PROCEDURAL DATABASE PROGRAMMING ( PL/SQL AND T-SQL)

Transcription

1 Technology & Information Management Instructor: Michael Kremer, Ph.D. Class 3 Database Programming PROCEDURAL DATABASE PROGRAMMING ( PL/SQL AND T-SQL)

2 AGENDA 3. Declaring Variables/Constants 4. Flow Control Structures 5. Exception Handling

3 Procedural Database Programming (PL/SQL and T-SQL) 3. DECLARING VARIABLES/CONSTANTS

4 3.1 VARIABLE DECLARATION STATEMENT 53 Variables are named placeholders in memory for quick retrieval during programming execution. Applications of variables: Reading/storing of values or even database data Evaluating of complex expressions and reusing Breaking down complex logic and storing intermediary results Declaring variables: Oracle: Syntax: Anchor datatypes, scalar or record: Can only define one variable per line SQL Server: Syntax: Declare multiple variables using one Declare statement

5 3.2 VARIABLE ASSIGNMENT STATEMENT 54 Assigning values to variables: Assigning database values: Retrieve only one value: Error is raised when SELECT returns more than one value.

6 3.3 CONSTANTS, DEFAULT VALUES, NOT NULL CONTSTAINT Declare and assign values to variables 55 Only in Oracle: Constants Variable with initialized value that cannot be changed. Only in Oracle: NOT NULL constraint for variables:

7 Procedural Database Programming (PL/SQL and T-SQL) 4. FLOW CONTROL STRUCTURES

8 4.1 DECISION STRUCTURES 56 Change the flow of code based on a condition. IF statement Conditional branching logic, for example, if salary is between $8,000 and $15,000, apply a bonus of $1,500.

9 4.1 DECISION STRUCTURES 57 Syntax of IF statement is quite different: When the condition evaluates to null, it is considered false. Use IS NULL predicate or IsNull() function (SQL Server) to test for null values if necessary. If condition is too complex assign it into a boolean variable.

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12 4.1 DECISION STRUCTURES 60 Short-Circuit IF Processing Evaluate boolean expression using optimization called short-circuiting AND operator (&&) stops evaluating second condition if a is not equal b. No need to evaluate second condition. Take advantage of this feature by placing low evaluation cost expression(more likely to lead to short circuiting) at the beginning. In Oracle and SQL Server short-circuiting is built-in with no special operators. Position expensive expressions in terms of memory or CPU utilization at the end of the set of conditions.

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14 4.1 DECISION STRUCTURES 62 SQL Server only: Does not strictly evaluate from left to right. Evaluates expression at compile-time if possible, known as contradiction detection.

15 4.1 DECISION STRUCTURES 63 CASE statement Powerful alternative to IF statement. Exists in non-procedural SQL in both SQL Server and Oracle. In Oracle only, CASE expression exists also as control flow statement. Two variations of CASE statement: Simple CASE: Processing branches based on single expression Searched CASE: Processing branches based on different expressions. Simple CASE Simple CASE statement breaks up condition into the left side (expression) listed at the top. Expression is then compared to various different results in each WHEN clause. Only relational operator is the equal sign (implied).

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17 4.1 DECISION STRUCTURES 65 The following basic rules apply for the CASE statement: Only one WHEN block is executed. If more than one expression evaluates to true, only the first WHEN block is processed/ ELSE clause is optional, but error is raised if no expression evaluates to true. WHEN clauses are evaluated from top to bottom. Searched CASE Expression and result of simple CASE statement are merged together. Allows for more flexibility to construct complex expressions, also additional relational operators besides equal sign are allowed. Order Dependent Logic Applies to any decision statement, conditions/expressions are evaluated from top to bottom. Once a condition is met, statements in that block are processed.

18 4.1 DECISION STRUCTURES 66 If more than one condition evaluates to true, and the order of blocks is changed, then a different block is processed. For salary = 20000, bonus is For salary = 21000, bonus is If you reorder the blocks, a different bonus is calculated. Order-dependent logic can be a subtle source of bugs. Similar example using ranges: For salary of 20000, both conditions are true. Reordering blocks yield different results. Main rule: Write condition expressions with no overlap, meaning for one value there is only one condition which evaluates to true.

19 4.2 LOOP STRUCTURES 67 Loops let you execute the same code repeatedly. Need to implement condition so that loop will finish at some point. SQL Server has one loop, Oracle has three. SQL Server is limited in terms of loops, for example you cannot construct a loop that is processed once irrespective of the condition. Simple Loop Only in Oracle. EXIT WHEN basically represents an If condition. If more complex exit logic is needed, then write custom If logic, especially when an Else block is needed.

