Oracle Learning - 12

Oracle/PLSQL: Sequences (Autonumber)

In Oracle, you can create an autonumber field by using sequences. A sequence is an object in Oracle that is used to generate a number sequence. This can be useful when you need to create a unique number to act as a primary key.
The syntax for a sequence is:
CREATE SEQUENCE sequence_name
MINVALUE value
MAXVALUE value
START WITH value
INCREMENT BY value
CACHE value;
For example:
CREATE SEQUENCE supplier_seq
MINVALUE 1
MAXVALUE 999999999999999999999999999
START WITH 1
INCREMENT BY 1
CACHE 20;
This would create a sequence object called supplier_seq. The first sequence number that it would use is 1 and each subsequent number would increment by 1 (ie: 2,3,4,...}. It will cache up to 20 values for performance.
If you omit the MAXVALUE option, your sequence will automatically default to:
MAXVALUE 999999999999999999999999999
So you can simplify your CREATE SEQUENCE command as follows:
CREATE SEQUENCE supplier_seq
MINVALUE 1
START WITH 1
INCREMENT BY 1
CACHE 20;
Now that you've created a sequence object to simulate an autonumber field, we'll cover how to retrieve a value from this sequence object. To retrieve the next value in the sequence order, you need to use nextval.
For example:
supplier_seq.nextval
This would retrieve the next value from supplier_seq. The nextval statement needs to be used in an SQL statement. For example:
INSERT INTO suppliers
(supplier_id, supplier_name)
VALUES
(supplier_seq.nextval, 'Kraft Foods');
This insert statement would insert a new record into the suppliers table. The supplier_id field would be assigned the next number from the supplier_seq sequence. The supplier_name field would be set to Kraft Foods.

Frequently Asked Questions

One common question about sequences is:
Question: While creating a sequence, what does cache and nocache options mean? For example, you could create a sequence with a cache of 20 as follows:
CREATE SEQUENCE supplier_seq
MINVALUE 1
START WITH 1
INCREMENT BY 1
CACHE 20;

Or you could create the same sequence with the nocache option:
CREATE SEQUENCE supplier_seq
MINVALUE 1
START WITH 1
INCREMENT BY 1
NOCACHE;

Answer: With respect to a sequence, the cache option specifies how many sequence values will be stored in memory for faster access.
The downside of creating a sequence with a cache is that if a system failure occurs, all cached sequence values that have not be used, will be "lost". This results in a "gap" in the assigned sequence values. When the system comes back up, Oracle will cache new numbers from where it left off in the sequence, ignoring the so called "lost" sequence values.
Note: To recover the lost sequence values, you can always execute an ALTER SEQUENCE command to reset the counter to the correct value.
Nocache means that none of the sequence values are stored in memory. This option may sacrifice some performance, however, you should not encounter a gap in the assigned sequence values.


Question: How do we set the LASTVALUE value in an Oracle Sequence?
Answer: You can change the LASTVALUE for an Oracle sequence, by executing an ALTER SEQUENCE command.
For example, if the last value used by the Oracle sequence was 100 and you would like to reset the sequence to serve 225 as the next value. You would execute the following commands.
alter sequence seq_name
increment by 124;
select seq_name.nextval from dual;
alter sequence seq_name
increment by 1;
Now, the next value to be served by the sequence will be 225.

Oracle/PLSQL: Commit

The syntax for the COMMIT statement is:
COMMIT [WORK] [COMMENT text];
The Commit statement commits all changes for the current session. Once a commit is issued, other users will be able to see your changes.
Oracle/PLSQL: Commit

The syntax for the COMMIT statement is:
COMMIT [WORK] [COMMENT text];
The Commit statement commits all changes for the current session. Once a commit is issued, other users will be able to see your changes.

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Oracle Learning - 11

Oracle/PLSQL: FOR Loop

The syntax for the FOR Loop is:
FOR loop_counter IN [REVERSE] lowest_number..highest_number
LOOP
{.statements.}
END LOOP;
You would use a FOR Loop when you want to execute the loop body a fixed number of times.

Let's take a look at an example.
FOR Lcntr IN 1..20
LOOP
LCalc := Lcntr * 31;
END LOOP;
This example will loop 20 times. The counter will start at 1 and end at 20.

