University of Helsinki/ CS 17.2.2003 Introduction to Databases, Spring 2003 Database programming

Programming Database Applications Embedded SQL

„ Databases are rarely used thru direct query „ SQL-statements are written among programming interfaces language statements. SQL-statements carry special (Exec SQL) marks that identify then as being SQL „ Typically they are used thru application „ The marks are used by a pre-compiler that programs substitutes the SQL statements with calls of library „ Techniques to use databases in application functions programs include: „ The result of the pre-compiler is then compiled with a – Embedded SQL standard programming language compiler – Database Application Programming interface „ 2-phase compilation Library (database API) „ Pre-compilers are available from dbms suppliers for – Other interface that hide the database some programming languages (Ada, C, C++, Cobol, Pascal,…, Java) – A special database programming language

Embedded SQL Example of Embedded SQL in Pascal

function avgSalary(dept:integer):real; var Marked SQL- statement among PrePre-- Function library n: integer; programming language compilercompiler (database functions) statements (C+SQL) salSum: integer; #include SQLCA.INC EXEC SQL BEGIN DECLARE SECTION

Standard programming var salary: integer; Language + ‘normal’ dpt: integer; Compiled ‘normal’ SQL statements as Compiled compilercompiler Parameters of function EXEC SQL END DECLARE SECTION programprogram calls

Example of Embedded SQL in Pascal Concepts in embedded SQL

begin „ Cursor EXEC SQL DECLARE salaryCursor CURSOR FOR SELECT salary from employee – Structure for processing the result of a query where department= :dpt; • Must be attached to a query (declare) n:=0; salSum:=0; dpt:= dept; • Is opened (open) = the attached query is executed using EXEC SQL open salaryCursor; the variable values of the execution time EXEC SQL fetch salaryCursor into :salary; • Is moved (fetch) = proceed to next record and transfer while sqlcode = 0 do begin data from the current record to program variables salSum := salSum + salary; • Is closed (close) = releases resources n := n + 1; EXEC SQL fetch salaryCursor into :salary; end; EXEC SQL close salaryCursor; if n > 0 then avgSalary := salSum/n else avgSalary := 0; end;.

© Harri Laine 1 University of Helsinki/ CS 17.2.2003 Introduction to Databases, Spring 2003 Database programming

Concepts in embedded SQL Programming using a database API

„ A database operation may fail. The success „ Application programming interface (API) is of the operation must be tested after each library that provides the services of the operation. DBMS. „ Success may be tested by checking the value „ Supplier specific libraries: Native API of the variable sqlstate (or sqlcode according – For example OracleCLI = Oracle Call Level to the older standard). Both return 0 if the Interface operation succeeded and an error code it „ Supplier independent libraries failed. – For example ODBC (Microsoft Open Database „ Success may be also tested by defining error Connection), JDBC Java database connection handlers (WHENEVER something do – Make it possible to change dbms and even to use Handler) many dbms in the same program

Programming using a database API JDBC

„ Supplier independent libraries require a dbms „ Java database connection (JDBC) provides only a specific driver to work with a particular dbms few classes: „ DriverManager „ ODBC is the most common API – This class provides services for attaching the dbms specific – All suppliers provide ODBC drivers. driver and for establishing connections to the database, – Parameters passed in C-language style – DBms drivers are able to register themselves. Thus it is enough to load the driver, for example, using classByName „ JDBC provides the same ideas as ODBC but service. in Java – Here is the code for explicit registration of the driver – Some details of ODBC are hidden within class – DriverManager.registerdriver( definitions. new oracle.jdbc.OracleDriver());

JDBC JDBC

„ Connection Connection con = – This class establishes a database connection DriverManager.getConnection( (session): • a connection between the application and the database – log in with some user account and password “jdbc:oracle:thin:@bodbacka.cs.helsinki.fi:1521:test”, – All database services need a connection – “scott”,”tiger”); Connection class connects service requests to the connection Establishes a connection using Oracle thin driver via – Connection should be closed when it is no longer port 1521 to computer bodbacka.cs.helsinki.fi and its needed test- database using user account scott and – DriverManager provides the method for creating a password tiger connection

© Harri Laine 2 University of Helsinki/ CS 17.2.2003 Introduction to Databases, Spring 2003 Database programming

JDBC JDBC

„ Statement „ ResultSet – Environment for executing database operations – The answers obtained by executing a query – Provides, for example, methods – Corresponds to the cursor of embedded SQL – executeQuery - to execute queries – Method Statement.executeQuery() creates a – executeUpdate - to execute other operations ResultSet object • executeQuery accepts the actual query as its parameter „ Connection provides a methos for creationg statements Statement stmt= con.createStatement(); ResultSet rs= stmt.executeQuery( “select name from employee”);

JDBC JDBC

„ Answer processing using the methods of – Data may be only in program variables. ResultSet ResultSet: provides data type specific methods getType to transfer data from active row to program getString – Method next() activates the next row of the for Strings, getBoolean for booleans, getInt, answer. It returns true if such a row exists and getDate,…. otherwise false. First call activates the first row of the answer. – These functions use the column name or the column sequence number as their parameter –esim: – String a= rs.getString(“address”); // column address – Int p= rs.getInt(3); // third column

JDBC JDBC

„ Get-functions are able to perform some data type Statement stmt= con.createStatement(); conversions, for example to change integers to ResultSet rs= stmt.executeQuery( strings or vice versa. If conversion fails an “select name, address, salary from employee ” + SQLException is thrown. All error conditions cause “ order by name”); SQLException to be thrown. while (rs.next()) { „ Null values need special treatment. getString returns System.out.println(rs.getString(1) +”, “+ java.NULL in case of SQL null value, but, for rs.getString(2)+”, “+rs.getString(3)); example, getInt returns 0 (zero). To test whether the } value was null there is method wasNull. It is Output : parametrized like the get-methods. Lahtinen Kalle, Katu 6, 12000 Mäki Manu, Kuja5, 20000

© Harri Laine 3 University of Helsinki/ CS 17.2.2003 Introduction to Databases, Spring 2003 Database programming

JDBC JDBC

„ Statement that do not produce a result (insert, „ Parametrized operation: delete, update, create, alter, …) are executed – Database operations may always be executed by using Statement.executeUpdate-method. giving them as parameters for the methods executeUpdate and executeQuery. Sometimes „ example almost the same operations is used repeatedly Int empUpdated= with only minor changes. In these cases it might stmt.executeUpdate(“update employee “+ be useful to compile the query only once and “set salary= salary + 10000 ” + reuse the compiled query. “ where name= ‘Laine Harri’ ”); – Class PreparedStatement supports this way of programming.

JDBC JDBC

PreparedStatement pst = registerDriver con.prepareStatement( DriverManager external driver “select name,address,salary “+ “from employee “+ “where name like ?” ); getConnection „ Question mark indicates a parameter, It can be used to substitute a constant value. Connection „ PreparedStatement provides data type specific set-methods for assigning values for these parameters (setString for strings) prepareStatement „ Set methods have two parameters: createStatement – The sequence number of the question mark in the SQL-operation setXXXX and Statement PreparedStatement – The value to substitute the parameter –Example; pst.setString(1,”Smi%”); executeQuery „ Prepared statement has executeQuery and execute update methods, but these don’t need any parameter. next ResultSet getYYYY

© Harri Laine 4