MAX is an aggregate function that evaluates the maximum of an
expression over a set of rows (see Aggregates (set functions)).
MAX is allowed only on expressions that evaluate to built-in data types (including
CHAR, VARCHAR, DATE, TIME, CHAR FOR BIT DATA, etc.).
Syntax
MAX ( [ DISTINCT | ALL ] Expression )
The
DISTINCT and ALL qualifiers eliminate or retain duplicates, but
these qualifiers have no effect in a MAX expression. Only one DISTINCT aggregate
expression per
SelectExpression is
allowed. For example, the following query is not allowed:
SELECT COUNT (DISTINCT flying_time), MAX (DISTINCT miles)
FROM Flights
The Expression can contain multiple
column references or expressions, but it cannot contain another aggregate
or subquery. It must evaluate to a built-in data type. You can therefore call
methods that evaluate to built-in data types. (For example, a method that
returns a java.lang.Integer or int evaluates to an INTEGER.)
If an expression evaluates to NULL, the aggregate skips that value.
The type's comparison rules determine the maximum value. For
CHAR and VARCHAR,
the number of blank spaces at the end of the value can affect how MAX is evaluated.
For example, if the values 'z' and 'z ' are both stored in a column, you cannot
control which one will be returned as the maximum, because blank spaces are
ignored for character comparisons.
The resulting data type is the same as the expression on
which it operates (it will never overflow).
-- find the latest date in the FlightAvailability table
SELECT MAX (flight_date) FROM FlightAvailability
-- find the longest flight originating from each airport,
-- but only when the longest flight is over 10 hours
SELECT MAX(flying_time), orig_airport
FROM Flights
GROUP BY orig_airport
HAVING MAX(flying_time) > 10
