EECE 433 Chapter 6, Spring 2012

Database Application Development Chapter 6 Some useful links http://msdn.microsoft.com/en-us/ library/tw738475.aspx http ://www.w3schools.com/ado/default.a sp http://www.w3schools.com/php/defa ult asp Overview Concepts covered in this lecture:  SQL in application code  Embedded SQL  Cursors  Dynamic SQL  JDBC  SQLJ  Stored procedures Content  Introduction and Cursor  Embedded SQL  JDBC  SQLJ  Stored Procedures  SQL/PSM Programming Models  Four Types:  SQL on console  Embedded SQL  Driver based (e.g. JDBC, ODBC)  Stored Procedures (standard: SQL/PSM) SQL Programming models Console > SELECT … Embedded SQL in C #include <sqlutil.h> EXEC SQL BEGIN DECLARE SECTION … Driver Based Application Driver 3 App Driver Mgr Driver 1 Driver 2 Data source 3 Data source 2 Data source 1 CREATE PROCEDURE IncreaseRating( IN sailor_sid INTEGER, IN increase INTEGER) UPDATE Sailors …… Stored Procedures 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 a statement to connect to the right database.  Two main integration approaches:  Embed SQL in the host language (Embedded SQL, SQLJ)  This is getting obselete.  Create special API to call SQL commands (JDBC) Impedance Mismatch and Cursor  SQL relations are (multi-) sets of records, with no a priori bound on the number of records.  No such data structure exist traditionally in procedural programming languages such as C++.  This mismatch with SQL is called impedance mismatch  SQL supports a mechanism called a cursor to handle this.  Cursor: A control structure to allows traversal Content  Introduction and Cursor  Embedded SQL  JDBC  SQLJ  Stored Procedures  SQL/PSM 1 Embedded SQL  Approach: Embed SQL in the host language.  A preprocessor converts the SQL statements into special API calls.  The preprocessor is DBMS-dependent. Each DBMS has its own pre-processor.  Then a regular compiler is used to compile the code.  Executable works with a particular DBMS for which the code was compiled.  Language constructs:  Declare host variables: EXEC SQL BEGIN (END) DECLARE SECTION  Connect to a database: EXEC SQL CONNECT  Execute query Statements: EXEC SQL Statement; • May need to open a CURSOR 1 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 EXEC SQL CONNECT TO {[server_name.]database_name} [AS connection_name] USER [login[.password] | $integrated] c_minrating = random(); EXEC SQL DECLARE sinfo CURSOR FOR SELECT S.sname, S.age FROM Sailors S WHERE S.rating > :c_minrating ORDER BY S.sname; EXEC SQL OPEN sinfo; do { EXEC SQL FETCH sinfo INTO :c_sname, :c_age; printf(“%s is %d years old\n”, c_sname, c_age); } while (SQLSTATE != ‘02000’); // while FOUND EXEC SQL CLOSE sinfo; Declare Variables Declare and Open a cursor Execute the query and traverse results Connect to DB 1 Dynamic SQL  Allows construction of SQL statements on-the-fly  Example: char c_sqlstring[]= {“DELETE FROM Sailors WHERE raiting>5”}; EXEC SQL PREPARE readytogo FROM :c_sqlstring; EXEC SQL EXECUTE readytogo; 1 Embedded SQL – overall program structure  Include required SQL environment files (use #include <sqlenv.h> and #include <sqlutil.h>  Declare Variables within host program (use EXEC SQL BEGIN/ END DECLARE)  Include SQLCA for error codes (use EXEC INCLUDE SQLCA)  Connect to database (use EXEC SSQL CONNECT)  Declare Cursor for multiple reads (use EXEC SQL DECLARE cursor_name CURSOR FOR sql_query)  Interact with database:  Read data from cursor (either copied or pointed by cursor) (use EXEC SQL FETCH cursor_name INTO :local_variable1, :local_variable2;)  Update data in DB (use EXEC SQL UPDATE…)  Read one row (use EXECX SQL SELECT..WHERE sid = some)  For dynamic query (use EXEC SQL PREPARE stmt_dyn FROM :str_stmt; and then EXEC SQL EXECUTE stmt_dyn FROM :input_var)  Close connection (use EXEC SQL CLOSE cursor_name)  Log off from database and free all resources (use EXEC SQL COMMIT RELEASE) 1 Content  Introduction and Cursor  Embedded SQL  JDBC  SQLJ  Stored Procedures  SQL/PSM 1 Database APIs: Alternative to embedding  Rather than modify compiler, add library with database calls (API)  Special standardized interface: procedures/objects  Pass SQL strings from language, presents result sets in a language-friendly way  At the high level, two key options: Sun’s JDBC: Java API and Microsoft’s ODBC  a “driver” traps