The FOR Loop can also loop in reverse. For example:
FOR Lcntr IN REVERSE 1..15
LOOP
LCalc := Lcntr * 31;
END LOOP;
This example will loop 15 times. The counter will start at 15 and end at 1. (loops backwards)

Oracle/PLSQL: CURSOR FOR Loop

The syntax for the CURSOR FOR Loop is:
FOR record_index in cursor_name
LOOP
{.statements.}
END LOOP;
You would use a CURSOR FOR Loop when you want to fetch and process every record in a cursor. The CURSOR FOR Loop will terminate when all of the records in the cursor have been fetched.
Here is an example of a function that uses a CURSOR FOR Loop:
CREATE OR REPLACE Function TotalIncome
( name_in IN varchar2 )
RETURN varchar2
IS
total_val number(6);

cursor c1 is
select monthly_income
from employees
where name = name_in;

BEGIN
total_val := 0;
FOR employee_rec in c1
LOOP
total_val := total_val + employee_rec.monthly_income;
END LOOP;

RETURN total_val;
END;
In this example, we've created a cursor called c1. The CURSOR FOR Loop will terminate after all records have been fetched from the cursor c1.
Oracle/PLSQL: While Loop

The syntax for the WHILE Loop is:
WHILE condition
LOOP
{.statements.}
END LOOP;
You would use a WHILE Loop when you are not sure how many times you will execute the loop body. Since the WHILE condition is evaluated before entering the loop, it is possible that the loop body may not execute even once.
Let's take a look at an example:
WHILE monthly_value <= 4000
LOOP
monthly_value := daily_value * 31;
END LOOP;
In this example, the WHILE Loop would terminate once the monthly_value exceeded 4000.

Oracle/PLSQL: Repeat Until Loop

Oracle doesn't have a Repeat Until loop, but you can emulate one. The syntax for emulating a REPEAT UNTIL Loop is:
LOOP
{.statements.}
EXIT WHEN boolean_condition;
END LOOP;
You would use an emulated REPEAT UNTIL Loop when you do not know how many times you want the loop body to execute. The REPEAT UNTIL Loop would terminate when a certain condition was met.
Let's take a look at an example:
LOOP
monthly_value := daily_value * 31;
EXIT WHEN monthly_value > 4000;
END LOOP;
In this example, the LOOP would repeat until the monthly_value exceeded 4000.
Oracle/PLSQL: Exit Statement

The syntax for the EXIT statement is:
EXIT [WHEN boolean_condition];
The EXIT statement is most commonly used to terminate LOOP statements.
Let's take a look at an example:
LOOP
monthly_value := daily_value * 31;
EXIT WHEN monthly_value > 4000;
END LOOP;
In this example, the LOOP would terminate when the monthly_value exceeded 4000.

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Oracle Learning - 10

Oracle/PLSQL: Case Statement

In Oracle 9i, you can use the case statement within an SQL statement. It has the functionality of an IF-THEN-ELSE statement.
The syntax for the case statement is:
CASE expression
WHEN condition_1 THEN result_1
WHEN condition_2 THEN result_2
...
WHEN condition_n THEN result_n
ELSE result END
expression is the value that you are comparing to the list of conditions. (ie: condition_1, condition_2, ... condition_n)
condition_1 to condition_n must all be the same datatype. Conditions are evaluated in the order listed. Once a condition is found to be true, the case statement will return the result and not evaluate the conditions any further.
result_1 to result_n must all be the same datatype. This is the value returned once a condition is found to be true.

Note:
If no condition is found to be true, then the case statement will return the value in the ELSE clause.
If the ELSE clause is omitted and no condition is found to be true, then the case statement will return NULL.
You can have up to 255 comparisons in a case statement. Each WHEN ... THEN clause is considered 2 comparisons.

For Example:
You could use the case statement in an SQL statement as follows:
select table_name,
CASE owner
WHEN 'SYS' THEN 'The owner is SYS'
WHEN 'SYSTEM' THEN 'The owner is SYSTEM'
ELSE 'The owner is another value' END
from all_tables;

The above case statement is equivalent to the following IF-THEN-ELSE statement:
IF owner = 'SYS' THEN
result := 'The owner is SYS';
ELSIF owner = 'SYSTEM' THEN
result := 'The owner is SYSTEM'';
ELSE
result := 'The owner is another value';
END IF;

The case statement will compare each owner value, one by one.

One thing to note is that the ELSE clause within the case statement is optional. You could have omitted it. Let's take a look at the SQL statement above with the ELSE clause omitted.
Your SQL statement would look as follows:
select table_name,
CASE owner
WHEN 'SYS' THEN 'The owner is SYS'
WHEN 'SYSTEM' THEN 'The owner is SYSTEM' END
from all_tables;
With the ELSE clause omitted, if no condition was found to be true, the case statement would return NULL.

