course: Database Applications (NDBI026) WS2015/16 RNDr. Michal Kopecký, Ph.D. Department of Software Engineering, Faculty of Mathematics and Physics, Charles University in Prague
Views Creating views Using Views Procedural Extensions Motivation Procedures Functions M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 2
View ~ named SELECT statement Represents a virtual table When querying the view the query is computed every time Similar to macro definition in C programming language M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 4
By statement CREATE VIEW view_name [col_name [, ]] AS SELECT [WITH [{CASCADE LOCAL} CHECK OPTION] M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 5
Example: CREATE VIEW Prague_Citizen AS SELECT * FROM Citizen WHERE City= Prague ; M. Kopecký Views and Procedural DBI026 -DB Extension Aplikace (NDBI026, - MFF Lect. UK 3) 6
Columns can be named explicitly If they are not, they inherit names from the SELECT statement (where all expressions must be named in this case) WITH CHECK OPTION ensures, that all inserted and/or updated rows become/remain visible through the view If this condition should be violated, the actualization is rejected M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 7
Example: CREATE VIEW Prague_Citizen AS SELECT * FROM Citizen WHERE City= Prague WITH CHECK OPTION; -- wrong actualization UPDATE Prague_Citizen SET City= Pilzen WHERE ID=7510151234; M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 8
Usage of views Predefined (and hidden to the user) join of more tables (split by normalization etc.) Individualization of visible data for different users Different users can see different rows in the same view according to their definition in the database. Different users can have defined completely different view in their default schema and so they can see even different structure (different columns) M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 9
Example: CREATE VIEW Emp_My_Dept AS SELECT E.Nr, E.Surname, E.Name, E.Job FROM Employee AS E, Employee AS X WHERE X.Login=CURRENT_USER AND E.DeptNr=X.DeptNr; M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 10
Some applications can have problems when the order of columns in the view changes (due binding variables to columns by their position instead of by name) can bring problems When creating vies it is advisable to assure, that future changes in table structures have minimal impact on the view structure Explicitly mention all needed columns in the SELECT statement It is useful to qualify all columns in SELECT statement by name/alias of given source (table/view) Explicitly name view columns M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 11
How to prevent shift of columns in views, that use the * symbol to show all columns? In the view on more than one table it is appropriate use * at most once, for columns of one table only, using syntax tab_name.* on the end of the column list in the SELECT clause. No further columns should be named afterwards. M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 12
Example: CREATE VIEW Emp_Dept AS SELECT E.EmpNr, E.Name, D.* FROM Employee AS E NATURAL RIGHT OUTER JOIN Department AS D; M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 13
It is not recommendable to use ORDER BY clause in the view definition The ORDER BY clause is not necessary every time Sometimes other ordering is required Often more views is joined together in the SELECT statement / other view and the ordering can cause the delays M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 14
When creating views, it is advisable to check the query by the Query optimizer, if the query can be and is optimized well It is not good idea use views containing GROUP BY in another queries and views. Additional conditions can cause complications to optimizer M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 15
