System Aspects of SQL

Transcription

1 System Aspects of SQL SQL Environment User Access Control SQL in Programming Environment Embedded SQL SQL and Java Transactions (Programmers View) SQL Environment: Introduction SQL server Supports operations on database elements Typically runs on large host machine SQL client Supports user connections to server Runs on (different) host machine Connection Channel between client and server 2 1

2 SQL Environment: Introduction Session All SQL operations performed while connection open Current catalog, current schema, authorized user SQL Environment SQL agent SQL Server Connection Session SQL Client Application Module: application program SQL agent: execution of module 3 SQL Environment: Module Types Generic SQL Interface: Module: each query or statement Embedded SQL: SQL statements within host-language program SQL statements pre-processed to function calls Calls executed at run-time True modules: Collection of stored procedures Host language code, SQL code 4 2

3 SQL Environment: ileges User Outside schema, handling implementation dependent Identification by Authorization ID (user name) Role Defines user group Inside schema, handling via SQL statements Identification by Authorization ID (role name) All users: special role PUBLIC Examples: CREATE ROLE Customer; CREATE ROLE Secretary WITH ADMIN Klaus; CREATE ROLE Movie_staff; CREATE ROLE Shop_owner; 5 User Access Control: Introduction Secrecy: Users should not be able to see things they are not supposed to. e.g., A student can t see other students grades. Integrity: Users should not be able to modify things they are not supposed to. e.g., Only instructors can assign grades. Availability: Users should be able to see and modify things they are allowed to. 6 3

4 User Access Control: Introduction Security policy specifies authorization Security mechanism enforces a security policy Two mechanisms at DBMS level Discretionary access control Concept of privileges for objects (tables and views) Mechanisms for giving and revoking users privileges Mandatory access control System-wide policies for DBS DB object have security class Rules on security classes govern access Used for specialized (e.g., military) applications 7 User Access Control: ileges ileges Right to perform SQL statement type on objects Assigned to roles (authorization IDs) Creator of object: all privileges DBMS: management of privileges and access rights ilege types: SELECT on table or view INSERT on table or view DELETE on table or view UPDATE on table or view REFERENCES: right to refer to relation in constraint USAGE: (SQL-92) right to use specified domain ALL PRIVILEGES: short form for all privileges 8 4

5 User Access Control: ileges Example INSERT INTO Format(name) SELECT format FROM Tape t WHERE t.format NOT IN (SELECT name FROM format); ileges: SELECT on Tape SELECT on Format INSERT on Format 9 User Access Control: ileges Grant privilege GRANT <privileges> ON <object> TO <users> [WITH GRANT OPTION] GRANT OPTION: Right to pass privilege on to other users Only owner can execute CREATE, ALTER, and DROP ilege to SELECT particular columns in a table GRANT <privileges> ON <tablename(<attributenames>)> TO <users> [WITH GRANT OPTION] 10 5

6 User Access Control: ileges Examples: GRANT INSERT, SELECT ON Movie TO Klaus Klaus can query Movie or insert tuples into it. GRANT DELETE ON Movie TO shop_owner WITH GRANT OPTION Anna can delete tuples, and also authorize others to do so GRANT UPDATE (pricepday) ON Movie TO movie_staff Staff can update (only) the price field of Movie tuples GRANT SELECT ON MovieView TO Customers This does NOT allow the customers to query Movie directly! 11 User Access Control: ileges on views Creator has privilege on view if privilege on all underlying tables Creator loses SELECT privilege on underlying table view is dropped Creator loses a privilege on underlying table creator loses privilege on view Creator loses a privilege held with grant option on underlying table users who were granted that privilege on the view lose privilege on view 12 6

7 User Access Control: ileges Revoke privilege Core SQL:1999 REVOKE <privileges> ON <object> FROM <users> RESTRICT RESTRICT: only revoke if non of the privileges have been granted by these users ilege given from different users must be revoked from all users to loose privilege 13 User Access Control: Examples Owner: GRANT Update ON Movie TO Klaus; Owner: GRANT Update ON Movie TO Anna; owner Klaus Anna Movie Owner: REVOKE Update ON Movie FROM Klaus RESTRICT; owner Klaus Anna Movie 14 7

