1 Database Application Development Chapter 6

2 Overview  SQL in application code  Embedded SQL  Cursors  Dynamic SQL  JDBC

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 (e.g., JDBC, ODBC, PHP …)

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 Embedded SQL

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 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 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 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 Test; printf (“Max value=%d\n”,value); }

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 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 Cursors  Can declare a cursor on a relation or 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 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 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 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 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 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 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 Embedding vs Database APIs 1. Embedding: Modify compiler (see discussion so far) 2. API : Provide library with standard database call interface

20 Database APIs

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

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

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 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 Executing SQL Statements 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 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 ResultSets A ResultSet is a 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 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 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 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 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 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 An 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 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 SQLJ Code Int sid; String name; Int rating; // named iterator #sql iterator Sailors(Int sid, String name, Int rating); Sailors sailors; // assume that the application sets rating #sailors = { SELECT sid, sname INTO :sid, :name FROM Sailors WHERE rating = :rating }; // retrieve results while (sailors.next()) { System.out.println(sailors.sid + “ “ + sailors.sname)); } sailors.close();

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 Sailors(Int, String, Int); Sailors sailors; #sailors = … while (true) { #sql {FETCH :sailors INTO :sid, :name} ; if (sailors.endFetch()) { break; } // process the sailor }

41 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