstring[] = {“DELETE FROM Sailors WHERE rating>5”}; -- parse, compile and bind to variable: EXEC SQL PREPARE readytogo FROM :c_sqlstring; EXEC SQL EXECUTE readytogo;

19. 19 Embedding vs Database APIs 1. Embedding: Modify compiler (see discussion thus far) 2. API : Provide library with standard database call interface

20. 20 Database APIs

21. 21 Database APIs  Special standardized interface to libraries of functions provided explicitly for SQL statements:  No preprocessor, instead host language compiler compiles code.  Pass SQL strings from PL language  Presents result sets in language-friendly way  Examples : Sun’s JDBC: Java API MS ODBC – Open DB Connection  Supposedly DBMS-neutral  “driver” traps calls & translates them into DBMS-specific code  Database can be across a network  Same executable works on different DBMSs without recompiling  Independent both at source code and at executable level

22. 22 JDBC Architecture: 4 Components  Application initiates and terminates connections, submits SQL statements  Driver manager load JDBC driver at run-time  Driver Registers with manager Connects to data source, transmits requests and returns/translates results and error codes into DBMS specific calls  Data source processes SQL statements

23. 23 JDBC Architecture Four types of drivers: Bridge:  Translates SQL commands into non-native API. Example: JDBC-ODBC bridge. Code for ODBC and JDBC driver needs to be available on each client. Direct translation to native API, non-Java driver:  Translates SQL commands to native API of data source. Need OS-specific binary on each client. Network bridge:  Send commands over network to middleware server that talks to data source. Needs only small JDBC driver at each client. Direction translation to native API via Java driver:  Converts JDBC calls directly to network protocol used by DBMS. Needs DBMS-specific Java driver at each client.

24. 24 JDBC Classes and Interfaces Steps to submit a database query:  Load JDBC driver  Connect to data source  Execute SQL statements

25. 25 JDBC Driver Management  All drivers managed by DriverManager class  Options for Loading JDBC driver:  In Java code (dynamic loading of class in java): Class.forName(“oracle/jdbc.driver.Oracledriver”);  When starting Java application: -Djdbc.drivers=oracle/jdbc.driver

26. 26 Connections in JDBC We interact with data source through sessions. Each connection identifies a logical session.  JDBC URL: jdbc: : Example: String url=“jdbc:oracle:www.bookstore.com:3083”; Connection con; try{ con = DriverManager.getConnection(url,usedId,password); } catch SQLException except { …}

27. 27 Connection Class Interface  public int getTransactionIsolation() void setTransactionIsolation(int level) Sets isolation level for current connection.  public boolean getAutoCommit() and void setAutoCommit(boolean b) If autocommit is set, then each SQL statement is considered its own transaction. Otherwise, a transaction is committed using commit(), or aborted using rollback().  public boolean isClosed() Checks whether connection is still open.

28. 28 Executing SQL Statements  Three ways of executing SQL statements:  Statement (static or dynamic SQL statements)  PreparedStatement (semi-static SQL statements)  CallableStatement (stored procedures)  PreparedStatement class: Precompiled & parameterized SQL statements:  Structure of query is fixed  Values of parameters are determined at run-time

29. 29 Executing SQL Statements (Contd.) String sql=“INSERT INTO Sailors VALUES(?,?,?,?)”; PreparedStatment pstmt=con.prepareStatement(sql); pstmt.clearParameters(); pstmt.setInt(1,sid); pstmt.setString(2,sname); pstmt.setInt(3, rating); pstmt.setFloat(4,age); // since no rows are returned, use executeUpdate() int numRows = pstmt.executeUpdate(); Where numRows is # of rows modified.