the calls and translates them into DBMS- specific code  database can be across a network 1 Good References  For ODBC: www.microsoft.com/data/odbc  For JDBC: java.sun.com/products/jdbc  Books: “ODBC developer’s guide” by Sanders, “JDBC DB access with Java…” by Hamilton Extra Info 1 Driver-based (e.g. JDBC) Architecture  Four architectural components:  Application (initiates and terminates connections, submits SQL statements)  Driver manager (load JDBC driver)  Driver: • connects to data source, • transmits requests and • returns/translates results and error codes)  Data source (processes SQL statements) Driver 3 App Driver Mgr Driver 1 Driver 2 Data source 3 Data source 2 Data source 1 1 JDBC Classes and Interfaces  JDBC is a collection of Java classes and interfaces that enables database access from programs written in Java.  It contains methods for:  Connecting to a data source  Executing SQL statements  Examining sets of results from SQL queries  Transaction management  Exception handling  Java application should include the java.sql package:  import java.sql.* 1 Steps to connect to the database 1. Load the JDBC driver 2. Connect to the data source 3. Execute SQL statements (use ResultSet) 2 STEP 1: Loading the JDBC driver - JDBC Driver Management  All drivers are managed by the Drivermanager class, which has methods to dynamically add/delete drivers  Loading a JDBC driver:  In the Java code: Class.forName(“oracle/jdbc.driver.Oracledriver”) ; (for SQL server) Driver d = (Driver)Class.forName("com.microsoft.jdbc.sqlserver.SQLServerDriver").newI nstance(); • This will automatically register the driver  Alternatively, when starting the Java application include on the command line: -Djdbc.drivers=oracle/jdbc.driver 2 STEP 2: Connections in JDBC  We interact with a data source through sessions. Each connection identifies a logical session.  Connections are specified through a JDBC URL, a URL that uses the jdbc protocol: jdbc:<subprotocol>:<otherParameters> Example: String url=“jdbc:oracle:www.bookstore.com:3083”; Connection con; try{ con = DriverManager.getConnection(url,userId,password); } catch (SQLException excpt) { …} 2 More on JDBC URL - example jdbc:oracle:<drivertype>:@<hostname>:<port >:<database_sid> <drivertype> - can be either "thin" or "oci" (thick) [oci - Oracle Call Interface, version of oci can also be appended , oci8 - Oracle 8 DB version] <hostname> - The machine name where the database is running (IP address can also be used). <port> - The port number where the database listener is running <database_sid> - The database SID of database  Example : jdbc:oracle:thin:@10.185.13.158:1521:ora9i  here 10.185.13.158 - The IP address of machine where db is running  1521 - Port where db listener is running  ora9i - The Database SID FYI only 2 Connection Class Interface  public int getTransactionIsolation() and void setTransactionIsolation(int level) Sets isolation level for the current connection. Manages how changes are visible to other concurrent transactions  public boolean getReadOnly() and void setReadOnly(boolean b) Specifies whether transactions in this connection are read-only  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() Ch k h th ti i till FYI only 2 STEP 3: Executing SQL Statements  Three different ways of executing SQL statements:  Statement (both static and dynamic SQL statements)  PreparedStatement (dynamic SQL statements)  CallableStatment (stored procedures)  Statement is the base class for the other two statement classes. 2 PreparedStatement Class  Dynamically generates pre-complied SQL code  Can have parameters but structure is fixed  Values of parameters are determined at run- time 2 Show example here See handout on driver based code 2 Example: Executing Prepared 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); // There are different ways for submitting the query string to the data source // If we know that no rows are returned, we can use executeUpdate() int numRows = pstmt.executeUpdate(); Preparing statement Executing statement 2 ExecuteQuery & ResultSets  PreparedStatement.executeUpdate only returns the number of affected records  PreparedStatement.executeQuery returns data, encapsulated in a ResultSet object (a cursor) Statement pstmt = NULL; Pstmt = con.createStatement(); ResultSet rs=pstmt.executeQuery(sql); // rs is now a cursor While (rs.next()) { // process the data } Declaring cursor in java Executing query returning multiple rows Process cursor content 2 Callable Statement JDBC: CallableStatement cstmt= con.prepareCall(“{call ShowSailors}); ResultSet rs = cstmt.executeQuery(); while (rs.next()) { … }  For calling Stored procedures 3 ResultSets (Contd.) 