M.P. Johnson, DBMS, Stern/NYU, Sp20041 C : Database Management Systems Lecture #15 Matthew P. Johnson Stern School of Business, NYU Spring, 2004

M.P. Johnson, DBMS, Stern/NYU, Sp Agenda Last time: Views, Constraints, Triggers This time: Begin programming for SQL  Embedded SQL Pro*C, SQLJ  PSMs  CLI SQL/CLI in C JDBC in Java DBI/DBDs in Perl Future:  Midterm on Thursday

M.P. Johnson, DBMS, Stern/NYU, Sp Future Next: PSMs  (Re)read 8.2 Next Thursday: Midterm  Practice SQL on sqlzoo Responsible (in this order) for 1. all material from class, 2. all material assigned reading Feedback is very important! Please fill out midterm course evals before MT  Else lose 30 points on MT  Details TBA by over week

M.P. Johnson, DBMS, Stern/NYU, Sp New topic: SQL Programming (8.1) Can write SQL queries in a SQL interpreter  Command prompt  SQL*Plus (sqlplus) in Oracle  mysql in MySQL Good for experimenting, not for anything non-trivial Better: use a standard programming language  Host language talks to SQL/DB

M.P. Johnson, DBMS, Stern/NYU, Sp Using two languages Q: Why not use just one language? “[W]e can quickly dispense with the idea…” (p351) Q: Why not do everything in the host lang.? A: What SQL provides is highly non-trivial  Query interpretation, optimizing  Queries stay constant across host languages Q: Why not do everything in SQL? A: Not designed as a general-purpose language  No recursion (no factorial!)  No, e.g., Swing library Germ of OO: modularize

M.P. Johnson, DBMS, Stern/NYU, Sp Impedance mismatch problem Big problem, though: impedance mismatch  Data structures in our app-programming lang. don’t automatically map onto those in SQL Different types/representations for data In SQL: tables with scalar fields In C: scalars, records (containing records…), pointers, arrays In Java: scalars, objects, references, arrays In Perl: scalars, lists/arrays, hashes

M.P. Johnson, DBMS, Stern/NYU, Sp SQL/host interface in embedded SQL So Q: how to transfer data between? A: Shared variables  Some vars in are program can be used by SQL  Prefix var with a :  After query, look here for received data SQL commands embedded in app. Code  Identified by EXEC SQL Source code is preprocessed before regular compilation  Result is a C (e.g.) program with library calls

M.P. Johnson, DBMS, Stern/NYU, Sp Programs with Embedded SQL Host language + Embedded SQL Preprocessor Host Language + function calls Host language compiler Executable Preprocessor Host language compiler Oracle’s Pro*C gcc prog.pc prog.c a.out

M.P. Johnson, DBMS, Stern/NYU, Sp Interface: SQL / Host Language Values get passed through shared variables.  Colons precede shared variables in SQL statements  EXEC SQL demarcates every SQL statement The variable SQLSTATE provides error messages and status reports  “00000” ~ success  “02000” ~ tuple not found Used in loops EXEC SQL BEGIN DECLARE SECTION; char productName[30]; char SQLSTATE[6]; EXEC SQL END DECLARE SECTION; EXEC SQL BEGIN DECLARE SECTION; char productName[30]; char SQLSTATE[6]; EXEC SQL END DECLARE SECTION;

M.P. Johnson, DBMS, Stern/NYU, Sp Embedded SQL example Context:  Product (pname, price, quantity, maker)  Purchase (buyer, seller, store, pname)  Company (cname, city)  Person(name, phone, city) Goal 1: Insert a new row in Purchase Goal 2: Look up price of product by name

M.P. Johnson, DBMS, Stern/NYU, Sp Embedded SQL example: insert void simpleInsert() { EXEC SQL BEGIN DECLARE SECTION; char n[20], c[30]; /* product-name, company-name */ int p, q; /* price, quantity */ char SQLSTATE[6]; EXEC SQL END DECLARE SECTION; /* get values for name, price and company somehow */ EXEC SQL INSERT INTO Product(pname, price, quantity, maker) VALUES (:n, :p, :q, :c); } void simpleInsert() { EXEC SQL BEGIN DECLARE SECTION; char n[20], c[30]; /* product-name, company-name */ int p, q; /* price, quantity */ char SQLSTATE[6]; EXEC SQL END DECLARE SECTION; /* get values for name, price and company somehow */ EXEC SQL INSERT INTO Product(pname, price, quantity, maker) VALUES (:n, :p, :q, :c); }

M.P. Johnson, DBMS, Stern/NYU, Sp Embedded SQL example: look-up int getWindowsPrice() { EXEC SQL BEGIN DECLARE SECTION; int p; char SQLSTATE[6]; EXEC SQL END DECLARE SECTION; EXEC SQLSELECT price INTO :p FROM Product WHERE Product.name = ‘Windows’; return p; } int getWindowsPrice() { EXEC SQL BEGIN DECLARE SECTION; int p; char SQLSTATE[6]; EXEC SQL END DECLARE SECTION; EXEC SQLSELECT price INTO :p FROM Product WHERE Product.name = ‘Windows’; return p; }

