Embedded SQL, Database APIs, and PL/SQL

Transcription

1 Embedded SQL, Database APIs, and PL/SQL Juliana Freire Some slides adapted from J. Ullman, L. Delcambre, R. Ramakrishnan, G. Lindstrom and Silberschatz, Korth and Sudarshan

2 SQL: Benefits Declarative languages: program is a prescription for what data is to be retrieved, rather than a procedure describing how to retrieve the data When we write an SQL select query, we do not make any assumptions about the order of evaluation Can be automatically optimized! Decision about order and evaluation plan is left to the optimized Optimizer has the resources to make sophisticated decisions

3 SQL: Limitations Not flexible enough for some applications Some queries cannot be expressed in SQL Non-declarative actions can t be done from SQL, e.g., printing a report, interacting with user/gui SQL queries may be just one small component of complex applications Trade-off: automatic optimization of queries expressed in powerful languages is hard

4 Solution SQL commands called from within a powerful host language program Three main integration approaches: Embed SQL in the host language (Embedded SQL) Create special API to call SQL commands (JDBC,ODBC) Allow external code to be executed from within SQL Query processing performed by the database Leverage database capabilities

5 Three-Tier Architecture A common environment for using a database has three tiers of processors: 1. Web servers --- talk to the user. 2. Application servers --- execute the business logic. 3. Database servers --- get what the app servers need from the database.

6 Three-Tier Architecture

7 Example: Amazon Database holds the information about products, customers, etc. Business logic includes things like what do I do after someone clicks checkout? Answer: Show the how will you pay for this? screen.

8 Environments, Connections, Queries The database is, in many DB-access languages, an environment Database servers maintain some number of connections, so app servers can ask queries or perform modifications The app server issues statements : queries and modifications, usually

9 Client and Servers in SQL Environment CONNECT TO <server name> AS <connection name> AUTHORIZATION <name and password> - A client may maintain several connections to the server, but only one can be active at any point in time - Operations performed while a connection is active form a session SQL Client Connection Session SQL Server

10 Three Kinds of Modules Module = application program Embedded SQL Generic SQL Interface True modules stored functions and procedures

11 Embedded SQL Embed SQL in the host language. A preprocessor converts the SQL statements into special API calls optimized access plans that are stored in the database Then a regular compiler is used to compile the code. SQL statements can refer to host variables (including special variables used to return status). Input: variable given a value by the host program to be used in a query Output: variable used to retrieve value from the database Language constructs: Connecting to a database: Declaring variables: Statements:

12 Declaring Variables

13 Executing Statements

14 Error/Status Reporting Each SQL statement executed returns a status code + additional information SQL communication area (SQLCA) Two special error variables: zero indicates normal execution negative indicates an error has occurred positive indicates warnings or special conditions, e.g., data not found Error class, e.g., syntax error Error subclass implementation dependent

15 Impedance Mismatch Updates are simple to embed: they execute, possibly modify the contents of the database, and return SQLCA structure indicating what happened Queries return data! 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 SQL supports a mechanism called a cursor to handle this.

16 Cursors Can declare a cursor on a relation or query statement (which generates a relation) Name associated with a query Can open a cursor, and repeatedly fetch a tuple then move the cursor, until all tuples have been retrieved Open causes the query to be evaluated EXEC SQL open c END-EXEC Fetch causes values of one tuple in the result to be placed on host variables EXEC SQL fetch c into :cn, :cc END-EXEC Repeated calls to fetch get successive tuples in the query result

17 Cursors (cont.) SQLSTATE gets set to to indicate no more data is available The close statement causes the database system to delete the temporary relation that holds the result of the query EXEC SQL close c END-EXEC Note: above details vary with language. Can use ORDER BY to control the order in which tuples are returned

18 Cursor: Example Define SQL query that gets names of sailors who ve reserved a boat of color desired_color, in alphabetical order, and declare a cursor for it When are input host variables bound? When open is executed! input variable

19 Embedding SQL in C: An Example input variable output variables

20 FETCH Statement What happens if updates are applied to table(s) referenced in a cursor after a result is fetched from a cursor? Important note [Chamberlin, A Complete Guide to DB2]: The result of a query associated with a cursor may be Completely materialized when the 1 st row is fetched, or Materialized one row at a time Choice depends on the access plan chose by the optimizer