8 User Access Control: Examples Owner: GRANT Update ON Movie TO Klaus WITH GRANT OPTION; Klaus: GRANT Update ON Movie TO Anna; owner Klaus Anna Movie Grant Owner: REVOKE Update ON Movie FROM Klaus RESTRICT; owner Klaus Anna Movie Grant Command fails! 15 User Access Control: ileges Revoke privilege enhanced SQL:1999 REVOKE [GRANT OPTION FOR] <privileges> ON <object> FROM <users> {RESTRICT CASCADE} CASCADE: revoke from all users that have been granted the privilege by these users RESTRICT: only revoke if non of the privileges have been granted by this user 16 8

9 User Access Control: Examples Owner: GRANT Update ON Movie TO Klaus WITH GRANT OPTION; Klaus: GRANT Update ON Movie TO Anna; owner Klaus Anna Movie Grant Owner: REVOKE Update ON Movie FROM Klaus CASCADE; owner Klaus Anna Movie 17 User Access Control: Examples Owner: GRANT Update ON Movie TO Klaus WITH GRANT OPTION; Klaus: GRANT Update ON Movie TO Anna; owner Klaus Anna Movie Grant Owner: REVOKE GRANT OPTION FOR Update ON Movie FROM Klaus CASCADE; owner Klaus Anna Movie 18 9

10 User Access Control: Examples Owner: GRANT Update ON Movie TO Klaus WITH GRAND OPTION; Owner: GRANT Update ON Movie TO Anna; Klaus: GRANT Update ON Movie TO Anna; owner Movie Klaus Anna Owner: REVOKE GRANT OPTION FOR Update ON Movie FROM Klaus CASCADE; owner Movie Klaus Anna 19 User Access Control: Object owners Schema owner: Right for create, drop, alter (no privilege, not grantable) All privileges on schema objects Object creator/owner: Create statement: current authorizationid is owner Enhanced SQL:1999 : owner needn't be creator Current user privileges in Oracle: SQL> SQL> SELECT * FROM FROM session_privs; PRIVILEGE CREATE SESSION ALTER ALTER SESSION CREATE TABLE TABLE

11 SQL in Programs: Introduction SQL Sub-language for data access Efficient database operations Host language: Control structures Complex computations User interface: output formatting, forms Transactions: DB interactions as unit of work SQL and host language needed 21 SQL in Programs: Impedance Mismatch Impedance Mismatch: differing data model of SQL and host language Problems: Set oriented operations vs manipulation of individuals Interconnection of program variables and SQL statements Compilation time of embedded SQL-statements 22 11

12 SQL in Programs: Program/DBS Communication 1. Fourth Generation Languages (4GL) Decreasing importance 2. Module Languages Standardized in SQL: Call level interface Most important approach Standardized in SQL: Component architectures Hiding the details of DB interaction Example: Enterprise Java Beans (EJB) 23 SQL in Programs: 1. 4GL Underlying assumption: application programs algorithmically simple sophisticated output formatting needed difficult to switch between different DBS Technical concept: Client workstation (presentation, requests, GUI) Database server Decreasing importance Proprietary protocol 24 12

13 SQL in Programs: 2. Modules Parameterized modules of SQL statements Standardized in SQL:1999 Compiled for a particular language Linked to application program Language Examples: COBOL, C, ADA,... Disadvantages: SQL code hidden in application and vice versa Not widely used Used in stored procedures (e.g., Oracle PL/SQL) Executed under control of DBS 25 SQL in Programs: 2. Modules (cont) Example: Returned state value Program Language variables MODULE demo demo NAMES NAMES are are ascii ascii LANGUAGE FORTRAN SCHEMA movie_db AUTHORIZATION PROCEDURE discount_op (SQLSTATE, :title VARCHAR(40), :discount DECIMAL(3,2)) UPDATE Movie Movie M SET SET pricepday = pricepday - :discount WHERE WHERE M.title = :title; PROCEDURE customerstate (SQLSTATE, :customer INTEGER) SELECT movie_id,tape_id,from_date FROM FROM Tape Tape T, T, Rental R WHERE WHERE R.member = customer AND AND R.tape_id = T.id; T.id; 26 13

14 SQL in Programs: 3. Call level interface Interface in standard programming languages Proprietary library routines, API Embedded C / Java /.. Standardized language extensions Standardized API Open Database connection (ODBC), Java Database Connectivity (JDBC) 27 SQL in Programs: 3. Call level interface Language/DBS specific library of procedures Example: MySQL C API Buffer for transferring commands and results API data types, e.g., MYSQL handle for db connections MYSQL_RES result set structure API functions, e.g., mysql_real_query() mysql_real_query(mysql *mysql, const char *query, unsigned int querylength) 28 14

