CS 235: Introduction to Databases Queries in PSM Cursors Fetching

Queries in PSM • The following rules apply to the use of CS 235: queries: 1. Queries returning a single value can be Introduction to Databases used in assignments 2. Queries returning a single tuple can be used Svetlozar Nestorov with INTO. 3. Queries returning several tuples can be Lecture Notes #15 used via a cursor. Cursors Fetching Tuples From a Cursor • A cursor serves as a tuple-variable that • Get next tuple: ranges over the tuples of the result of a FETCH c INTO a1, a2, …, ak; query. – a1, a2, …, ak are the attributes of the result of DECLARE c CURSOR FOR (<query>); the query of c. • Opening a cursor evaluates <query>. – c is moved to the next tuple. OPEN c; • A cursor is used by creating a loop around • Closed with CLOSE c; FETCH. End of Cursor Cursor Structure • SQL operations return status in DECLARE c CURSOR FOR… SQLSTATE (in PSM). … • FETCH returns ‘02000’ in SQLSTATE cursorLoop: LOOP … when no more tuples are found. FETCH c INTO…; • Useful declaration: IF NotFound THEN LEAVE cursorLoop; DECLARE NotFound CONDITION FOR END IF; SQLSTATE ‘02000’ … END LOOP; 1 Cursor Example Example • Write a procedure that makes free all beers BEGIN OPEN c; sold for more than $5 at Spoon. menuLoop: LOOP CREATE PROCEDURE FreeBeer() FETCH c INTO aBeer, aPrice; IF NotFound THEN LEAVE menuLoop END IF; DECLARE aBeer VARCHAR[30]; IF aPrice > 5.00 THEN DECLARE aPrice REAL; UPDATE Sells DECLARE NotFound CONDITION FOR SET price = 0 WHERE bar = ‘Spoon’ and beer = aBeer; SQLSTATE ‘02000’; END IF; DECLARE CURSOR c FOR END LOOP; SELECT beer, price FROM Sells WHERE bar = CLOSE c; ‘Spoon’; END; MySQL Routines Procedures • MySQL’s version of PSM (Persistent, CREATE PROCEDURE <name>(<arglist>) Stored Modules). BEGIN – Stored procedures. <declarations> – Functions. <statements> • Brand new feature (in 5.0). END; – Adheres to standards (similar to IBM’s DB2, different from Oracle PL/SQL). – Bugs possible (bugs.mysql.com) Functions Arguments CREATE PROCEDURE <name>(<arglist>) • Argument list has name-mode-type triples. RETURNS <type> – Mode: IN, OUT, or INOUT for read-only, write- BEGIN only, read/write, respectively. <declarations> – Types: standard SQL. <statements> END; 2 Example Declarations • A procedure to add a beer and price to Spoon’s menu: • Variables DELIMITER // CREATE PROCEDURE addSpoonMenu( • Conditions IN b CHAR(20), •Cursors IN p REAL) BEGIN • Handlers INSERT INTO Sells • Must be declared in this order! VALUES(‘Spoon', b, p); END;// DELIMITER ; CALL addSpoonMenu(‘Guinness’, 7.50); Conditions Handlers DECLARE <condName> • Define what to do in case of errors (or conditions) CONDITION FOR SQLSTATE <errorStr> DECLARE { EXIT | CONTINUE } DECLARE <condName> HANDLER FOR CONDITION FOR <errorNumber> {<errorNum> | SQLSTATE <errorStr> | <condName> } • The following conditions are predefined: SQL statement – NOT FOUND (no more rows) – SQLEXCEPTION (error) • Common practice: set a flag for CONTINUE handlers and – SQLWARNING (warning) check inside stored procedure. Body Constructs Queries in Routines • Assignments: 1. Single-row selects allow retrieval into a SET<variable> = <expression> – Variables must be declared. variable of the result of a query that is • Branches guaranteed to produce one tuple. IF <condition> THEN <statement(s)> 2. Cursors allow the retrieval of many ELSE tuples, with the cursor and a loop used to <statement(s)> END IF; process each in turn. 3 Cursors in MySQL Example (1/3) • The cursor declaration is: • The FreeBeer in MySQL: DECLARE <curName> CURSOR FOR <query>; CREATE PROCEDURE FreeBeer() BEGIN • Fetching is done with: DECLARE aBeer CHAR(20); FETCH c INTO <variables>; DECLARE aPrice REAL; DECLARE flag INT DEFAULT 0; Example (2/3) Example (3/3) DECLARE menu CURSOR FOR SELECT beer, price OPEN menu; FROM Sells REPEAT FETCH menu INTO aBeer, aPrice; WHERE bar = ‘Spoon’; IF aPrice > 5.00 THEN DECLARE CONTINUE HANDLER UPDATE Sells SET price = 0 FOR NOT FOUND WHERE bar = ‘Spoon’ AND beer = aBeer; SET flag = 1; END IF; UNTIL flag = 1 END REPEAT; CLOSE menu; END;// 4.

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