21 Updates Through Cursors Can update tuples fetched by cursor by declaring that the cursor is for update" declare c cursor for select * from account where branch-name = ʻPerryridgeʼ for update" To update tuple at the current location of cursor" update account set balance = balance where current of c# Apart from their method of finding the row to be updated/deleted, positioned updates behave exactly the same as UPDATE and DELETE statements that contain a search condition#

22 Dynamic SQL Sometimes, we can t predict in advance what SQL statements will be needed Dynamic SQL allows construction of SQL statements on-the-fly Need to PREPARE/EXECUTE Example: Run-time overhead use only when essential Incurs the cost of access path selection (optimization) at run time Static SQL prepares statements in advance well-suited for applications that perform repetitive queries/transactions

23 Database APIs: Generic Interface 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 Embedded vs. Generic API: the difference is more a matter of look and feel than of substance Sun s JDBC: Java API Supposedly DBMS-neutral a driver traps the calls and translates them into DBMSspecific code database can be across a network

24 Database APIs: Generic Interface

25 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) See textbook for details! Java applet Java application JDBC API JDBC Driver Manager Driver Driver Oracle DB2

26 XSB-Oracle Interface Allow calls to Oracle from Prolog XSB-specific calls: db_open(oracle(name, Pass)) write('% Connected to Oracle as '), write(name), writeln('...'),nl, db_sql('drop table DEPT'), db_create_table('dept', 'DEPTNO NUMBER(2),DNAME VARCHAR2(14)'), db_import( DEPT'( DEPTNO', DNAME'), deptall), db_insert(dept_put(a1,a2),(deptall(a1,a2))), dept_put(1, computer science ), dept_put(1, biology ),

27 Stored Procedures/Functions Issues with accessing DB from remote application: JDBC incurs data transfer overheads DB resources are tied up, e.g., open cursors Advantageous to execute some of the application logic inside the database Minimize data transfer Utilize full power of database server Stored procedure is a program executed within the process space of the database Can be run by multiple users

28 Oracle s PL/SQL Oracle's procedural extension SQL. Combines the data manipulating power of SQL with the data processing power of procedural languages. Stored objects Procedures, functions packages Some uses: External procedures: PL/SQL code invoked from other languages E.g., using special tags, you can embed PL/SQL scripts into HTML source code Triggers: procedures invoked by insert, delete, update

29 Procedures In Oracle s PL/SQL SQL> create or replace procedure sum_salary 2 is 3 cursor c1 is 4 select * from employee; 5 sal_sum integer; 6 begin 7 sal_sum := 0; 8 for emp_rec in c1 loop 9 sal_sum := sal_sum + emp_rec.salary; 10 end loop; 11 dbms_output.put_line('salary sum: ' sal_sum); 12 end; 13 / Procedure created. SQL> exec sum_salary Salary sum: PL/SQL procedure successfully completed.

30 Functions In PL/SQL SQL> create or replace function f(i in integer) 2 return integer 3 is 4 begin 5 return i*i; 6 end; 7 / Function created. SQL> exec dbms_output.put_line(f(13)); 169 PL/SQL procedure successfully completed.

31 PL/SQL: Overview Block-structured language: procedures, functions, and anonymous blocks Block has three parts: a declarative part, an executable part, and an exception-handling part. [DECLARE] --- declarations BEGIN --- statements [EXCEPTION HANDLERS] END; May contain sub-blocks

32 PL/SQL: Variables and Constants Declared vars and constants can be used in SQL and procedural statements Types: SQL datatypes: CHAR, DATE, or NUMBER PL/SQL datatypes: BOOLEAN or BINARY_INTEGER E.g., part_no NUMBER(4); in_stock BOOLEAN; dept_rec dept%rowtype; my_title books.title%type Assigning values to vars: tax := price * tax_rate %ROWTYPE is used to declare a record with the same types as found in the specified database table, view or cursor %TYPE is used to declare a field with the same type as that of a specified table's column SELECT sal * 0.10 INTO bonus FROM emp WHERE empno = emp_id;