Example: CREATE VIEW Citizen_Counts AS SELECT City, COUNT(*) AS Size FROM Citizen GROUP BY City; SELECT * FROM Citizen_Counts WHERE City= Pilzen ; Optimizer gets SELECT * FROM ( SELECT City, COUNT(*) AS Size FROM Citizen GROUP BY City ) ASCitizen_Counts WHERE City= Pilzen ; Should change it to SELECT City, COUNT(*) AS Size FROM Citizen WHERE City= Pilzen GROUP BY City; M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 16
By statement DROP VIEW view_name; M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 17
CREATE VIEW vw_name [(column [, ])] AS SELECT [WITH {CHECK OPTION READ ONLY}]; List of columns in brackets WITH READ ONLY disallows any actualization through the view CREATE VIEW vw_name [(column [, ])] [WITH ENCRYPTION] AS SELECT [WITH CHECK OPION]; M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 18
INFORMATION_SCHEMA. VIEWS USER_VIEWS USER_TAB_COLUMNS M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 19
In standard SQL the server obtains and evaluates individual SQL statements. In given example 2n packets is sent to the server and 2n packets is sent back. n x for (;;) { FETCH; if ( ) break; if ( ) INSERT ; else UPDATE ; }; 1 1 1 1 1 1 M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 21
Using procedural extension it is possible to send complete piece of code including the application logic and process it on the server In given example only one packed is sent in both ways. LOOP FETCH; EXIT WHEN ( ); IF ( ) THEN INSERT ; ELSE UPDATE ; END LOOP; 1 1 M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 22
Savings of the communication channel Less number of send/received packets One packet can contain more than one statement Substantially less amount of data send over the network Data can be processed locally on the server without transferring them to the client Thinner and less expensive client can be used The code can be stored on the server The code can be shared by more/all client applications M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 23
Extension of the server capability by other procedures and functions Extended data protection Triggers Allowed data manipulation can be encapsulated in procedures The application need not have privileges to manipulate data directly M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 24
Significantly less portability than the table (relational schema) definition and DML language usage (INSERT, UPDATE, DELETE) Standardized in ANSI SQL-99 Lot of proprietary non-compatible solutions M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 25
Blocks of code with fixed structure (based on PL/1) Statements have to be terminated by semicolons [DECLARE declarations] BEGIN statements [EXCEPTION exception handling] END; Sequence of statements (based on C/C#) Statements need not to be terminated by semicolons DECLARE sections are understood as statements BEGIN END as well M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 26
DECLARE @ratio FLOAT SELECT @ratio = numerator/denominator FROM Fractions WHERE numerator=123 IF @ratio > 0 UPDATE Fractions SET Result = @ratio WHERE numerator=123 DECLARE ratio NUMBER; BEGIN SELECT numerator/denominator INTO ratio FROM Fractions WHERE numerator=123; IF ratio > 0 THEN UPDATE Fractions SET Result=ratio WHERE numerator=123; END IF; END; M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 27
name [CONSTANT] type [:= expr]; Basic types Standard types BOOLEAN containing TRUE, FALSE, NULL @name [AS] type Basic types Standard types M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 28
Explicitly by name (as in table column definition) X NUMERIC(7,2); By copying from another variable Y X%Type; By copying from table column type E EMP.Ename%Type; By copying from table row type R EMP%RowType; Record, containing all fields, corresponding to columns Access using dot notation M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 29
Empty statement NULL; Assignment var := expr; SQL statement UPDATE Emp SET Sal = Sal*1.05; DELETE FROM Emp WHERE EmpNo=1; Assignment SET @var = expr SQL příkaz UPDATE Emp SET Sal = Sal*1.05; DELETE FROM Emp WHERE EmpNo=1 M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 30
SELECT, returning exactly one row SELECT expr 1 [, ] INTO var 1 [, ] FROM WHERE ; SELECT, returning exactly one row SELECT @var 1 = expr 1 [, ] FROM WHERE ; M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 31