M.P. Johnson, DBMS, Stern/NYU, Sp Embedded SQL example: look-up What about search for arbitrary product? Q: Will this work? int getPrice(char *name) { EXEC SQL BEGIN DECLARE SECTION; int p; char SQLSTATE[6]; EXEC SQL END DECLARE SECTION; EXEC SQLSELECT price INTO :p FROM Product WHERE Product.name = :name; return p; } int getPrice(char *name) { EXEC SQL BEGIN DECLARE SECTION; int p; char SQLSTATE[6]; EXEC SQL END DECLARE SECTION; EXEC SQLSELECT price INTO :p FROM Product WHERE Product.name = :name; return p; }

M.P. Johnson, DBMS, Stern/NYU, Sp Embedded SQL example: look-up int getPrice(char *name) { EXEC SQL BEGIN DECLARE SECTION; char n[20]; int p; char SQLSTATE[6]; EXEC SQL END DECLARE SECTION; strcpy(n, name); /* copy name to local var */ EXEC SQLSELECT price INTO :p FROM Product WHERE Product.name = :n; return p; } int getPrice(char *name) { EXEC SQL BEGIN DECLARE SECTION; char n[20]; int p; char SQLSTATE[6]; EXEC SQL END DECLARE SECTION; strcpy(n, name); /* copy name to local var */ EXEC SQLSELECT price INTO :p FROM Product WHERE Product.name = :n; return p; }

M.P. Johnson, DBMS, Stern/NYU, Sp Cursors For product’s price, looked up single (scalar) value Q: What if we SELECT multiple fields?  E.g., find all info for some product A: Just list destination vars separated by commas Q: What if find multiple rows?  E.g., find all products above a certain price Use a cursor to step through the results  Each result placed in an array Using cursors: 1. Declare the cursor 2. Open the cursor 3. Fetch tuples one by one 4. Close the cursor

M.P. Johnson, DBMS, Stern/NYU, Sp Cursor loop structure Each time around loop, we  Do a FETCH to obtain next row  Examine SQLSTATE to check success  Can say: What is NO_MORE_TUPLES? #define NO_MORE_TUPLES !(strcmp(SQLSTATE,”02000”)) if(NO_MORE_TUPLES) break;

M.P. Johnson, DBMS, Stern/NYU, Sp Multiple-row look-up example void product2XML() { EXEC SQL BEGIN DECLARE SECTION; char n[20], c[30]; int p, q; char SQLSTATE[6]; EXEC SQL END DECLARE SECTION; EXEC SQL DECLARE crs CURSOR FOR SELECT pname, price, quantity, maker FROM Product; EXEC SQL OPEN crs;... void product2XML() { EXEC SQL BEGIN DECLARE SECTION; char n[20], c[30]; int p, q; char SQLSTATE[6]; EXEC SQL END DECLARE SECTION; EXEC SQL DECLARE crs CURSOR FOR SELECT pname, price, quantity, maker FROM Product; EXEC SQL OPEN crs;...

M.P. Johnson, DBMS, Stern/NYU, Sp Multiple look-up example printf(“ \n”); while (1) { EXEC SQL FETCH FROM crs INTO :n, :p, :q,:c; if (NO_MORE_TUPLES) break; printf(“ \n”); printf(“ %s \n”, n); printf(“ %d \n”, p); printf(“ %d \n”, q); printf(“ %s \n”, c); printf(“ \n”); } EXECT SQL CLOSE crs; printf(“ \n”); } printf(“ \n”); while (1) { EXEC SQL FETCH FROM crs INTO :n, :p, :q,:c; if (NO_MORE_TUPLES) break; printf(“ \n”); printf(“ %s \n”, n); printf(“ %d \n”, p); printf(“ %d \n”, q); printf(“ %s \n”, c); printf(“ \n”); } EXECT SQL CLOSE crs; printf(“ \n”); }

M.P. Johnson, DBMS, Stern/NYU, Sp Cursor on query not table EXEC SQL DECLARE c CURSOR FOR SELECT beer, price FROM Sells WHERE bar = 'Joe''s Bar'; EXEC SQL OPEN CURSOR c; while(1) { EXEC SQL FETCH c INTO :theBeer, :thePrice; if(NOT FOUND) break; /* format and print beer and price */ } EXEC SQL CLOSE CURSOR c; EXEC SQL DECLARE c CURSOR FOR SELECT beer, price FROM Sells WHERE bar = 'Joe''s Bar'; EXEC SQL OPEN CURSOR c; while(1) { EXEC SQL FETCH c INTO :theBeer, :thePrice; if(NOT FOUND) break; /* format and print beer and price */ } EXEC SQL CLOSE CURSOR c;