33 DECLARE PL/SQL: Variables and Constants v_emprecord emp%rowtype; BEGIN SELECT * INTO v_emprecord FROM emp WHERE ROWNUM = 1; DBMS_OUTPUT.PUT_LINE('Name = ' v_emprecord.ename); DBMS_OUTPUT.PUT_LINE('Salary = ' v_emprecord.sal); END;

34 PL/SQL: Cursors PL/SQL implicitly declares a cursor for all SQL data manipulation statements Explicitly declare a cursor for queries that return multiple rows DECLARE CURSOR c1 IS SELECT empno, ename, job FROM emp WHERE deptno = 20; Use OPEN, FETCH, and CLOSE to control cursor. OPEN executes the query associated with the cursor, identifies the result set, and positions the cursor before the first row. FETCH retrieves the current row and advances the cursor to the next row. CLOSE disables the cursor after the last row has been processed

35 PL/SQL: Cursors FOR Loops Simplify coding no need to use OPEN, FETCH, and CLOSE DECLARE CURSOR c1 IS SELECT ename, sal, hiredate, deptno FROM emp;... BEGIN FOR emp_rec IN c1 LOOP... salary_total := salary_total + emp_rec.sal; END LOOP; END

36 PL/SQL: Error Handling Exception is raised when an error occurs Exceptions Internally defined: division by zero and out of memory User defined: DECLARE insufficient_funds EXCEPTION; BEGIN IF THEN RAISE insufficient_funds; END IF; Improved readability BEGIN SELECT... SELECT... SELECT EXCEPTION WHEN NO_DATA_FOUND THEN -- catches all 'no data found' errors

37 PL/SQL: Procedures specification PROCEDURE debit_account (acct_id INTEGER, amount REAL) IS old_balance REAL; new_balance REAL; declarative overdrawn EXCEPTION; BEGIN SELECT bal INTO old_balance execution FROM accts WHERE acct_no = acct_id; new_balance := old_balance - amount; IF new_balance < 0 THEN RAISE overdrawn; ELSE UPDATE accts SET bal = new_balance WHERE acct_no = acct_id; END IF; EXCEPTION WHEN overdrawn exception THEN... END debit_account; body

38 PL/SQL: Procedures SQL> create or replace procedure sum_salary is 3 cursor c1 is declaration 4 select * from employee; 5 sal_sum integer; 6 begin 7 sal_sum := 0; 8 for emp_rec in c1 loop 9 sal_sum := sal_sum + emp_rec.salary; 10 end loop; 11 dbms_output.put_line('salary sum: ' sal_sum); 12 end; 13 / Procedure created. SQL> exec sum_salary Salary sum: PL/SQL procedure successfully completed. Procedure stored in DB execution

39 PL/SQL: Procedures SQL> create or replace procedure sum_salary is 3 cursor c1 is 4 select * from employee; 5 sal_sum integer; 6 begin 7 sal_sum := 0; 8 for emp_rec in c1 loop 9 sal_sum := sal_sum + emp_rec.salary; 10 that database. end loop; 11 - Call procedure dbms_output.put_line('salary from trigger sum: ' sal_sum); 12 end; 13 - From / SQLPlus Procedure created. SQL> exec sum_salary Salary sum: PL/SQL procedure successfully completed. Procedure stored in DB declaration Once compiled and stored in the data dictionary, it is a schema object, which can be referenced by any number of applications connected to execution - SQL> CALL create_dept('finance', 'NEW YORK');

40 PL/SQL: Functions Function = subprogram that computes a value SQL> create or replace function f(i in integer) 2 return integer 3 is 4 begin 5 return i*i; 6 end; 7 / Function created. SQL> exec dbms_output.put_line(f(13)); 169 PL/SQL procedure successfully completed.

41 What are the advantages of PL/SQL? Performance: reduced communication overheads JDBC and other APIs need to send one SQL command at a time high overhead PL/SQL: sends one block at a time Stored procedures: Executed in server close to the data Re-use application logic by multiple users Figure from PL/SQL User's Guide and Reference Release 2 (9.2)

42 PL/SQL On-line Documentation On-line documentation appdev.101/b10807/toc.htm Additional information available at SQL 2003 introduced Persistent, Stored Modules (SQL/PSM) General purpose language for creating stored procedures See textbook, chapter 9.4

43 Summary Embedded SQL allows execution of parameterized static queries within a host language Dynamic SQL allows execution of completely adhoc 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

44 Additional References Google JDBC, google PL/SQL, JDBC Overview: jdbc/overview.html XSB: A Complete Guide to DB2 Universal Database. Don Chamberlin.