CREATE [OR REPLACE] PROCEDURE procname [(param_declaration [, ])] {AS IS} block_without_keyword_declare; Parameter declaration name [IN] [OUT] type [{:= DEFAULT} expression] IN parameter can be read inside the procedure body OUT parameter can be set inside the procedure body Procedure call from the SQL console (in PL/SQL simply by name) EXEC procname[(expression [, ])] Parameters are assigned by their order EXEC procname[(param_name=>expression [, ]) Parameters are assigned by name M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 32
CREATE PROCEDURE UpdEmpSal ( xempno NUMBER, xsal NUMBER, xcomm NUMBER DEFAULT NULL ) AS The size of parameter types is not declared BEGIN UPDATE Emp SET Sal = xsal, Comm = xcomm WHERE EmpNo=xEmpNo; END; / EXEC UpdEmpSal(1234,25500) -- without commissions; EXEC UpdEmpSal(4321,24000,1500); M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 33
{CREATE ALTER} PROCEDURE procname [; number] [param_declaration [, ]] [WITH RECOMPILE] AS statements [;] Parameter declaration @name type [= expression] [OUT[PUT]] OUT[PUT] - parameter is an output parameter number allows creations of more versions of the same procedure Procedure call EXEC[UTE] procname [expression [, ]] Parameters are assigned by their order EXEC[UTE] jmproc [@name=expression [, ]] Parameters are assigned by name M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 34
CREATE PROCEDURE UpdEmpSal @xempno INTEGER, @xsal INTEGER, @xcomm INTEGER = NULL AS UPDATE Emp SET Sal = @xsal, Comm = @xcomm WHERE EmpNo=@xEmpNo; EXEC UpdEmpSal @xempno = 1234, @xsal = 25500; GO EXEC UpdEmpSal 4321, 24000, 1500; GO M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 35
CREATE [OR REPLACE] FUNCTION func_name [(param_declaration [, ])] RETURN type {AS IS} block_without_keyword_declare; Return of the result RETURN expression; Ends the function execution Function has to return some value, even in case of NULL value. If not, an exception is thrown. Function execution the same as in case of any standard function SELECT func_name[(expression [, ])] FROM ; Parameters are assigned by their order SELECT func_name[(name=>expression [, ])] FROM ; Parameters are assigned by name M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 36
CREATE FUNCTION Polynomial ( x NUMBER, a NUMBER DEFAULT 0, b NUMBER DEFAULT 0, c NUMBER DEFAULT 0 ) RETURN NUMBER AS BEGIN RETURN (a*x+b)*x+c; END; / SELECT Polynomial(1, b=>2) FROM DUAL; -- 2*x, x=1, i.e. 2 SELECT Polynomial(2, b=>3, c=>5) FROM DUAL; -- 3*x+5, x=2, i.e. 11 SELECT Polynomial(2, 1, 1, 1) FROM DUAL; -- x^2+x+1, x=2, i.e. 7 M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 37
{CREATE ALTER} FUNCTION func_name [(param_declaration [, ])] RETURNS typ [AS] BEGIN RETURN expression; END [;] Function execution the same as in case of any standard function, but with explicit schema prefix Brackets are obligatory even if no parameters are declared SELECT sch_name.func_name([expression [, ]]) FROM ; Parameters are assigned by their order SELECT sch_name.func_name([@name=expression [, ]]) FROM ; Parameters are assigned by name M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 38
CREATE FUNCTION Polynomial ( @x FLOAT, @a FLOAT = 0, @b FLOAT = 0, @c FLOAT = 0 ) RETURNS FLOAT AS BEGIN RETURN (@a*@x+@b)*@x+@c; END; GO SELECT sch_name.polynomial(1, default, 2, default); -- 2*x, x=1, i.e. 2 SELECT sch_name.polynomial(2, default, @b=3, @c=5); -- 3*x+5, x=2, i.e. 11 SELECT sch_name.polynomial(2, 1, 1, 1); -- x^2+x+1, x=2, i.e. 7 M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 39
CREATE FUNCTION func_name [(param_declaration [, ])] RETURNS TABLE [AS] RETURN [(] SELECT [)] Can be used as a view (this time with parameters) SELECT * FROM sch_name.func_name([expression [, ]]); M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 40
CREATE FUNCTION func_name [(param_declaration [, ])] RETURNS @varname TABLE (table definition) [AS] BEGIN body that inserts data into pseudo-table @varname RETURN END [;] Can be used as a view (this time with parameters) SELECT * FROM func_name([expression [, ]]); M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 41