15 SQL in Programs: Embedded SQL Direct SQL: SQL interpreter accepts and executes SQL commands SQL in host language: Program in programming language (C, Java, ) Parts of program in SQL statements Most implementations: call level interface used Most popular: Embedded C (Oracle: PRO*C) Java support SQLJ = Embedded Java JDBC = Standardized call interface for Java 29 SQL in Programs: Embedded SQL Program with "native" and SQL-like statements Pre-compiler = Preprocessor creates native code Calls to DBS resources included Programmer: embedded SQL or function calls Host language + Embedded SQL SQL library Preprocessor Host language + Function Calls Host language compiler Object-code program 30 15

16 SQL in Programs: Static/dynamic embedding Static embedding: SQL commands known in advance SQL-compilation and language binding at pre-compile time Dynamic SQL: SQL-String compiled at runtime variable bindings at runtime 31 SQL in Programs: Embedded SQL Concepts: Well defined type mapping (for different languages) Syntax for embedded SQL statements EXEC SQL {SELECT title FROM...} Binding to host language variables EXEC SQL {SELECT id FROM Movie WHERE titel = :titlestring};... Exception handling WHENEVER <condition> <action> SQLSTATE 32 16

17 SQL in Programs: Embedded SQL SQL / Host Language Interface: Embedded SQL-statement: EXEC SQL <sql statement> Shared variables: :<variablename> (access in SQL) <variablename> (access in host language) Exception handling: SQLSTATE (SQL function execution status) e.g., no problem answer tuple not found 33 SQL in Programs: Embedded SQL Shared variable declaration Syntax: EXEC SQL BEGIN DECLARE SECTION; EXEC SQL END DECLARE SECTION; Declaration in host language Use variable types in common Example: EXEC SQL BEGIN DECLARE SECTION; integer movie_number; integer tape_number; EXEC SQL END DECLARE SECTION; 34 17

18 SQL in Programs: Embedded SQL Single row results: direct insert into variable Syntax: EXEC SQL SELECT <attributename> INTO :<sharedvariable> FROM <tablenames> WHERE <condition> Multiple row results: Use of cursors on result set 35 SQL in Programs: Cursor concept Important concept Cursor: Name of SQL statement and Handle for processing the result set record by record Defined at runtime Opened at runtime (SQL-statement executed) Used in most language embeddings of SQL e.g., ESQL-C, PL/SQL, JDBC 36 18

19 SQL in Programs: Cursor concept No binding of result attributes to variables Allows traversal of result set row by row 1. Cursor declaration 2. Cursor initialisation 3. Fetch tuples 4. Close cursor no DECLARE yes OPEN FETCH EMPTY? CLOSE 37 SQL in Programs: Cursor concept Cursor declaration: EXEC SQL DECLARE <cursorname> CURSOR FOR <query> Cursor initialisation: EXEC SQL OPEN <cursorname>; binds input variables executes query puts first results into communication area positions cursor before first row of the result set 38 19

20 SQL in Programs: Cursor concept Fetch tuples: EXEC SQL FETCH <cursorname> INTO :<shared variable>; Puts next results into communication area Positions cursor before before next row of the result set Assigns tuple to shared variables Sets SQLSTATE 39 SQL in Programs: Example #include <stdio.h> /* /* declare host variables */ */ EXEC SQL SQL BEGIN DECLARE SECTION; char userid[12] = "ABEL/xyz"; char movie_name[10]; int int movie_number; int int tape_number; char temp[32]; void sql_error(); EXEC SQL SQL END END DECLARE SECTION; /* /* include the the SQL SQL Communication Area */ */ #include <sqlca.h> 40 20

21 SQL in Programs: Example /* /* main program */ */ main() { movie_number = 200; /* /* handle errors */ */ EXEC SQL SQL WHENEVER SQLERROR do do sql_error("oracle error"); /* /* connect to to Oracle */ */ EXEC SQL SQL CONNECT :userid; printf("connected.\n"); 41 SQL in Programs: Example /* /* declare a cursor */ */ EXEC SQL SQL DECLARE movie_cursor CURSOR FOR FOR SELECT m.title FROM movie m, m, tape t WHERE t.id = :tape_number AND AND t.movie_id = m.id; /* /* get get user data */ */ printf( Tape number? "); "); gets(temp); tape_number = atoi(temp); 42 21

