Chapter 9 SQL in a server environment

Transcription

1 Chapter 9 SQL in a server environment SQL in a Programming Environment embedded SQL persistent stored modules Database-Connection Libraries Call-level interface (CLI) JDBC PHP

2 SQL in Real Programs We have seen only how SQL is used at the generic query interface --- an environment where we sit at a terminal and ask queries of a database. Reality is almost always different: conventional programs interacting with SQL.

3 Options 1. SQL statements are embedded in a host language (e.g., C). 2. Code in a specialized language is stored in the database itself (e.g., PSM, PL/SQL). 3. Connection tools are used to allow a conventional language to access a database (e.g., CLI, JDBC, PHP/DB).

4 SQL in a Programming Environment Embedded SQL: add to a conventional programming language (C for example, we called host language ), certain statements that represent SQL operation. Host language+embedded SQL code?

5 System Implementation Host Language + Embedded SQL Preprocessing Host Language + Function calls Host-language compiler SQL library Object-code program How to identify SQL statements? How to move data between SQL and a conventional programming language? Mismatch problem exists?

6 How to recognize SQL statements (the Interface between SQL statements and programming language) Each embedded SQL statement introduced with EXEC SQL Shared variables : exchange data between SQL and a host language. When they are referred by a SQL statement, these shared variables are prefixed by a colon, but they appear without colon in hostlanguage statements. EXEC SQL BEGIN / END DECLARE SECTION to declare shared variables.

7 the Interface between SQL statements and programming language SQL define an array of characters SQLSTATE that is set every time the system is called. SQLSTATE connects the host-language program with the SQL execution system : no error 02000: could not be found

8 Implementations of SQLSTATE SQL defines an array of characters SQLSTATE that is set every time the system is called. Errors are signaled there Different systems use different way Oracle provides us with a header file sqlca.h that declares a communication area and defines macros to access it, such as NOT FOUND. Sybase provides SQLCA with sqlcode 0:success, <0: fail, 100: not found

9 Example: Find the price for a given beer at a given bar Sells (bar, beer, price) EXEC SQL BEGIN DECLARATION SECTION CHAR thebar[21], thebeer[21]; Float theprice; EXEC SQL END DECLARAE SECTION EXEC SQL SELECT price INTO :theprice FROM sells WHERE beer = :thebeer AND bar =:thebar;

10 Queries produce sets of tuples as a result, while none of the major host languages supports a set data type directly. So, cursors are used. A cursor declaration: EXEC SQL DECLARE <cursor> CURSOR FOR <query> A statement EXEC SQL OPEN<cursor> : the cursor is ready to retrieve the first tuple of the relation over which the cursor ranges. EXEC SQL FETCH FROM < cursor > INTO <list of variables> EXEC SQL CLOSE <cursor>: the cursor is no longer ranges over tuples of the relation.

11 Cursor Example Void worthranges() { int i,digits, counts[15]; EXEC SQL BEGIN DECLARE SECTION; int worth; char SQLSTATE[6]; EXEC SQL END DECLARE SECTION; EXEC SQL DECLARE execcursor CURSOR FOR SELECT networth FROM MovieExec; EXEC SQL OPEN execcursor; while (1) { EXEC SQL FETCH FROM execcursor INTO :worth; if (NO_MORE_TUPLES) BREAK; else.. } EXEC SQL CLOSE execcursor; }

12 More about cursor: The order in which tuples are fetched from the relation can be specified. The effect of changes to the relation that the cursor ranges over can be limited. The motion of the cursor through the list of tuples can be varied.

13 Modification by cursor With Where clause WHERE CURRENT OF followed by the name of the cursor. Define NO_MORE_TUPLES!(strc mp(sqlstate, )) e.g... EXEC SQL OPEN execcursor; while (1) { EXEC SQL FETCH FROM execcursor INTO :execname,:execaddr,:certno,:worth; if (NO_MORE_TUPLES) BREAK; IF (WORTH < 1000) EXEC SQL DELETE FROM MovieExec WHERE CURRENT OF execcursor; else.. EXEC SQL CLOSE execcursor;

14 Protecting against concurrent updates EXEC SQL DECLARE execcursor INSENSITIVE CURSOR FOR SELECT networth FROM MovieExec; The SQL system will guarantee that changes to relation MovieExec made between one opening and closing of execcursor will not affect the set of tuples fetched. Insensitive cursors could be expensive, systems spend a lot of time to manage data access.