30. 30 ResultSets  PreparedStatement.executeUpdate only returns number of affected records  PreparedStatement.executeQuery returns data, encapsulated in a ResultSet object (a cursor) ResultSet rs=pstmt.executeQuery(sql); // rs is now a cursor While (rs.next()) { // process the data }

31. 31 ResultSets A ResultSet is a very powerful cursor:  previous() : moves one row back  absolute(int num) : moves to the row with the specified number  relative (int num) : moves forward or backward  first() and last()

32. 32 Matching Java and SQL Data Types getTimestamp()java.sql.TimeStampTIMESTAMP getTime()java.sql.TimeTIME getDate()java.sql.DateDATE getFloat()DoubleREAL getInt()IntegerINTEGER getDouble()DoubleFLOAT getDouble()DoubleDOUBLE getString()StringVARCHAR getString()StringCHAR getBoolean()BooleanBIT ResultSet get methodJava classSQL Type

33. 33 JDBC: Exceptions and Warnings  Most of java.sql can throw an SQLException if an error occurs.  SQLWarning is a subclass of SQLException; not as severe (they are not thrown and their existence has to be explicitly tested)

34. 34 Warning and Exceptions (Contd.) try { stmt=con.createStatement(); warning=con.getWarnings(); while(warning != null) { // handle SQLWarnings; warning = warning.getNextWarning(): } con.clearWarnings(); stmt.executeUpdate(queryString); warning = con.getWarnings(); … } //end try catch( SQLException SQLe) { // handle the exception }

35. 35 Examining Database Metadata DatabaseMetaData object gives information about database system and catalog. DatabaseMetaData md = con.getMetaData(); // print information about the driver: System.out.println( “Name:” + md.getDriverName() + “version: ” + md.getDriverVersion());

36. 36 Database Metadata (Contd.) DatabaseMetaData md=con.getMetaData(); ResultSet trs=md.getTables(null,null,null,null); String tableName; While(trs.next()) { tableName = trs.getString(“TABLE_NAME”); System.out.println(“Table: “ + tableName); //print all attributes ResultSet crs = md.getColumns(null,null,tableName, null); while (crs.next()) { System.out.println(crs.getString(“COLUMN_NAME” + “, “); }

37. 37 A (Semi-)Complete Example Connection con = // connect DriverManager.getConnection ( url, ”login", ”pass“ ); Statement stmt = con.createStatement(); // set up stmt String query = "SELECT name, rating FROM Sailors"; ResultSet rs = stmt.executeQuery(query); try { // handle exceptions // loop through result tuples while (rs.next()) { String s = rs.getString(“name"); Int n = rs.getFloat(“rating"); System.out.println(s + " " + n); } } catch(SQLException ex) { System.out.println(ex.getMessage () + ex.getSQLState () + ex.getErrorCode ()); }

38. 38 SQLJ : SQL-Java Complements JDBC with a (semi-) static query model: Compiler can perform syntax checks, strong type checks, consistency of the query with the schema  All arguments always bound to the same variable: #sql = { SELECT name, rating INTO :name, :rating FROM Books WHERE sid = :sid;}  Compare to JDBC: sid=rs.getInt(1); if (sid==1) {sname=rs.getString(2);} else { sname2=rs.getString(2);}  SQLJ (part of the SQL standard) versus embedded SQL (vendor-specific)

39. 39 SQLJ Code String title, Float price, String author; // named iterator #sql iterator Books (String title, Float price); Books books; // assume that the application sets author #books = { SELECT title, price INTO :title, :price FROM Books WHERE author = :author }; // retrieve results while (books.next()) { System.out.println(books.title + “ “ + books.price)); } books.close();

40. 40 SQLJ Iterators Two types of iterators (“cursors”):  Named iterator  Need both variable type and name, and then allows retrieval of columns by name.  See example on previous slide.  Positional iterator  Need only variable type, and then uses FETCH.. INTO construct: #sql iterator Books (String title, Float price); Books books; #books = … while (true) { #sql {FETCH :books INTO :title, :price} ; if (books.endFetch()) { break; } // process the book }

41. 41 Stored Procedures

42. 42 Stored Procedures  What is a stored procedure:  Program executed through single SQL statement  Executed in process space of the server  Advantages:  Can encapsulate application logic while staying “close” to the data  Reuse of application logic by different users  Avoid tuple-at-a-time return of records through cursors