IF condition THEN statementsif [ELSIF condition THEN statementselsif1 [ELSIF condition THEN statementselsif2 ]] [ELSE statementselse ] END IF; Condition evaluated as NULL is considered as not met IF condition statementif blockif [ELSE statementelse blockelse] Condition evaluated as NULL is considered as not met M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 42
WHILE condition LOOP [EXIT WHEN condition] END LOOP; While-loop with optional exit in the middle WHILE condition statementwhile blockwhile BREAK exits the loop CONTINUE exits the iteration M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 43
LOOP [EXIT WHEN condition] END LOOP; General loop with exit in the middle The exit can be written also in form IF condition THEN exit; END IF; M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 44
FOR I IN 1..10 LOOP [EXIT WHEN condition] END LOOP; FOR I IN REVERSE 10..1 LOOP END LOOP; M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 45
GOTO label; <<label>> statement; The label must be followed by statement, at least by empty statement NULL; GOTO label label: M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 46
Named blocks <<label>> DECLARE BEGIN END [label]; Named loops <<label>> LOOP END LOOP [label]; Allow access to overloaded variables using qualification by block name Allow exit from more loop levels at once: EXIT label WHEN M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 47
SELECT expr [, ] INTO varname [, ] FROM WHERE ; Has to return exactly one row, otherwise exception NO_DATA_FOUND TOO_MANY_ROWS DECLARE xempno EMP.EmpNo%TYPE; xename EMP.EName%TYPE; y Emp%ROWTYPE; BEGIN SELECT EmpNo, EName INTO xempno, xename FROM EMP WHERE EmpNo=1; SELECT * INTO y FROM EMP WHERE EmpNo=2; END; M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 48
SELECT @varname=expr [, ] FROM WHERE ; Has to return exactly one row, otherwise exception DECLARE @xempno INT, @xename VARCHAR(30) SELECT @xempno = EmpNo, @xename = EName FROM EMP WHERE EmpNo=1 M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 49
Multi-row SELECT statements without INTO can be processed only using cursors Cursors have attributes, accessible by syntax cursorname%attrname IsOpen BOOLEAN Found BOOLEAN NotFound BOOLEAN RowCount NUMBER DECLARE CURSOR C IS SELECT * FROM EMP; R C%RowType; BEGIN OPEN C; LOOP FETCH C INTO R; EXIT WHEN NOT C%Found; END LOOP; CLOSE C; END; M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 50
Multi-row SELECT statements can be processed only using cursors After each FETCH the @@FETCH_STATUS should be checked 0 row was successfully fetched -1 fetch after the end of cursor -2 row is missing DECLARE C CURSOR FOR SELECT * FROM EMP; OPEN C; FETCH NEXT FROM C; WHILE @@FETCH_STATUS=0 BEGIN FETCH NEXT FROM C; END; CLOSE C; DEALLOCATE C; M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 51
More simple cursor processiong using cursor FOR loops This variable is not used Automatically declared variable visible only inside the loop DECLARE CURSOR C IS SELECT * FROM EMP; R C%RowType; BEGIN FOR R IN C LOOP END LOOP; END; M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 52
The SELECT can be used directly in FOR LOOP declaration BEGIN FOR R IN ( SELECT * FROM EMP ) LOOP END LOOP; END; M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 53
Declaration CURSOR C(J VARCHAR2, D NUMBER) IS SELECT * FROM EMP WHERE Job=J AND DeptNo=D; Usage OPEN C( Manager,10); CLOSE C; FOR R IN C( Manager,10) LOOP END LOOP; M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 54
Declaration C [SCROLL] CURSOR FOR SELECT ; Data fetching FETCH {NEXT PRIOR ABSOLUTE n RELATIVE n LAST FIRST} FROM C [INTO @varname [, ]] IF the cursor is not declared with SCROLL keyword (recommended when not necessary), only NEXT modifier is allowed M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 55
Allows handle exceptional states after SQL statement execution Exception are very common They have to be handled properly M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 56