22 SQL in Programs: Example /* /* open the the cursor and and identify the the result set set */ */ EXEC SQL SQL OPEN movie_cursor; /* /* fetch and and process data in in a loop exit when no no more data */ */ EXEC SQL SQL WHENEVER NOT NOT FOUND DO DO break; while (1){ EXEC SQL SQL FETCH movie_cursor INTO :movie_name; } 43 SQL in Programs: Example /* /* close cursor before another SQL SQL statement is is executed */ */ EXEC SQL SQL CLOSE movie_cursor; } EXEC SQL SQL COMMIT WORK RELEASE; exit(0); 44 22

23 SQL in Programs: Positioned Update Step through set of rows and update or delete Syntax: Example: EXEC SQL DECLARE <cursorname> CURSOR FOR <query> FOR UPDATE ON <attribute>; WHERE CURRENT OF <cursorname> EXEC SQL DECLARE mycurs CURSOR FOR SELECT id,length,title FROM Movie FOR UPDATE ON length EXEC SQL UPDATE Movie SET lenght = length + 1 WHERE CURRENT OF mycurs; 45 SQL in Programs: Cursor Options Ordering tuples Use ORDER BY in query Cursor motion SCROLL CURSOR Relative to current position: PRIOR/NEXT/RELATIVE<nr> e.g., FETCH <cursorname> PRIOR INTO... Absolute position: first/last/absolute<nr> Limit effect of changes Performance: cursor FOR READ ONLY Concurrent access: INSENSITIVE CURSOR FOR 46 23

24 SQL in Programs: Cursor sensitivity Example: EXEC SQL DECLARE mycurs INSENSITIVE CURSOR FOR SELECT id,length,title FROM Movie FOR UPDATE ON length WHERE id >100; EXEC SQL OPEN... EXEC SQL FETCH mycurs INTO... UPDATE Movie SET lenght = length + 20 WHERE CURRENT OF mycurs; Changes not visible in result set Visible if cursor closed and reopened 47 SQL in Programs: Dynamic SQL Statements not known at compile time Statements computed by host language User input of query Tasks at run-time: Pass query string to SQL system Translate to executable statement Execute statement Use Prepared Statements 48 24

25 SQL in Programs: Dynamic SQL Step 1: EXEC SQL PREPARE <SQLvariable> FROM <string> String: SQL statement SQLvariable: assigned SQL statement Parse and prepare statement for execution Step 2: EXEC SQL EXECUTE <SQLvariable> Execute statement SQLvariable 49 SQL in Programs: Dynamic SQL Example: void readquery(){ EXEC SQL SQL BEGIN DECLARE SECTION; char *query; EXEC SQL SQL END END DECLARE SECTION; /* /* prompt user for for query allocate space make :query point to to query*/ EXEC SQL SQL PREPARE SQLquery FROM :query; EXEC SQL SQL EXECUTE SQLquery; } 50 25

26 SQL in Programs: Dynamic SQL Multiple execution: Prepare once Execute many times Single execution: Combination of step 1 an 2 EXEC SQL EXECUTE IMMEDIATE <string> Example: EXEC SQL SQL EXECUTE IMMEDIATE :query; 51 SQL in Programs: SQL & Java SQLJ Embedded SQL for Java Compiles to JDBC method call Defined and implemented by major DBS companies (Oracle in particular) JDBC Java call-level interface (API) for SQL DBS DB vendor independent Supports static and dynamic SQL Implemented by nearly all DB vendors 52 26

27 SQL in Programs: SQLJ Part 1: SQLJ Embedded SQL Mostly reviewed and implemented Integrated with JDBC API Oracle has placed Translator source into public domain Part 2: SQLJ Stored Procedures and UDFs Using Java static methods as SQL stored procedures & functions Leverages JDBC API Part 3: SQLJ Data Types Pure Java Classes as SQL ADTs Alternative to SQL:1999 Abstract Data Types 53 SQL in Programs: SQLJ Example // // Part of of a SQLJ program, one one method: public void changemovie(int movieid, int int newtape) { string mtitle; int int tnumber; #sql { SELECT m.title, count(t.id) INTO :mtitle, :tnumber FROM movie m, m, tape t WHERE m.id = :movieid AND AND m.id = t.movie_id }; }; if if (tnumber < 3) 3) #sql {INSERT INTO tape VALUES (:newtape, 'DVD', :movieid)}; } 54 27