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() 3 Matching Java and SQL Data Types SQL Type Java class ResultSet get method BIT Boolean getBoolean() CHAR String getString() VARCHAR String getString() DOUBLE Double getDouble() FLOAT Double getDouble() INTEGER Integer getInt() REAL Double getFloat() DATE java.sql.Date getDate() TIME java.sql.Time getTime() TIMESTAMP java.sql.TimeStamp getTimestamp() These (accessor) methods are used to retrieve values from the current row of the query result referenced by the ResultSet object. There are two forms for each method: by column index or by column name. Briefly 3 JDBC: Exceptions and Warnings  Similar to SQLSTATE in embedded SQL. For error handling.  Most of java.sql can throw an SQLException if an error occurs.  It supports the following methods:  getMessage( )  getSQLState( )  getNextException( )  SQLWarning is a subclass of SQLException; not as severe (they are not thrown and their existence has to be explicitly tested)  Connection, statement, and resultSet objects all have a getWarnings( ) FYI only 3 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 = stmt.getWarnings(); … } //end try catch( SQLException SQLe) { // handle the exception } FYI only 3 Examining Database Metadata DatabaseMetaData object gives information about the database system and the catalog. DatabaseMetaData md = con.getMetaData(); // print information about the driver: System.out.println( “Name:” + md.getDriverName() + “version: ” + md.getDriverVersion()); Metadata has 134 methods! 3 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 //iterating on columns not rows ResultSet crs = md.getColumns(null,null,tableName, null); while (crs.next()) { System.out.println(crs.getString(“COLUMN_NAME” + “, “); } } SKIP 3 A (Semi-)Complete Example //Load driver 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"); //gets the value for the string in column “name” Int n = rs.getIntrating"); System.out.println(s + " " + n); } } catch(SQLException ex) { System.out.println(ex.getMessage () + ex.getSQLState () + ex.getErrorCode ()); } 3 Content  Introduction and Cursor  Embedded SQL  JDBC  SQLJ  Stored Procedures  SQL/PSM 3 SQLJ  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 mybooks = { SELECT name, rating INTO :name, :rating FROM Books WHERE sid = :sid;  Compare to JDBC (note getString(2) can be bound to diff variables.) 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) 3 SQLJ Iterator (cursor) Example Code Int sid; String name; Int rating; // named iterator #sql iterator Sailors(Int sid, String name, Int rating); //Declare iterator class Sailors sailors; //Instantiate Iterator // assume that the application sets rating //Initialize iterator #sql sailors = { SELECT sid, sname INTO :sid, :name FROM Sailors WHERE rating = :rating }; // retrieve results. Iteratively read from the iterator rows while (sailors.next()) { System.out.println(sailors.sid + “ “ + sailors.sname)); } sailors.close(); //close iterator 4 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, :rating} ; if (sailors.endFetch()) { break; } // process the sailor } Note similarity to embedded SQL 4 Content  Introduction and Cursor  Embedded SQL  JDBC  SQLJ  Stored Procedures  SQL/PSM 4 Stored Procedures  What is a stored procedure:  Program executed through a single SQL statement  Executed in the 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 4 NOTE on Syntax!  That Syntax for Stored Procedures has differences for different DBMS.  The following slides are based on textbook and external references (applicable mostly with MySQL).  The goal is to give you multiple flavors, while emphasizing the capability of Stored Procedures.  