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Oracle Concepts - PL/SQL IF/THEN/ELSE Statements

Oracle Tips by Burleson Consulting


The IF/THEN statement checks a Boolean value or expression and if true, executes the statements in the THEN clause.  If the condition is false, the statements in the THEN clause are skipped and execution jumps to the END IF, and the expression that is checked must return a true or false.  It can be a simple Boolean variable or a compound expression joined with AND/OR clauses, and the expression can even be a PL/SQL function that returns a Boolean value. 

There is no requirement to surround the expression with parenthesis but most developers use parentheses for clarity.  The THEN clause can contain a single or multiple statements, or a nested PL/SQL block.  Here is an example of a basic IF/THEN statement with a THEN clause.

if v_numb > 5 then

  v_numb  := 5;

  v_other := 10;

end if;

In the statements above, the Boolean condition (v_numb > 5) must be true before the THEN clause is executed.  If v_numb is equal to or less than 5, or if it evaluates to NULL, the program control jumps to the statement after the END IF clause.  Note that the THEN clause can contain any number of valid PL/SQL statements.  The variable v_other and v_numb will not change unless the condition is true. 

Note:  All the IF/THEN statements must end with an END IF clause.  The PL/SQL engine will continue to include statements in the THEN clause until it encounters an END IF.  If you get a compile error that states: ?found xxxx when expecting IF?, the compiler encountered an END statement before it encountered the END IF.  Simply find the end of you IF statement and close it with an END IF to correct the problem.

Sometimes the program flow will want to branch one direction if the condition is true and another direction if the condition is false, and this is handled within the IF/THEN/ELSE statement.

Like the IF/THEN statement, the THEN clause statements will only be executed if the condition is true.  However if the condition is false, the statements in the ELSE clause are executed.  This is an either-or situation.

if n_numb > 5 then
  v_status := ?large?;
  v _status := ?small?;
end if;

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.

After executing the example, the variable v_status will be defined as either large or small.  Note that the above statement could also be written as two separate IF statements.

if n_numb > 5 then v_status := ?large?; end if;

if n_numb <=5 then v_status := ?small?; end if;

Programmatically, the results are the same, however the two IF statements required two evaluations, while the IF/THEN/ELSE statement requires only one evaluation. 

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.

If you like Oracle tuning, you may enjoy the new book "Oracle Tuning: The Definitive Reference", over 900 pages of BC's favorite tuning tips & scripts. 

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