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Oracle Concepts - Displaying PL/SQL
Output
Oracle Tips by Burleson Consulting |
Displaying PL/SQL Output
Another change with PL/SQL from SQL is that the database
does not return the output. PL/SQL code normally will change data, insert
values, and so forth, inside the database. It will not normally display results
back to the user. To do this we use a procedure called dbms_output.put_line to
place the results in a buffer that SQL*Plus will retrieve and display. SQL*Plus
must be told to retrieve data from this buffer in order to display the results.
The SQL*Plus command “set serveroutput on” causes SQL*Plus to retrieve and
display the buffer.
SQL> declare
2 v_line
varchar2(40);
3 begin
4 v_line :=
'Hello World';
5
dbms_output.put_line (v_line);
6 end;
7 /
PL/SQL procedure
successfully completed.
SQL> set
serveroutput on
SQL> declare
2 v_line
varchar2(40);
3 begin
4 v_line :=
'Hello World';
5
dbms_output.put_line (v_line);
6 end;
7 /
Hello World
PL/SQL procedure
successfully completed.
The first time the script is run, the result was just a
notice that the script completed successfully. Once we set serverouput on and
rerun the script, the results are shown.
As discussed earlier, this is an anonymous block of
PL/SQL code. It is sent to the database, compiled and executed, then SQL*Plus
retrieves the results. The script is stored in the SQL*Plus buffer and can be
rerun by executing the forward slash.
SQL> /
Hello World
PL/SQL procedure
successfully completed.
The script is not stored in the database (like a stored
or named procedure). It must be resent to the database and compiled each time
it is executed.
For the complete story, we recommend the book “Easy
Oracle PL/SQL Programming”. Once you have mastered basic SQL you are ready
for the advanced book “Oracle
PL/SQL Tuning” by Dr. Timothy Hall.
As with SQL statements, SQL*Plus variables can be used
to make the PL/SQL script dynamic. Just as with a SQL statement, the variables
are local to SQL*Plus and are substituted before the code is sent to the
database.
SQL> declare
2 v_line
varchar2(40);
3 begin
4 v_line :=
'Hello &name';
5
dbms_output.put_line (v_line);
6 end;
7 /
Enter value for
name: John
old 4: v_line
:= 'Hello &name';
new 4: v_line
:= 'Hello John';
Hello John
The SQL*Plus accept command is a more flexible method of
embedding dynamic data in the script.
SQL> accept
v_string prompt "Enter Your First Name: "
Enter Your First
Name: Thomas
SQL> declare
2 v_line
varchar2(40):= '&v_string';
3 begin
4 v_line :=
'Hello '||v_line;
5
dbms_output.put_line (v_line);
6 end;
7 /
old 2: v_line
varchar2(40):= '&v_string';
new 2: v_line
varchar2(40):= 'Thomas';
Hello Thomas
PL/SQL procedure
successfully completed.
Let’s look at this script a little closer. The first
line is the SQL*Plus accept command to get the SQL*Plus variable v_string. This
line must be executed alone, not part of the PL/SQL block. At the prompt the
name Thomas was entered. Now the script is run but it is slightly modified from
previous examples.
SQL> declare
2 v_line
varchar2(40):= '&v_string';
The variable v_line is declared as a varchar2(40) and is
given a default value that equals v_string. The PL/SQL assignment operator (:=)
is used to assign the value. Hence, v_line is a bucket that gets assigned the
string ‘Thomas’. A developer would read an assignment statement in English as
“v_line gets v_string” to indicate the assignment. Let’s examine a more complex
assignment statement.
4 v_line := 'Hello
'||v_line;
Line 4 uses the concatenate operator to append ‘Hello ‘
to the front of v_line and then assigns it back to the variable v_line. The
variable v_line now contains the string ‘Hello Thomas’. Line 5 places the value
of v_line in the buffer to be retrieved by SQL*Plus.
old 2: v_line
varchar2(40):= '&v_string';
new 2: v_line
varchar2(40):= 'Thomas';
These two lines demonstrate SQL*Plus’s verify function
showing us what is substituted before the code is sent to the database for
execution. This information can be switched on/off with the verify command
SQL> set verify on
SQL> set verify
off
PL/SQL Variable Declaration and Conversion
In the previous examples a variable v_line was defined.
All variables are defined in the declaration section of the block. Variables
are defined in the form:
variableName
datatype := defaultvalue;
Below are examples of variables. Variables can be
defined as any valid datatype to include user defined datatypes and records.
declare
v_str1
varchar2(80);
v_str2
varchar2(30) := ‘Hello World’;
d_today date;
n_sales
number;
n_order
number(8);
begin
A constant is defined the same way a variable is with
the key word constant.
c_standard constant number :=
90;
Notice that a constant must be assigned a value and the
above statement does four things:
* Names the variable c_standard
* Defines c_standard as a constant
* Defines c_standard as a numeric datatype
* Assigns a value of 90 to c_standard
With PL/SQL constants, note that a constant value can’t
be changed unless it is redefined in a subsequent block.
In the examples above our two variables are defined as
numbers, and we are now ready to see how to include the precision and scale for
a number. As in SQL, PL/SQL supports mathematical operations and has a large
library of mathematical functions, covering everything from advanced
multivariate statistics to Newtonian Calculus. PL/SQL also supports single-row
functions to convert numbers to characters and characters to numbers.
This is an excerpt from the bestselling "Easy
Oracle Jumpstart" by Robert Freeman and Steve Karam (Oracle ACE and Oracle
Certified Master). It’s only $19.95 when you buy it directly from the
publisher
here.
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