20 4.2 LOOP STRUCTURES 68 Use an If statement in conjunction with the EXIT statement. You can place the EXIT or EXIT WHEN anywhere within the Loop structure. This flexibility allows for writing loops where statements are executed before a condition is tested. FOR LOOP Numeric counter process within the statement definition. Notice the two periods between lower and upper bound, they are part of the statement. REVERSE keyword is optional. Increment/Decrement is always 1.

21 4.2 LOOP STRUCTURES 69 Rules for FOR LOOP: WHILE Loop Do not declare loop variable, it is implicitly declared as an Integer. Scope of loop variable is limited to loop, cannot reference variable outside the loop. Expressions for lower, upper bounds are evaluated once at the beginning. Never change values of loop variable or the boundary values, either you will receive a compile error or instructions are ignored. Using the Reverse keyword will automatically decrement from upper to lower bound, do not switch the values. Exists also in SQL Server. SQL Server Syntax: While loop is a conditional loop that continues to execute as long as the Boolean expression is true. Use While loop when you do not know how many times to loop. Use BREAK to stop the loop, CONTINUE to skip current loop

22 4.2 LOOP STRUCTURES 70 Oracle syntax is much simpler.

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24 Procedural Database Programming (PL/SQL and T-SQL) 5. EXCEPTION HANDLING

25 5.1 INTRODUCTION TO EXCEPTION HANDLING 72 Three kinds of errors in a programming language: Compile-time errors: Incorrect syntax Run-time errors: Invalid condition at run time Logical error (bug): Does not produce expected results. Exception handling is kind of a control flow statement. When an error occurs, control of the program is passed into the exception (Oracle) or catch (SQL Server) block. Syntax is different, concepts are the same. Point of no return when control is passed into exception/catch block: Sometimes it is desired to simply skip an error and continue with program execution Design sub blocks of code and implement separate exception handling for each block. Two kinds of exceptions: System exception (raised by the database software) Programmer-defined exception (Business logic rules)

26 5.1 INTRODUCTION TO EXCEPTION HANDLING 73 Commonly used exception terms Ensure that exception/catch code blocks are simple and solid in order not to raise an exception by itself. At most use If or Case statements to test for various different error situations.

27 5.2 BASIC EXCEPTION HANDLING 74 Basic syntax for SQL Server: Try and End Try block contains actual program unit. If an exception is raised, program control is passed into the Catch section. Errors trapped by the Catch block are not returned to the calling application. Use SELECT result sets, RAISERROR, or PRINT to return information. Try..Catch constructs can be nested. Either a Try block or a Catch block can contain nested Try..Catch constructs. Try...Catch constructs catch unhandled errors from stored procedures or triggers executed by the code in the Try block. Alternatively, the stored procedures or triggers can contain their own Try..Catch constructs.

28 5.1 INTRODUCTION TO EXCEPTION HANDLING 75 GOTO statements cannot be used to enter a Try or Catch block. Use GOTO statements only to jump within the Try or the Catch block, if necessary. Before SQL Server 2008 use global for error handling. A value of 0 means no error has is reset to 0 after each successful statement! This means you cannot have any statements between the code where you expect an error and the code that checks the value Also once you for a value it is reset to 0. In order to handle this deficiency, store the error code in a local variable. Use this syntax in a user-defined function.

29 5.1 INTRODUCTION TO EXCEPTION HANDLING 76 You can have multiple exception handlers in a single exception section. Exception handlers are structured much like a CASE statement.

30 5.1 INTRODUCTION TO EXCEPTION HANDLING 77 Predefined Oracle Exceptions

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33 5.3 PROGRAMMER-DEFINED EXCEPTIONS 80 System exceptions are automatically raised, but programmer-defined exceptions must be manually raised. RaiseError method is used to raise programmerdefined exceptions. RaiseError passes control into Catch section. When using msg_id, then add custom message into system table sysmessages in the master database (13000 <msg_id< ) When passing a string (msg_string), then error number is New in SQL Server 2012:

34 5.1 INTRODUCTION TO EXCEPTION HANDLING 81 Differences between RaiseError and Throw statements: Severity level can assume following values: Select a value between 11 and 18 in order to have control passed into the Catch block. State value must be in the range of You can use different values for same error used in different program units, for example. Oracle Syntax: Major difference is that you must declare named exceptions. Then use Raise statement and exception_name to raise the error.

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37 5.4 BUILT-IN ERROR FUNCTIONS 84 Main Oracle Error Functions

38 5.1 INTRODUCTION TO EXCEPTION HANDLING 85 SQL Server Error Functions You can use these functions only in within the Exception/Catch section of your program. These functions return error information specific to the scope of the current block in which they are referenced. They will return null if called outside the Exception/Catch section.

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