The WHERE clause, if present, is applied to the output as a condition to list, for example:
SELECT *
FROM CUSTOMER
WHERE NAME LIKE 'Russell';
prints the details of all customers named 'Russell' (double quotes are not accepted).
Output may be obtained from more than one table; all the tables are joined, and the WHERE condition is applied to the resulting joined table (which has n1*n2*... entries if the individual tables have n1, n2,... records). In practice, the WHERE condition is usually used to give equality joins between tables so that only one line is printed for each entry in the first table, for example:
SELECT *
FROM order_header, customer
WHERE customer.customer_number=order_header.customer_number;
prints one line for each order, giving details of the order and of the corresponding customer.