28 SQL in Programs: SQL & Java Java in Web context (2 tier architecture): Java application Business Logic (application) JDBC Proprietary protocol of DBMS DBMS Database Server 55 SQL in Programs: SQL & Java Java in Web context (3 tier architecture): Java applet or WWW Browser GUI HTTP, RMI, CORBA, Application server Business Logic (application) JDBC DBMS Proprietary protocol of DBMS Database Server 56 28

29 SQL in Programs: JDBC 1. Preparation import java.sql.*; 2. Load a driver many vendor products Class.forName( "oracle.jdbc.driver.oracledriver"); String url = url JDBC-Driver and host information 57 SQL in Programs: JDBC 3. Set up connection database(s) Connection con = DriverManager.getConnection( "jdbc:oracle:thin:@<host>:<port>:<db>", <username>,<password>); Several connections at a time possible 4. Create statement object Statement stmt = con.createstatement(); Similar to channel for sending queries to database 58 29

30 SQL in Programs: JDBC 5. Send SQL query string ResultSet rs = stmt.executequery( <query>" ); results in ResultSet object 6. Process results one after the other processed with "hidden cursor" while (rs.next()){ for (i = 1; i <= numcols; i++){ if (i > 1) System.out.print(","); System.out.print(rs.getString(i)); } } 59 SQL in Programs: JDBC Example #import java.io.*; #import java.sql.*; #import java.util.*; Preparation 2. Load driver 3. Connect to database Class.forName("oracle.jdbc.driver.OracleDriver"); String url = "jdbc:oracle:thin:@kuh:1521:introkuh"; Protocol Sub-protocol Oracle-spec. Host Port Connection con = DriverManager.getConnection ( url, user", passwort"); 60 30

31 SQL in Programs: JDBC Example. 4. Create SQL-statement.. 5. execute statement. Statement stmt = con.createstatement(); ResultSet rs = stmt.executequery( "SELECT id, title FROM movie"); 6. Process results while (rs.next()) { String n = rs.getint( id"); String n = rs.getstring( title"); System.out.println(s + ": " + n); } } 61 SQL in Programs: JDBC variable binding No explicit cursor Several methods in JDBC e.g., boolean next(), void close(), <JavaType> get<javatype>(), boolean wasnull() Access result data by position or by name By position: By name: String s = rs.getstring(2); String rs.getstring ("b") ; 62 31

32 SQL in Programs: JDBC variable binding Example: java.sql.statement stmt = con.createstatement(); ResultSet rs1 rs1 = stmt.executequery ("SELECT id, id, title FROM movie"); while (rs1.next()) { int int mid mid = rs1.getint( id"); String mt mt = rs1.getstring( title"); System.out.println("ROW:" + mid mid + " " + mt);} ResultSet rs2 rs2 = stmt.executequery ("SELECT id, id, movie_id FROM tape"); while (rs2.next()) { int int tid tid = rs2.getint(1); int int tmid = rs2.getint(2); System.out.println("ROW:" + tid tid + " " + tmid);} 63 SQL in Programs: Prepared statements Pass input parameters Use prepared statement java.sql.preparedstatement prepstmt = con.preparestatement(<query>); Statement compiled Missing values in query:? Set value: prepstmt.setstring(<position>, <value>); 64 32

33 SQL in Programs: Prepared statements String mtitle;... java.sql.preparedstatement prepstmt = con.preparestatement( "SELECT count(*) FROM Movie m, m, Tape t WHERE t.movie_id = m.id AND AND m.title =? ); ); prepstmt.setstring(1, mtitle); ResultSet rs rs = prepstmt.executequery() ; while (rs.next()){ int int i = r.getint(1); // // by by position, no no name available System.out.println("Number of of tapes for for " + mtitle + " is: is: " +i) +i) } 65 SQL in Programs: Positioned update Positioned update needs cursor name Define cursor (JDBC 1) public void setcursorname(string name) throws SQLException Use for updates and deletes Define cursor (JDBC2) more flexible (anonymous) cursor handling setcursorname not implemented in Oracle Driver 66 33