Oracle/PLSQL: GOTO Statement

The GOTO statement causes the code to branch to the label after the GOTO statement.
For example:
GOTO label_name;

Then later in the code, you would place your label and code associated with that label.
Label_name: {statements

Oracle/PLSQL: Loop Statement

The syntax for the LOOP statement is:
LOOP
{.statements.}
END LOOP;
You would use a LOOP statement when you are not sure how many times you want the loop body to execute and you want the loop body to execute at least once.
The LOOP statement is terminated when it encounters either an EXIT statement or when it encounters an EXIT WHEN statement that evaluated to TRUE.
Let's take a look at an example:
LOOP
monthly_value := daily_value * 31;
EXIT WHEN monthly_value > 4000;
END LOOP;
In this example, the LOOP would terminate when the monthly_value exceeded 4000.

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Oracle Learning - 9

Oracle/PLSQL: IS NULL

In other languages, a null value is found using the = null syntax. However in PLSQL to check if a value is null, you must use the "IS NULL" syntax.
To check for equality on a null value, you must use "IS NULL".
For example,
IF Lvalue IS NULL then
.
END IF;
If Lvalue contains a null value, the "IF" expression will evaluate to TRUE.

You can also use "IS NULL" in an SQL statement. For example:
select * from suppliers
where supplier_name IS NULL;
This will return all records from the suppliers table where the supplier_name contains a null value.

Oracle/PLSQL: IS NOT NULL

In other languages, a not null value is found using the != null syntax. However in PLSQL to check if a value is not null, you must use the "IS NOT NULL" syntax.
For example,
IF Lvalue IS NOT NULL then
.
END IF;
If Lvalue does not contain a null value, the "IF" expression will evaluate to TRUE.

You can also use "IS NOT NULL" in an SQL statement. For example:
select * from suppliers
where supplier_name IS NOT NULL;
This will return all records from the suppliers table where the supplier_name does not contain a null value.

Oracle/PLSQL: IF-THEN-ELSE Statement

There are three different syntaxes for these types of statements.
Syntax #1: IF-THEN
IF condition THEN
{...statements...}
END IF;

Syntax #2: IF-THEN-ELSE
IF condition THEN
{...statements...}
ELSE
{...statements...}
END IF;

Syntax #3: IF-THEN-ELSIF
IF condition THEN
{...statements...}
ELSIF condition THEN
{...statements...}
ELSE
{...statements...}
END IF;

Here is an example of a function that uses the IF-THEN-ELSE statement:
CREATE OR REPLACE Function IncomeLevel
( name_in IN varchar2 )
RETURN varchar2
IS
monthly_value number(6);
ILevel varchar2(20);
cursor c1 is
select monthly_income
from employees
where name = name_in;
BEGIN
open c1;
fetch c1 into monthly_value;
close c1;
IF monthly_value <= 4000 THEN ILevel := 'Low Income'; ELSIF monthly_value > 4000 and monthly_value <= 7000 THEN ILevel := 'Avg Income'; ELSIF monthly_value > 7000 and monthly_value <= 15000 THEN
ILevel := 'Moderate Income';
ELSE
ILevel := 'High Income';
END IF;
RETURN ILevel;
END;
In this example, we've created a function called IncomeLevel. It has one parameter called name_in and it returns a varchar2. The function will return the income level based on the employee's name.

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Oracle Learning - 8

SQL: CREATE Table

The basic syntax for a CREATE TABLE is:
CREATE TABLE table_name
(column1 datatype null/not null,
column2 datatype null/not null,
...
);
Each column must have a datatype. The column should either be defined as "null" or "not null" and if this value is left blank, the database assumes "null" as the default.

For example:

CREATE TABLE supplier
( supplier_id numeric(10) not null,
supplier_name varchar2(50) not null,
contact_name varchar2(50)
)

SQL: CREATE Table from another table

You can also create a table from an existing table by copying the existing table's columns.
It is important to note that when creating a table in this way, the new table will be populated with the records from the existing table (based on the SELECT Statement).

Syntax #1 - Copying all columns from another table
The basic syntax is:
CREATE TABLE new_table
AS (SELECT * FROM old_table);

For example:
CREATE TABLE suppliers
AS (SELECT *
FROM companies
WHERE id > 1000);
This would create a new table called suppliers that included all columns from the companies table.
If there were records in the companies table, then the new suppliers table would also contain the records selected by the SELECT statement.

Syntax #2 - Copying selected columns from another table
The basic syntax is:
CREATE TABLE new_table
AS (SELECT column_1, column2, ... column_n FROM old_table);

For example:
CREATE TABLE suppliers
AS (SELECT id, address, city, state, zip
FROM companies
WHERE id > 1000);
This would create a new table called suppliers, but the new table would only include the specified columns from the companies table.
Again, if there were records in the companies table, then the new suppliers table would also contain the records selected by the SELECT statement.

