What is a Search Condition?
Go Up to Restricting Row Retrieval with WHERE
Because the WHERE
clause specifies the type of data a query is searching for it is often called a search condition. A query examines each row in a table to see if it meets the criteria specified in the search condition. If it does, the row qualifies for retrieval.
When a row is compared to a search condition, one of three values is returned:
- True: A row meets the conditions specified in the
WHERE
clause. - False: A row fails to meet the conditions specified in the
WHERE
clause. - Unknown: A column tested in the
WHERE
clause contains an unknown value that could not be evaluated because of aNULL
comparison.
Most search conditions, no matter how complex, evaluate to True or False. An expression that evaluates to True or False—like the search condition in the WHERE
clause—is called a boolean expression.