43. 43 SQL/PSM Declare a stored procedure: CREATE PROCEDURE name(p1, p2, …, pn) local variable declarations procedure code; Declare a function: CREATE FUNCTION name (p1, …, pn) RETURNS sqlDataType local variable declarations function code;

44. 44 Stored Procedures: Examples CREATE PROCEDURE ShowNumReservations SELECT S.sid, S.sname, COUNT(*) FROM Sailors S, Reserves R WHERE S.sid = R.sid GROUP BY S.sid, S.sname

45. 45 Stored Procedures: Examples Stored procedures can have parameters:  is one of {IN, OUT, INOUT} eg: IN val1 int CREATE PROCEDURE IncreaseRating( sailor_sid IN INTEGER, increase IN INTEGER) UPDATE Sailors SET rating = rating + increase WHERE sid = sailor_sid

46. 46 Defining a stored procedure CREATE PROCEDURE [( )] ; A parameter in the paramList is specified as: is one of {IN, OUT, INOUT} eg: val1 IN int You can drop procedure by DROP PROCEDURE In PL/SQL, you can replace procedure by CREATE OR REPLACE PROCEDURE …

47. 47 PL/SQL Engine

48. 48 Stored Procedures: Examples Stored procedure do not have to be in SQL: CREATE PROCEDURE TopSailors( IN num INTEGER) LANGUAGE JAVA EXTERNAL NAME “file:///c:/storedProcs/rank.jar”

49. 49 Example: Procedure in PSM CREATE PROCEDURE testProcedure BEGIN INSERT INTO Student VALUES (5, ‘Joe’); END; Oracle PL/SQL: CREATE PROCEDURE testProcedure I S BEGIN INSERT INTO Student VALUES (5, ‘Joe’); END;. run;

50. 50 Calling Stored Procedures EXEC SQL BEGIN DECLARE SECTION Int sid; Int rating; EXEC SQL END DECLARE SECTION // now increase the rating of this sailor EXEC CALL IncreaseRating(:sid,:rating); Embedded SQL:

51. 51 Calling Stored Procedures JDBC: CallableStatement cstmt= con.prepareCall(“{call ShowSailors}); ResultSet rs = cstmt.executeQuery(); while (rs.next()) { … } SQLJ: #sql iterator ShowSailors(…); ShowSailors showsailors; #sql showsailors={CALL ShowSailors}; while (showsailors.next()) { … }

52. 52 More about Procedures  If there is an error in your procedure, Oracle will give you a warning. Use command SHOW ERRORS to show the errors in your procedure.  Calling Procedures call [( )];

53. 53 PL/SQL

54. 54 Local Declarations CREATE PROCEDURE testProcedure (num IN int, name IN varchar) IS num1 int; -- local variable BEGIN num1 := 10; INSERT INTO Student VALUES (num1, name); END;. run;

55. 55 Other PL/SQL features Assignment statements: PL/SQL :=

56. 56 Control Structures: IF THEN ELSE IF THEN ELSIF THEN ELSIF … ELSE END IF;

57. 57 Loops LOOP END LOOP; To exit from a loop use EXIT;

58. 58 Loops: Example CREATE PROCEDURE testProcedure (num IN int, name IN varchar) IS num1 int; BEGIN num1 := 10; LOOP INSERT INTO Student VALUES (num1, name); num1 := num1 + 1; IF (num1 > 15) THEN EXIT; END IF; END LOOP; END;. run;

59. 59 FOR Loops FOR i in [REVERSE].. LOOP END LOOP Example: FOR i in 1.. 5 LOOP INSERT INTO Student (sNumber) values (10 + i); END LOOP;

60. 60 WHILE LOOPS WHILE LOOP END LOOP;

61. 61 Functions CREATE FUNCTION [( )] RETURNS type AS BEGIN ; END; You can call a function as part of an SQL expression

62. 62 Functions: Example CREATE FUNCTION testFunction RETURN int IS num1 int; BEGIN SELECT MAX (sNumber) INTO num1 FROM Student; RETURN num1; END;. run; SELECT * from Student where sNumber = testFunction ();