15 Scrolling Cursors EXEC SQL DECLARE execcursor SCROLL CURSOR FOR MovieExec; The cursor may be used in a manner other than moving forward in the order of tuples. Follow FETCH by one of several options that tell where to find the desired tuple. Those options are NEXT, PRIOR, FIRST, LAST and so on.

16 Need for Dynamic SQL Most applications use specific queries and modification statements to interact with the database. The DBMS compiles EXEC SQL statements into specific procedure calls and produces an ordinary host-language program that uses a library. Sometimes we don t know what it needs to do until it runs?

17 Dynamic SQL Preparing a query: EXEC SQL PREPARE <query-name> FROM <text of the query>; Executing a query: EXEC SQL EXECUTE <query-name>; Prepare = optimize query. Prepare once, execute many times.

18 Example: A Generic Interface EXEC SQL BEGIN DECLARE SECTION; char query[max_length]; EXEC SQL END DECLARE SECTION; while(1) { /* issue SQL> prompt */ /* read user s query into array query */ EXEC SQL PREPARE q FROM :query; EXEC SQL EXECUTE q; } q is an SQL variable representing the optimized form of whatever statement is typed into :query

19 Execute-Immediate If we are only going to execute the query once, we can combine the PREPARE and EXECUTE steps into one. Use: EXEC SQL EXECUTE IMMEDIATE <text>;

20 Example: Generic Interface Again EXEC SQL BEGIN DECLARE SECTION; char query[max_length]; EXEC SQL END DECLARE SECTION; while(1) { /* issue SQL> prompt */ /* read user s query into array query */ EXEC SQL EXECUTE IMMEDIATE :query; }

21 Stored Procedures PSM, or persistent stored modules, allows us to store procedures as database schema elements. PSM = a mixture of conventional statements (if, while, etc.) and SQL. Lets us do things we cannot do in SQL alone.

22 Procedures Stored in the Schema Aim Provide a way for the user to store with a database schema some functions or procedures that can be used in SQL queries or other SQL statements.

23 Creating PSM Functions and Procedures Procedure Declarations CREATE PROCEDURE <name>(<arglist>) local declarations; procedure body; Function Declarations CREATE FUNCTION <name> (<parameters>) RETURNS <type> local declarations function body;

24 Example: CREATE PROCEDURE move ( IN oldaddr VARCHAR [255], IN newaddr VARCHAR [255] UPDATE MOVIEsTAR SET address = newaddr WHERE address = oldaddr; ) The parameters of a procedure are triples of mode-name-type IN = procedure uses value, does not change value. OUT = procedure changes, does not use. INOUT = both.

25 Function Declaration Function parameter may only be of mode IN, the only way to obtain information from a function is through its return-value.

26 Example: Stored Procedure Let s write a procedure that takes two arguments b and p, and adds a tuple to Sells that has bar = Joe s Bar, beer = b, and price = p. Used by Joe to add to his menu more easily.

27 The Procedure CREATE PROCEDURE JoeMenu ( ) IN b IN p CHAR(20), REAL INSERT INTO Sells VALUES( Joe s Bar, b, p); Parameters are both read-only, not changed The body --- a single insertion

28 Invoking Procedures Use SQL/PSM statement CALL, with the name of the desired procedure and arguments. Example: CALL JoeMenu( Moosedrool, 5.00);

29 Where to call? CALL <procedure name> (<argument list>); From a host-language program, e.g. EXEC SQL CALL foo(:x,3); As a statement of another PSM function or procedure As an SQL command issued to the generic SQL interface, e.g. CALL foo(1,3)

30 Invoking Functions It is not permitted to call a function. Use the function name and suitable arguments as part of an expression. Functions used in SQL expressions where a value of their return type is appropriate.

31 Simple statements in PSM Return statement in a function: RETURN <expression>; declare local variables : DECLARE <name><type>; Assignments: SET <variable>=<expression>; SET b = Bud ; Groups of statements: BEGIN END Separate by semicolons. Branching statements: If then else, Loops: for-loops, loops,

32 Example: IF Let s rate bars by how many customers they have, based on Frequents(drinker, bar). <100 customers: unpopular customers: average. >= 200 customers: popular. Function Rate(b) rates bar b.