Syntax #3 - Copying selected columns from multiple tables
The basic syntax is:
CREATE TABLE new_table
AS (SELECT column_1, column2, ... column_n
FROM old_table_1, old_table_2, ... old_table_n);


For example:
CREATE TABLE suppliers
AS (SELECT companies.id, companies.address, categories.cat_type
FROM companies, categories
WHERE companies.id = categories.id
AND companies.id > 1000);
This would create a new table called suppliers based on columns from both the companies and categories tables.

SQL: ALTER Table

The ALTER TABLE command allows you to add, modify, or drop a column from an existing table.

Adding column(s) to a table
Syntax #1
To add a column to an existing table, the ALTER TABLE syntax is:
ALTER TABLE table_name
ADD column_name column-definition;
For example:
ALTER TABLE supplier
ADD supplier_name varchar2(50);
This will add a column called supplier_name to the supplier table.

Syntax #2
To add multiple columns to an existing table, the ALTER TABLE syntax is:

ALTER TABLE table_name
ADD ( column_1 column-definition,
column_2 column-definition,
...
column_n column_definition );
For example:

ALTER TABLE supplier
ADD ( supplier_name varchar2(50),
city varchar2(45) );
This will add two columns (supplier_name and city) to the supplier table.

Modifying column(s) in a table
Syntax #1
To modify a column in an existing table, the ALTER TABLE syntax is:
ALTER TABLE table_name
MODIFY column_name column_type;
For example:
ALTER TABLE supplier
MODIFY supplier_name varchar2(100) not null;
This will modify the column called supplier_name to be a data type of varchar2(100) and force the column to not allow null values.

Syntax #2
To modify multiple columns in an existing table, the ALTER TABLE syntax is:

ALTER TABLE table_name
MODIFY ( column_1 column_type,
column_2 column_type,
...
column_n column_type );
For example:

ALTER TABLE supplier
MODIFY ( supplier_name varchar2(100) not null,
city varchar2(75) );
This will modify both the supplier_name and city columns.

Drop column(s) in a table
Syntax #1
To drop a column in an existing table, the ALTER TABLE syntax is:
ALTER TABLE table_name
DROP COLUMN column_name;
For example:
ALTER TABLE supplier
DROP COLUMN supplier_name;
This will drop the column called supplier_name from the table called supplier.

Rename column(s) in a table
(NEW in Oracle 9i Release 2)
Syntax #1
Starting in Oracle 9i Release 2, you can now rename a column.
To rename a column in an existing table, the ALTER TABLE syntax is:
ALTER TABLE table_name
RENAME COLUMN old_name to new_name;
For example:
ALTER TABLE supplier
RENAME COLUMN supplier_name to sname;
This will rename the column called supplier_name to sname.

SQL: DROP Table

The basic syntax for a DROP TABLE is:
DROP TABLE table_name;

For example:
DROP TABLE supplier;
This would drop table called supplier.


SQL: Global Temporary tables

Global temporary tables are distinct within SQL sessions.
The basic syntax is:
CREATE GLOBAL TEMPORARY TABLE table_name ( ...);

For example:

CREATE GLOBAL TEMPORARY TABLE supplier
( supplier_id numeric(10) not null,
supplier_name varchar2(50) not null,
contact_name varchar2(50)
)
This would create a global temporary table called supplier .

SQL: Local Temporary tables

Local temporary tables are distinct within modules and embedded SQL programs within SQL sessions.
The basic syntax is:
DECLARE LOCAL TEMPORARY TABLE table_name ( ...);

SQL: VIEWS

A view is, in essence, a virtual table. It does not physically exist. Rather, it is created by a query joining one or more tables.
The syntax for a VIEW is:
CREATE VIEW view_name AS
SELECT columns
FROM table
WHERE predicates;


For example:
CREATE VIEW sup_orders AS
SELECT supplier.supplier_id, orders.quantity, orders.price
FROM supplier, orders
WHERE supplier.supplier_id = orders.supplier_id
and supplier.supplier_name = 'IBM';
This would create a virtual table based on the result set of the select statement. You can now query the view as follows:
SELECT *
FROM sup_orders;

Frequently Asked Questions

Question: Can you update the data in a view?
Answer: A view is created by joining one or more tables. When you update record(s) in a view, it updates the records in the underlying tables that make up the view.
So, yes, you can update the data in a view providing you have the proper privileges to the underlying tables.

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