Exception types Intended to be handled on the server or in the client application Standard named exceptions Are generated by the server, have assigned a name Standard unnamed exceptions Are generated by the server, have not assigned any name Intended to be handled on the server User defined internal exceptions Defined by the user, have name, have no description Intended to be handled in the client application User defined application exceptions Defined by the user, have no name, have description M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 57
Basic exceptions NO_DATA_FOUND TOO_MANY_ROWS DUP_VAL_ON_INDEX INVALID_NUMBER ZERO_DIVIDE Catches any not previously handled exception Catching EXCEPTION WHEN name1 [OR name2 ] THEN statements; [WHEN name3 [OR ] THEN statements;] [WHEN OTHERS THEN statements;] Block don t catches exceptions, thrown in the DECLARE section DECLARE x NUMBER := 0; y NUMBER := 1/x; /* ZERO_DIVIDE!! */ Such exceptions can be handled at leas one level higher M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 58
Can be named by the user and handled as usual DECLARE X EXCEPTION; PRAGMA EXCEPTION_INIT(X,-54) /* ORA-00054 ~ Resource locked */ Catching EXCEPTION WHEN X THEN ; EXCEPTION WHEN OTHERS THEN ; Any handled exception can be re-thrown, if it should be handled again in higher levels or in the client application EXCEPTION WHEN THEN RAISE; M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 59
BEGIN TRY statements END TRY BEGIN CATCH statement END CATCH Functions for exception identification ERROR_NUMBER() ERROR_SEVERITY() ERROR_STATE() ERROR_PROCEDURE() ERROR_LINE() ERROR_MESSAGE() M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 60
BEGIN TRY statements END TRY BEGIN CATCH statements END CATCH Not detected are Warnings with ERROR_SEVERITY<=10 Severe errors with ERROR_SEVERITY>20 that stops the session execution In higher levels are caught Compilation errors M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 61
BEGIN TRY SELECT 1/0; -- zero divide END TRY BEGIN CATCH SELECT @ErrorNumber = ERROR_NUMBER(), @ErrorSeverity = ERROR_SEVERITY(), @ErrorState = ERROR_STATE(), @ErrorProcedure = ERROR_PROCEDURE(), @ErrorLine = ERROR_LINE(), @ErrorMessage = ERROR_MESSAGE(); END CATCH; GO M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 62
Declaration DECLARE X EXCEPTION; Throwing RAISE X; Catching EXCEPTION [WHEN X THEN ;] WHEN OTHERS ; Inner.X not caught, is propagated outside Other Outer.X not handles Inner.X Problem with overloading <<Outer>> DECLARE X EXCEPTION; BEGIN <<Inner>> DECLARE X EXCEPTION; BEGIN RAISE X; END Inner; EXCEPTION WHEN X THEN ; END Outer; M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 63
Throwing RAISE_APPLICATION_ERROR( -20001, -- exception number <-20100;-20001> Description, -- text that describes the exception cause ); If the third parameter FALSE is added, the exception is only put to the exception stack, but is not thrown. It allows return more exceptions at once, where the lower one is a cause of the upper one M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 64
CREATE PROCEDURE InsEmp ( xempno NUMBER, xename VARCHAR2, xsal NUMBER DEFAULT NULL) AS xs NUMBER; BEGIN xs := xsal; IF xs IS NULL THEN SELECT MIN(Sal) INTO xs FROM EMP; END IF; INSERT INTO Emp(EmpNo,EName,Sal) VALUES(xEmpNo,xEName,xS); EXCEPTION WHEN DUP_VAL_ON_INDEX THEN RAISE_APPLICATION_ERROR( -20011, Employee xempno already exists! ); END; / EXEC InsEmp(1234, Smith ); M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 65
Throwing RAISERROR({ msg_id -- exception number, >= 50000 msg_str -- message text @msg_var -- message text in variable }, severity, state, argument [, ]) Individual messages can be prepared by sp_addmessage procedure and then referenced only by msg_id msg_str can contain references to parameters, similarly to C- function printf, and the values can be defined in arguments Severity is the severity of the exception. Severity 19-25 can be thrown only by member of sysadmin group/role. M. Kopecký Views and Procedural Extension (NDBI026, Lect. 3) 66