45 Bonus Material

46 JDBC Java Database Connectivity (JDBC) is a library used to access a database server using Java as the host language 46

47 Making a Connection The JDBC classes import java.sql.*; Class.forName(com.mysql.jdbc.Driver); Connection mycon = DriverManager.getConnection( ); Loaded by forname URL of the database your name, and password go here. The driver for mysql; others exist 47

48 Statements JDBC provides two classes: 1. Statement = an object that can accept a string that is a SQL statement and can execute such a string. 2. PreparedStatement = an object that has an associated SQL statement ready to execute. 48

49 Creating Statements The Connection class has methods to create Statements and PreparedStatements. Statement stat1 = mycon.createstatement(); PreparedStatement stat2 = mycon.createstatement( SELECT beer, price FROM Sells + WHERE bar = Joe s Bar ); createstatement with no argument returns a Statement; with one argument it returns a PreparedStatement.

50 Executing SQL Statements JDBC distinguishes queries from modifications, which it calls updates. Statement and PreparedStatement each have methods executequery and executeupdate. For Statements: one argument: the query or modification to be executed. For PreparedStatements: no argument.

51 Example: Update stat1 is a Statement. We can use it to insert a tuple as: stat1.executeupdate( INSERT INTO Sells + VALUES( Brass Rail, Bud,3.00) );

52 Example: Query stat2 is a PreparedStatement holding the query SELECT beer, price FROM Sells WHERE bar = Joe s Bar. executequery returns an object of class ResultSet we ll examine it later. The query: ResultSet menu = stat2.executequery();

53 Accessing the ResultSet An object of type ResultSet is something like a cursor. Method next() advances the cursor to the next tuple. The first time next() is applied, it gets the first tuple. If there are no more tuples, next() returns the value false.

54 Accessing Components of Tuples When a ResultSet is referring to a tuple, we can get the components of that tuple by applying certain methods to the ResultSet. Method getx (i ), where X is some type, and i is the component number, returns the value of that component. The value must have type X.

55 Example: Accessing Components Menu = ResultSet for query SELECT beer, price FROM Sells WHERE bar = Joe s Bar. Access beer and price from each tuple by: while ( menu.next() ) { } thebeer = Menu.getString(1); theprice = Menu.getFloat(2); /*something with thebeer and theprice*/

56 User-Defined Functions (UDF) Create additional functions to supplement the built-in function supplied by DBMS Users can utilize UDFs in SQL queries Scalar functions are applied to a column or expression and operate on a single value Table functions, when invoked, return an entire table

57 UDF Examples get_bal returns the balance of the specified account: CREATE FUNCTION get_bal(acc_no IN NUMBER) RETURN NUMBER IS acc_bal NUMBER(11,2); BEGIN SELECT order_total INTO acc_bal FROM orders WHERE customer_id = acc_no; RETURN(acc_bal); END; get_bal can be used in a SQL statement: SELECT get_bal(165) FROM dual dual is a table which is created by oracle along with the data dictionary. It consists of exactly one column whose name is dummy and one record. The value of that record is X.

58 UDF Examples CREATE OR REPLACE FUNCTION new_name(pid IN NUMBER) RETURN VARCHAR2 IS nname VARCHAR(40); BEGIN SELECT 'Pres' name INTO nname FROM test WHERE test.id = pid; RETURN(nname); END; select new_name(2) from dual

59 UDF Examples CREATE OR REPLACE FUNCTION mydouble(n IN NUMBER) RETURN NUMBER IS dn NUMBER; BEGIN dn := n*2; RETURN(dn); END; CREATE OR REPLACE FUNCTION addtitle(name IN VARCHAR) RETURN VARCHAR IS tn VARCHAR(40); BEGIN tn := 'Pres. ' name; RETURN(tn); END;