63. 63 PL/SQL Example CREATE OR REPLACE FUNCTION rateSailor (sailorId IN INTEGER) RETURN INT IS rating INTEGER; numRes INTEGER; BEGIN SELECT COUNT(*) INTO numRes FROM Reserves R WHERE R.sid = sailorId; IF (numRes > 10) THEN rating :=1; ELSE rating := 0; END IF; RETURN rating; END;. run;

64. 64 Cursors When we execute a statement, a relation is returned. It is stored in private work area for the statement. Cursor is a pointer to this area. To create a cursor CURSOR c_customers is SELECT * from CUSTOMERS;

65. 65 Cursors We can open the cursor. OPEN c_customers; We can select data from the cursor. FETCH c_customers into customers_rec; And we can close the cursor. CLOSE c_customers;

66. 66 Implicit & Explicit Cursors Every SQL data manipulation statements including queries that return only one row is an implicit cursor. An explicit cursor is what we create. For queries that return more than one row, you must declare an explicit cursor

67. 67 CREATE OR REPLACE PROCEDURE copyProcedure IS stID INT; name VARCHAR (10); CURSOR myCursor IS SELECT * FROM STUDENT; BEGIN OPEN myCursor; LOOP FETCH myCursor INTO stID, name; EXIT WHEN myCURSOR%NOTFOUND; INSERT INTO newStudent VALUES (stID, name); END LOOP; CLOSE myCursor; END;

68. 68 Cursor Attributes The SQL cursor attributes are :-  %ROWCOUNT: The number of rows processed by a SQL statement.  %FOUND : TRUE if at least one row was processed.  %NOTFOUND : TRUE if no rows were processed.  %ISOPEN : TRUE if cursor is open or FALSE if cursor has not been opened or has been closed. Only used with explicit cursors.

69. 69 Advanced Explicit Cursor Concepts

70. 70 Cursor that uses parameters CURSOR c_students (p_Department classes.department%TYPE p_Course classes.department%TYPE ) IS SELECT * FROM classes WHERE department = p_Department AND course = p_Course; To call the cursor OPEN c_students('CS',101);

71. 71 Cursors for update The syntax for this parameter in the SELECT statement is: SELECT... FROM... FOR UPDATE [OF column_reference] [NOWAIT] where column_reference is a column in the table against which the query is performed. A list of columns can also be used.

72. 72 Example…for update DECLARE CURSOR c_AllStudents IS SELECT * FROM students FOR UPDATE OF first_name, last_name; Or the cursor can select every column by not specifing a range DECLARE CURSOR c_AllStudents IS SELECT * FROM students FOR UPDATE;

73. 73 NOWAIT If another session already has locks on the rows in the active set, then the SELECT FOR UPDATE will hang until the other session releases the lock. To handle this situation the parameter NOWAIT is available, which in case the rows are locked,OPEN will return the error ORA-54 resource busy and acquire with NOWAIT specified

74. 74 Other Info Re Oracle :  Oracle stores procedures and functions in catalog as relational tables:  Check user_procedures  Check user_functions  You may run queries against them such as: describe user_procedures; select object_name from user_procedures;

75. 75 Summary of PL/SQL Constructs  Local variables (DECLARE)  RETURN values for FUNCTION  Branches and loops:  IF (condition) THEN statements; ELSIF (condition) statements; … ELSE statements; END IF;  LOOP statements; END LOOP  Queries can be parts of expressions  Can use cursors naturally without “EXEC SQL”

76. 76 Summary of Whirlwind Tour  Embedded SQL allows execution of parameterized static queries within host language  Dynamic SQL allows execution of completely ad-hoc queries within a host language  Cursor mechanism allows retrieval of one record at a time and bridges impedance mismatch between HL and SQL  APIs such as JDBC introduce a layer of abstraction between application and DBMS  SQLJ: Static model, queries checked at compile-time.  Stored procedures execute application logic directly at the server  SQL/PSM standard for writing stored procedures (and also for triggers)