Search Conditions 
The SQL Optimizer can only optimize data access for those SQL statements that contain a search condition. A search condition is defined with a WHERE clause.
SELECT name FROM customer
WHERE cno = 7020
You can specify a search condition for the following SQL statement types: SELECT, UPDATE, DELETE, INSERT.
Some search condition restrictions apply to UPDATE statements.
The form of the search condition has a significant effect on the costs incurred by executing the SQL statement. When determining costs for an SQL statement, the SQL Optimizer only considers the following search conditions:
If the search conditions in an SQL statement cannot be transformed into one of the forms listed above, the SQL Optimizer cannot determine a search strategy for them and uses the sequential search instead.
If search conditions are linked using a number of identical Boolean operators, the sequence of these conditions does not affect how the SQL Optimizer determines the search strategy.
See also:
SQL Reference Manual, Table Expression (table_expression)