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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.

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