Class 8 Database Programming PROCEDURAL … · PROCEDURAL DATABASE PROGRAMMING ... Try to avoid using cursors whenever possible ... for both UPDATE and DELETE statements inside a

PROCEDURAL DATABASE PROGRAMMING

( PL/SQL AND T-SQL)

Database Programming

Instructor: Michael Kremer, Ph.D.Technology & Information Management

Class 8

AGENDA

10. Cursors

10.1 Cursor Basics

10.2 Cursor Parameters

10.3 Cursor Variables

10.4 Data Updates using Cursors

11. Dynamic SQL

11.1 Overview of Dynamic SQL

11.2 Basic Dynamic SQL

Procedural Database Programming (PL/SQL and T-SQL)

10. CURSORS

10.1 CURSOR BASICS

Cursor Runner (Latin), it runs through a result set

If possible, avoid cursors for the following reasons:

Cursors use a lot of overhead, often much more than an equivalent set-

based approach

Cursors override SQL Server’s built-in query optimizations, often making

them much slower than an equivalent set-based solution

In its simplest form, a cursor is a pointer to a result set

To create a cursor, you declare it like a variable and assign a SQL

Select statement to it

Then you open, fetch, and

eventually close it

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10.1 CURSOR BASICS

Previous steps are generalized not exact syntax

Common terminology when working with cursors:

Static SQL: A SQL statement is static if it is fully specified, or fixed, at the

time the code containing that statement is compiled

Dynamic SQL: A SQL statement is dynamic if it is constructed at runtime

and then executed, so you do not completely specify the SQL statement in

the code you write

Result set: This is the set of rows identified by the database as fulfilling

the request for data specified by the SQL statement

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10.1 CURSOR BASICS

Common terminology when working with cursors (continued):

Implicit cursor: The programming environment declares and manages an

implicit cursor every time you execute a SQL DML statement or a SELECT

that returns a single row from the database directly into a procedural data

structure

Explicit cursor: This is a SELECT statement that you declare as a cursor

explicitly in your application code

Cursor variable: This is a variable you declare that references or points to

a cursor object in the database

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10.1 CURSOR BASICS

Declaring Cursors

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10.1 CURSOR BASICS

SQL Server Cursor

Options

Compatibility

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10.1 CURSOR BASICS

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10.1 CURSOR BASICS

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10.1 CURSOR BASICS

Opening Cursors

When you open a cursor, SQL statement is processed at that

time.

In Oracle, data is read at the time when the cursor is opened

(read-consistency model), even if changes occur afterwards. In

SQL Server, you can specify this behavior

Much simpler syntax in Oracle

Test whether cursor is

already open:

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10.1 CURSOR BASICS

Fetching Cursors

A cursor is a pointer to a virtual table defined by the SELECT

statement

Fetching is the

process of

retrieving one row at a time

into your program’s current

scope and processing it

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10.1 CURSOR BASICS

Use function @@FETCH_STATUS

to test whether there are

more rows to fetch

If you fetch past the last row, no exception is raised

Use the cursor attribute %FOUND or %NOTFOUND to test

whether a row was found or not

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10.1 CURSOR BASICS

Closing Cursors

It is important to close a cursor in order to release system

resources

In Oracle, make sure to test whether a cursor is actually open

before you close it

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10.1 CURSOR BASICS

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10.1 CURSOR BASICS

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10.1 CURSOR BASICS

Cursor Optimization Tips

Try to avoid using cursors whenever possible

Do not forget to close cursor when its result set is not needed

Do not forget to deallocate SQL Server cursor when the data structures

comprising the cursor are not needed

Try to reduce the number of records to process in the cursor

Try to reduce the number of columns to process in the cursor

Use READ ONLY cursors, whenever possible, instead of updatable cursors

Try avoid using insensitive, static and keyset cursors, whenever possible

Use FAST_FORWARD cursors, whenever possible

Use FORWARD_ONLY cursors, if you need updatable cursor and the

FETCH NEXT will be the only used fetch option

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10.2 CURSOR PARAMETERS

To make your cursor more dynamic, add parameters to the

SELECT statement, mostly to the WHERE clause

Then pass a parameter into the program unit and use that

parameter to build the cursor dynamically

In SQL Server, you must use passed in

parameter directly in your SQL

statement

In Oracle, you add parameter to cursor

list declaration

Only when you open the cursor, you

pass in the parameter

That allows for further manipulation

of parameters in your program unit

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10.2 CURSOR VARIABLES

A cursor variable is a variable that points to or references an

underlying cursor

Unlike an explicit cursor, which names a work area for the result

set, a cursor variable is a reference to that work area

The cursor variable can be opened for any query, even for

different queries within a single program execution

The most important benefit of the cursor variable is that it

provides a mechanism

for passing results of

queries between

different program units,

even between client and server programs

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In Oracle, you have to declare a cursor type first

Then declare a cursorvariable

In SQL Server, a cursorvariable can only be usedas an OUTPUT mode

In Oracle, you can use a cursor variable for all modes, that is, IN, OUT, or IN OUT

Remember that the value of a cursor variable is the reference to the cursor object, not the state of the cursor object

In other words, the value of a cursor variable does not change after you fetch from or close a cursor

Only two operations, in fact, may change the value of a cursor variable (that is, the cursor object to which the variable points):

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10.2 DATA UPDATES USING CURSORS

In general, cursors are used to read data mainly

When you issue a SELECT statement against the database to query some records, no locks are placed on the selected rows

There are times, however, when you will want to lock a set of records even before you change them in your program

FOR UPDATE clause

Oracle offers the FOR UPDATE clause of the SELECT statement to perform this locking

In SQL Server, you can use the FOR UPDATE clause only in a cursor

If you use multiple table SELECT statements, all tables involved are locked

When specifying individual columns, only rows in those tables referenced by these columns are locked

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CURRENT OF clause

Oracle and SQL Server provide the WHERE CURRENT OF clause for both UPDATE and DELETE statements inside a cursor

This clause allows you to easily make changes to the most recently fetched row of data

Notice that the WHERE CURRENT OFclause references the cursor, not the record into which the next fetched row is deposited

To delete a row from the database for the most recently fetched record:

Most important advantage to using WHERE CURRENT OF to change the last row fetched is that you do not have to code in two (or more) places the criteria used to uniquely identify a row in a table

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Procedural Database Programming (PL/SQL and T-SQL)

11. DYNAMIC SQL

11.1 OVERVIEW OF DYNAMIC SQL

SQL statements that are constructed and executed at runtime

Use dynamic SQL sparingly and only when necessary (caching,

performance)

Applications of dynamic SQL:

Execute DDL statements

Support ad hoc query and update requirements of web-based

applications.

Softcode business rules and formulas.

Reporting requirements

Oracle: Old method package DBMS_SQL, newer method Execute

Immediate

SQL Server: Procedure sp_executesql, preferred way as it has an

interface (IN, OUT parameters)

Simpler method is Execute

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11.1 OVERVIEW OF DYNAMIC SQL

Assume we have orders tables that are separated by year. The

year is appended to the table name, such as Orders2010,

Orders2011, etc.

Year is passed

as a parameter

Dynamically build

SQL statement.

Same concept applies to DDL statements:

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11.2 BASIC DYNAMIC SQL

Syntax to use dynamic SQL

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In Oracle, you can also use dynamic SQL in a cursor. This is

pretty powerful and represents ultimate flexibility

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Class 8 Database Programming PROCEDURAL … · PROCEDURAL DATABASE PROGRAMMING ... Try to avoid using cursors whenever possible ... for both UPDATE and DELETE statements inside a

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