34 SQL in Programs: Positioned update JDBC 2.0 Result set scrollable and updateable Example: Statement stmt = con.createstatement( ResultSet.TYPE_SCROLL_SENSITIVE, ResultSet.CONCUR_UPDATABLE); stmt.setfetchsize(25); ResultSet rs = stmt.executequery( "SELECT id, title FROM movie"); rs.first(); rs.updatestring( title, xxxxx ); rs.updaterow(); 67 SQL in Programs: Transactions Transaction: Collection of one or more database operations executed atomically (either all operations or none ) Programmers view: Everything between beginning of a sequence of operations on the database and COMMIT or ROLLBACK No explicit "transaction begin" command... OPEN MyCurs;... ; COMMIT; Begin of first transaction (first SQL command in program) End of first transaction 68 34

35 SQL in Programs: Transactions COMMIT Effects on database made permanent ROLLBACK Aborts transaction All changes in transaction undone (rolled back) Programmers View: Auto-commit mode: each SQL-command is a transaction Various transaction isolation levels 69 SQL in Programs: Transactions Transaction manager: Isolate concurrent users from each other Problems: Lost update: same object concurrently updated by two users, one update lost Dirty read: object value changed by transaction which aborts later Non-repeatable read: same object has different value within same transaction Phantom tuples: non-repeatable read caused by insertions or deletions 70 35

36 SQL in Programs: Isolation levels READ UNCOMMITTED SET TRANSACTION READ ONLY, ISOLATION LEVEL READ UNCOMMITTED Allows read access to uncommitted transactions Transaction has to be read only Lowest locking overhead Unpleasant effects may occur Example: TA1 increases the prices of some movies in DB by 5% TA2 scrolls through all movies, sees new prices 71 SQL in Programs: Isolation levels READ COMMITTED SET TRANSACTION ISOLATION LEVEL READ COMMITTED Allows read access to committed transactions only Long write locks, no or short read locks Non-repeatable reads Example: TA1 Read(a) x=x+a Read(a) y:=y-a TA2 Wrong balance Write a=a-10 commit 72 36

37 SQL in Programs: Isolation levels REPEATABLE READ SET TRANSACTION ISOLATION LEVEL REPEATABLE READ Allows read access to committed transactions only All data isolated from concurrent writes Read and write locks long term until end of TA Phantom tuples may occur 73 SQL in Programs: Isolation levels SERIALIZABLE SET TRANSACTION ISOLATION LEVEL SERIALIZABLE Allows read access to committed transactions only All data isolated from concurrent writes No phantom tuples inserted into the read set by other transaction Standard default 74 37

38 SQL in Programs: Transactions and JDBC Transactional properties of connections TRANSACTION_NONE (not implemented) TRANSACTION_READ_UNCOMMITTED TRANSACTION_READ_COMMITTED TRANSACTION_REPEATABLE_READ TRANSACTION_SERIALIZABLE Methods: public void settransactionisolation(int level) throws SQLExceptionpublic void setautocommit(boolean autocommit) public void commit() throws SQLException public void rollback() throws SQLException 75 SQL in Programs: Visibility of changes ResultSet rs = stmt1.executequery( "SELECT id, length FROM movie"); int i = stmt2.executeupdate ( DELETE FROM movie ); rs.first(); rs.updatestring( title, xxxxx ); rs.updaterow(); Scroll-insensitive result set no change by other result sets even in the same TA are visible Updates in result set r visible for operations on r Deletes / inserts (!) in result set r not visible Sensitive result set: depending on connection isolation level 76 38

39 SQL in Programs: Exception handling Abort transaction when error: EXEC SQL WHENEVER SQLERROR CONTINUE; EXEC SQL ROLLBACK WORK RELEASE; WHENEVER SQLERROR CONTINUE prevents ROLLBACK from (infinite) invocation of routine Example: void sql_error(msg){ char buf[500]; int int buflen, msglen; EXEC SQL SQL WHENEVER SQLERROR CONTINUE; EXEC SQL SQL ROLLBACK WORK RELEASE; buflen = sizeof (buf); sqlglm(buf, &buflen, &msglen); printf("%s\n", msg); printf("%*.s\n", msglen, buf); exit(1); } 77 SQL in Programs: Summary Access Rights Means to ensure data security ileges to roles Program DB communication: Fourth Generation Languages (4GL) Module Languages Call level interface Component architectures Transactions in programs Isolation levels Begin, end transaction 78 39