M.P. Johnson, DBMS, Stern/NYU, Sp More on Cursors Cursors can modify a relation as well as read it. Cursors can be protected against changes to the underlying relations Can determine the order in which the cursor will get tuples by the ORDER BY keyword in the SQL query The cursor can be a scrolling one: can go forward, backward  +n, -n, Abs(n), Abs(-n) Cursors can traverse both stored tables and queries

M.P. Johnson, DBMS, Stern/NYU, Sp Modifications with cursors As we traverse through result set, can modify the current row NB: In regular SQL, usually modify sets of rows (UPDATE WHERE …) With cursors, we update the last row fetched Simple example: in Product table, we decide we want  to raise (i.e., double) all our prices  Unless price < 100, in which case they’re deleted

M.P. Johnson, DBMS, Stern/NYU, Sp Modification by cursor example void doublePrices() { EXEC SQL BEGIN DECLARE SECTION; int p; char SQLSTATE[6]; EXEC SQL END DECLARE SECTION; EXEC SQL DECLARE crs CURSOR FOR SELECT price FROM Product; EXEC SQL OPEN crs;... void doublePrices() { EXEC SQL BEGIN DECLARE SECTION; int p; char SQLSTATE[6]; EXEC SQL END DECLARE SECTION; EXEC SQL DECLARE crs CURSOR FOR SELECT price FROM Product; EXEC SQL OPEN crs;...

M.P. Johnson, DBMS, Stern/NYU, Sp Modification by cursor example while (1) { EXEC SQL FETCH FROM crs INTO :p; if (NO_MORE_TUPLES) break; if (p < 100) EXEC SQL DELETE FROM Product WHERE CURRENT OF Product; else EXEC SQL UPDATE Product SET price = 2*price; WHERE CURRENT OF Product; } EXECT SQL CLOSE crs; } while (1) { EXEC SQL FETCH FROM crs INTO :p; if (NO_MORE_TUPLES) break; if (p < 100) EXEC SQL DELETE FROM Product WHERE CURRENT OF Product; else EXEC SQL UPDATE Product SET price = 2*price; WHERE CURRENT OF Product; } EXECT SQL CLOSE crs; }

M.P. Johnson, DBMS, Stern/NYU, Sp A mention of concurrent access What if the DB changes while our cursor is looping?  I.e., after we opened the cursor, but while we’re fetching Should we see the changes? Maybe, maybe not make these changes invisible by declaring insensitive Q: How is this accomplished? One crude way: delay any changes until all insensitive cursors close Good idea: indicate read-only cursors so they won’t be held up: EXEC SQL DECLARE crs INSENSITIVE CURSOR FOR SELECT price FROM Product; EXEC SQL DECLARE crs INSENSITIVE CURSOR FOR SELECT price FROM Product; EXEC SQL DECLARE crs CURSOR FOR SELECT price FROM Product; FOR READ ONLY; EXEC SQL DECLARE crs CURSOR FOR SELECT price FROM Product; FOR READ ONLY;

M.P. Johnson, DBMS, Stern/NYU, Sp Scrolling cursors Usually cursor just walks through rows 1 by 1 Other options:  NEXT (default) or PREVIOUS  FIRST or LAST  RELATIVE +/-n RELATIVE 1 ~ NEXT RELATIVE –1 ~ ?  ABSOLUTE +/-n ABSOLUTE 1 ~ FIRST (not 0!) ABSOLUTE –1 ~ LAST To use these, declare as SCROLL cursor EXEC SQL DECLARE crs SCROLL CURSOR FOR Product; EXEC SQL DECLARE crs SCROLL CURSOR FOR Product;

M.P. Johnson, DBMS, Stern/NYU, Sp Pro*C on sales Example script: sample1.pc  See Pro*C tutorialPro*C tutorial Pro*C compiler is proc  Must include /oracle/precomp/public  Must link with shared library /oracle/lib/libclntsh.so  Includes makefile proc.mk, but may require modifications sales% cp /oracle/precomp/demo/proc/sample1.pc. sales% proc sample1.pc sales% gcc -osample1 -I/oracle/precomp/public /oracle/lib/libclntsh.so sample1.c sales% sample1 sales% cp /oracle/precomp/demo/proc/sample1.pc. sales% proc sample1.pc sales% gcc -osample1 -I/oracle/precomp/public /oracle/lib/libclntsh.so sample1.c sales% sample1

M.P. Johnson, DBMS, Stern/NYU, Sp Indeterminate-speed review Look at integrity violations in Go over some R.A./SQL questions from hw…  SQL questions located at qlquestions.txt qlquestions.txt  Some “solutions” available at olns.txt olns.txt  Test by running at