33 Example: IF (continued) CREATE FUNCTION Rate (IN b CHAR(20) ) BEGIN END; RETURNS CHAR(10) DECLARE cust INTEGER; SET cust = (SELECT COUNT(*) FROM Frequents WHERE bar = b); IF cust < 100 THEN RETURN unpopular ELSEIF cust < 200 THEN RETURN average ELSE RETURN popular END IF; Number of customers of bar b Nested IF statement

34 Loops Basic form: <loop name>: LOOP <statements> END LOOP; Exit from a loop by: LEAVE <loop name>

35 Example: Exiting a Loop loop1: LOOP... LEAVE loop1;... END LOOP; If this statement is executed... Control winds up here

36 Other Loop Forms WHILE <condition> DO <statements> END WHILE; REPEAT <statements> UNTIL <condition> END REPEAT;

37 Queries General SELECT-FROM-WHERE queries are not permitted in PSM. There are three ways to get the effect of a query: 1. Queries producing one value can be the expression in an assignment. 2. Single-row SELECT... INTO. 3. Cursors.

38 Example: Assignment/Query Using local variable p and Sells(bar, beer, price), we can get the price Joe charges for Bud by: SET p = (SELECT price FROM Sells WHERE bar = Joe s Bar AND beer = Bud );

39 SELECT... INTO Another way to get the value of a query that returns one tuple is by placing INTO <variable> after the SELECT clause. Example: SELECT price INTO p FROM Sells WHERE bar = Joe s Bar AND beer = Bud ;

40 Cursors A cursor is essentially a tuple-variable that ranges over all tuples in the result of some query. Declare a cursor c by: DECLARE c CURSOR FOR <query>;

41 Opening and Closing Cursors To use cursor c, we must issue the command: OPEN c; The query of c is evaluated, and c is set to point to the first tuple of the result. When finished with c, issue command: CLOSE c;

42 Fetching Tuples From a Cursor To get the next tuple from cursor c, issue command: FETCH FROM c INTO x1, x2,,xn ; The x s are a list of variables, one for each component of the tuples referred to by c. c is moved automatically to the next tuple.

43 Breaking Cursor Loops (1) The usual way to use a cursor is to create a loop with a FETCH statement, and do something with each tuple fetched. A tricky point is how we get out of the loop when the cursor has no more tuples to deliver.

44 Breaking Cursor Loops (2) Each SQL operation returns a status, which is a 5-digit character string. For example, = Everything OK, and = Failed to find a tuple. In PSM, we can get the value of the status in a variable called SQLSTATE.

45 Breaking Cursor Loops (3) We may declare a condition, which is a boolean variable that is true if and only if SQLSTATE has a particular value. Example: We can declare condition NotFound to represent by: DECLARE NotFound CONDITION FOR SQLSTATE ;

46 Breaking Cursor Loops (4) The structure of a cursor loop is thus: cursorloop: LOOP FETCH c INTO ; IF NotFound THEN LEAVE cursorloop; END IF; END LOOP;

47 Exceptions in PSM Where to go: 1) continue:execute the CREATE FUNCTION GetYear(t statement VARCHAR[255]) after the one that RETURNS INTEGER raised the exception. DECLARE Not_Found 2) CONDITION Exit:leave the FOR BEGIN END SQLSTATE block.the statement after the ; block is executed next. DECLARE Too_Mamy 3) CONDITION Undo: not executed FOR SQLSTATE the ; statement within the block and exit like 2) BEGIN DECLARE EXIT HANDLER FOR Not_Found,Too_Many RETURN NULL;// handler declaration RETURN (SELECT year FROM Movie WHERE title=t); END;

48 Components of Exception handler in PSM A list of exception conditions that invoke the handler when raised. Code to be executed when one of the associated exceptions is raised. An indication of where to go after the handler has finished its work. DELARE <where to go> HANDLER FOR <condition list> <statement>

49 Example: Cursor in PSM Let s write a procedure that examines Sells(bar, beer, price), and raises by $1 the price of all beers at Joe s Bar that are under $3. Yes, we could write this as a simple UPDATE, but the details are instructive anyway.