Syntax specific to the lab (SQL server) will be covered in the Lab. 4 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 Stored procedures can have parameters:  Three different modes: IN, OUT, INOUT CREATE PROCEDURE IncreaseRating( IN sailor_sid INTEGER, IN increase INTEGER) UPDATE Sailors SET rating = rating + increase WHERE sid = sailor_sid 4 Calling Stored Procedures  Interactive CALL storedProcedure (arguments…)  Using embedded SQL: 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); 4 Calling Stored Procedures (Contd.) 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()) { … } 4 Content  Introduction and Cursor  Embedded SQL  JDBC  SQLJ  Stored Procedures  SQL/PSM 4 SQL/PSM (Persistent Stored Modules) Most DBMSs allow users to write stored procedures in a simple, general-purpose language (close to SQL)  SQL/PSM standard is a representative 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; 4 Main SQL/PSM Constructs CREATE FUNCTION rate Sailor (IN sailorId INTEGER) RETURNS INTEGER //Return values DECLARE rating INTEGER // Declare variables DECLARE numRes INTEGER SET numRes = (SELECT COUNT(*) //Assign variables FROM Reserves R WHERE R.sid = sailorId) IF (numRes > 10) THEN rating =1; //IF-THEN branch ELSE rating = 0; END IF; RETURN rating; //Return values 5 Main SQL/PSM Constructs (Contd.)  Local variables (DECLARE)  RETURN values for FUNCTION  Assign variables with SET  Branches and loops:  IF (condition) THEN statements; ELSEIF (condition) statements; … ELSE statements; END IF;  LOOP statements; END LOOP  Queries can be parts of expressions  Can use cursors naturally without “EXEC SQL” 5 Summary  Embedded SQL allows execution of parametrized static queries within a 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 host language and SQL  APIs such as JDBC introduce a layer of abstraction between application and DBMS 5 Summary (Contd.)  SQLJ: Static model, queries checked a compile-time.  Stored procedures execute application logic directly at the server  SQL/PSM standard for writing stored procedures 5 Backup 5 Content  Introduction and Cursor  Embedded SQL  JDBC  SQLJ  Stored Procedures  SQL/PSM 5 Embedded SQL  Approach: Embed SQL in the host language.  A preprocessor converts the SQL statements into special API calls.  The preprocessor is DBMS-dependent. Each DBMS has its own pre-processor.  Then a regular compiler is used to compile the code.  Language constructs:  Declare host variables: EXEC SQL BEGIN (END) DECLARE SECTION  Connect to a database: EXEC SQL CONNECT  Execute query Statements: EXEC SQL Statement; • M d t CURSOR 5 EXEC SQL CONNECT  EXEC SQL CONNECT;  SQL server syntax:  EXEC SQL CONNECT TO {[server_name.]database_name} [AS connection_name] USER [login[.password] | $integrated]  Reference: http://msdn.microsoft.com/en-us/library/aa225245(SQL.80).aspx 5 Embedded SQL: Variables  Host variables must be declared as follows: EXEC SQL BEGIN DECLARE SECTION char c_sname[20]; long c_sid; short c_rating; float c_age; EXEC SQL END DECLARE SECTION  Such host variables must be prefixed a column (:) in SQL statements.  SQL defines the standard for matching types of variables between SQL and host language. E.g. c_sid has the type INTEGER in SQL 5 Embedded SQL: Exceptions  Error codes are made available through including the following data structure (SQLCA = SQL Communication Area)  EXEC SQL INCLUDE SQLCA;  Two special “error” variables which are part of the sqlca data structure can be used to check for errors  sqlca.sqlcode (long, is negative if an error has occurred)  sqlca.sqlstate (char[6], predefined codes for common errors)  Can use the condition SQL statement to check for errors:  EXEC SQL WHENEVER [SQLERROR | NOT FOUND] [CONTINUE | GOTO statement] 5 Cursors  Can declare a cursor on a relation or query statement (which generates a relation).  Can open a cursor, and repeatedly fetch a tuple then move the cursor, until all tuples have been retrieved.  Can use a special clause, called ORDER BY, in queries that are accessed through a cursor, to control the order in which tuples are returned. • Fields in ORDER BY clause must also appear in SELECT clause.  The ORDER BY clause, which orders answer tuples, is only allowed in the context of a cursor.  Can also modify/delete tuple pointed to by a cursor 6 Cursor that gets names of sailors who’ve reserved a red boat, in alphabetical order  Note that it is illegal to replace S.sname by, say, S.sid in the ORDER BY clause! (Why?) 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