50 The Needed Declarations CREATE PROCEDURE JoeGouge( ) DECLARE thebeer CHAR(20); DECLARE theprice REAL; DECLARE NotFound CONDITION FOR SQLSTATE ; DECLARE c CURSOR FOR (SELECT beer, price FROM Sells WHERE bar = Joe s Bar ); Used to hold beer-price pairs when fetching through cursor c Returns Joe s menu

51 The Procedure Body BEGIN OPEN c; menuloop: LOOP FETCH c INTO thebeer, theprice; IF NotFound THEN LEAVE menuloop END IF; IF theprice < 3.00 THEN UPDATE Sells SET price = theprice+1.00 WHERE bar = Joe s Bar AND beer = thebeer; END IF; END LOOP; CLOSE c; END; Check if the recent FETCH failed to get a tuple If Joe charges less than $3 for the beer, raise it s price at Joe s Bar by $1.

52 Database connection The third approach to connecting databases to conventional languages is to use library calls. 1. C + CLI 2. Java + JDBC 3. PHP + PEAR/DB

53 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.

54 DBMS environment: the framework under which data may exist and SQL operation on data may be executed. catalog schema catalog cluster catalog environment Schemas: collections of tables, views, assertions, domains and so on. Catalog: collections of schemas, information about all the schemas in the catalog. Clusters: each user has an associated cluster, so in a sense, a cluster is the database as seen by a particular user.

55 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.

56 Diagram to Remember Environment Connection Statement

57 SQL/CLI Instead of using a preprocessor (as in embedded SQL), we can use a library of functions. The library for C is called SQL/CLI = Call- Level Interface. Embedded SQL s preprocessor will translate the EXEC SQL statements into CLI or similar calls, anyway.

58 Data Structures C connects to the database by structs of the following types: 1. Environments : represent the DBMS installation. 2. Connections : logins to the database. 3. Statements : SQL statements to be passed to a connection. 4. Descriptions : records about tuples from a query, or parameters of a statement.

59 JDBC Java Database Connectivity (JDBC) is a library similar to SQL/CLI, but with Java as the host language. Like CLI, but with a few differences for us to cover.

60 Making a Connection import java.sql.*; The JDBC classes 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

61 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.

62 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.

63 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.

64 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)

65 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();

66 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.

67 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.

68 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*/

69 PHP (personal home page) A scripting language to be used for actions within HTML text. Indicated by <? PHP code?>. DB library exists within PEAR (PHP Extension and Application Repository). Include with include(db.php).

70 Variables in PHP Must begin with $. OK not to declare a type for a variable. But you give a variable a value that belongs to a class, in which case, methods of that class are available to it.

71 String Values PHP solves a very important problem for languages that commonly construct strings as values: How do I tell whether a substring needs to be interpreted as a variable and replaced by its value? PHP solution: Double quotes means replace; single quotes means don t.

72 Example: Replace or Not? $100 = one hundred dollars ; $sue = You owe me $100. ; $joe = You owe me $100. ; Value of $sue is You owe me $100, while the value of $joe is You owe me one hundred dollars.

73 PHP Arrays Two kinds: numeric and associative. Numeric arrays are ordinary, indexed 0,1, Example: $a = array( Paul, George, John, Ringo ); Then $a[0] is Paul, $a[1] is George, and so on.

74 Associative Arrays Elements of an associative array $a are pairs x => y, where x is a key string and y is any value. If x => y is an element of $a, then $a[x] is y.

75 Example: Associative Arrays An environment can be expressed as an associative array, e.g.: $myenv = array( phptype => oracle, hostspec => database => cs145db, username => ullman, password => notmypw );

76 Making a Connection With the DB library imported and the array $myenv available: include(db.php); $mycon = DB::connect($myEnv); Function connect in the DB library Class is Connection because it is returned by DB::connect(). <vendor>://<user name>:<password><host name>/<database name>

77 Executing SQL Statements Method query applies to a Connection object. It takes a string argument and returns a result. Could be an error code or the relation returned by a query.

78 Remember this variable is replaced by its value. Example: Executing a Query Find all the bars that sell a beer given by the variable $beer. Method application $beer = Bud ; $result = $mycon->query( SELECT bar FROM Sells. WHERE beer = $beer ; ); Concatenatio in PHP

79 Cursors in PHP The result of a query is the tuples returned. Method fetchrow applies to the result and returns the next tuple, or FALSE if there is none.

80 Example: Cursors while ($bar = $result->fetchrow()) { // do something with $bar }

81 Summary Embedded SQL (shared variables, EXEC SQL, Cursor), Dynamic SQL SQL/PSM Call-level Interface (SQL/CLI) JDBC PHP