# MAX function

An aggregate function that returns the maximum value from a set of numbers. Opposite of
the `MIN`

function. Its single argument can be numeric column, or the numeric
result of a function or expression applied to the column value. Rows with a
`NULL`

value for the specified column are ignored. If the table is empty, or
all the values supplied to `MAX`

are `NULL`

, `MAX`

returns `NULL`

.

**Syntax:**

`MAX([DISTINCT | ALL] ``expression`) [OVER (`analytic_clause`)]

When the query contains a `GROUP BY`

clause, returns one value for each combination of
grouping values.

**Restrictions:** In Impala 2.0 and higher, this function can be used as an analytic function, but with restrictions on any window clause.
For `MAX()`

and `MIN()`

, the window clause is only allowed if the start
bound is `UNBOUNDED PRECEDING`

.

**Return type:** Same as the input value, except for `CHAR`

and `VARCHAR`

arguments which produce a `STRING`

result

**Usage notes:**

If you frequently run aggregate functions such
as `MIN()`

, `MAX()`

, and `COUNT(DISTINCT)`

on
partition key columns, consider enabling the `OPTIMIZE_PARTITION_KEY_SCANS`

query option, which optimizes such queries. See the OPTIMIZE_PARTITION_KEY_SCANS query
option topic for the kinds of queries that this option applies to, and slight
differences in how partitions are evaluated when this query option is enabled.

**Complex type considerations:**

To access a column with a complex type (`ARRAY`

, `STRUCT`

, or `MAP`

)
in an aggregation function, you unpack the individual elements using join notation in the query,
and then apply the function to the final scalar item, field, key, or value at the bottom of any nested type hierarchy in the column.
See Complex types for details about using complex types in Impala.

`ARRAY`

of `STRUCT`

items).
The array is unpacked inside the query using join notation.
The array elements are referenced using the `ITEM`

pseudocolumn, and the structure fields inside the array elements
are referenced using dot notation.
Numeric values such as `SUM()`

and `AVG()`

are computed using the numeric `R_NATIONKEY`

field, and
the general-purpose `MAX()`

and `MIN()`

values are computed from the string `N_NAME`

field.
```
describe region;
+-------------+-------------------------+---------+
| name | type | comment |
+-------------+-------------------------+---------+
| r_regionkey | smallint | |
| r_name | string | |
| r_comment | string | |
| r_nations | array<struct< | |
| | n_nationkey:smallint, | |
| | n_name:string, | |
| | n_comment:string | |
| | >> | |
+-------------+-------------------------+---------+
select r_name, r_nations.item.n_nationkey
from region, region.r_nations as r_nations
order by r_name, r_nations.item.n_nationkey;
+-------------+------------------+
| r_name | item.n_nationkey |
+-------------+------------------+
| AFRICA | 0 |
| AFRICA | 5 |
| AFRICA | 14 |
| AFRICA | 15 |
| AFRICA | 16 |
| AMERICA | 1 |
| AMERICA | 2 |
| AMERICA | 3 |
| AMERICA | 17 |
| AMERICA | 24 |
| ASIA | 8 |
| ASIA | 9 |
| ASIA | 12 |
| ASIA | 18 |
| ASIA | 21 |
| EUROPE | 6 |
| EUROPE | 7 |
| EUROPE | 19 |
| EUROPE | 22 |
| EUROPE | 23 |
| MIDDLE EAST | 4 |
| MIDDLE EAST | 10 |
| MIDDLE EAST | 11 |
| MIDDLE EAST | 13 |
| MIDDLE EAST | 20 |
+-------------+------------------+
select
r_name,
count(r_nations.item.n_nationkey) as count,
sum(r_nations.item.n_nationkey) as sum,
avg(r_nations.item.n_nationkey) as avg,
min(r_nations.item.n_name) as minimum,
max(r_nations.item.n_name) as maximum,
ndv(r_nations.item.n_nationkey) as distinct_vals
from
region, region.r_nations as r_nations
group by r_name
order by r_name;
+-------------+-------+-----+------+-----------+----------------+---------------+
| r_name | count | sum | avg | minimum | maximum | distinct_vals |
+-------------+-------+-----+------+-----------+----------------+---------------+
| AFRICA | 5 | 50 | 10 | ALGERIA | MOZAMBIQUE | 5 |
| AMERICA | 5 | 47 | 9.4 | ARGENTINA | UNITED STATES | 5 |
| ASIA | 5 | 68 | 13.6 | CHINA | VIETNAM | 5 |
| EUROPE | 5 | 77 | 15.4 | FRANCE | UNITED KINGDOM | 5 |
| MIDDLE EAST | 5 | 58 | 11.6 | EGYPT | SAUDI ARABIA | 5 |
+-------------+-------+-----+------+-----------+----------------+---------------+
```

**Examples:**

```
-- Find the largest value for this column in the table.
select max(c1) from t1;
-- Find the largest value for this column from a subset of the table.
select max(c1) from t1 where month = 'January' and year = '2013';
-- Find the largest value from a set of numeric function results.
select max(length(s)) from t1;
-- Can also be used in combination with DISTINCT and/or GROUP BY.
-- Return more than one result.
select month, year, max(purchase_price) from store_stats group by month, year;
-- Filter the input to eliminate duplicates before performing the calculation.
select max(distinct x) from t1;
```

`MAX()`

in an analytic context. They use a table
containing integers from 1 to 10. Notice how the `MAX()`

is reported for each input value, as
opposed to the `GROUP BY`

clause which condenses the result set.
```
select x, property, max(x) over (partition by property) as max from int_t where property in ('odd','even');
+----+----------+-----+
| x | property | max |
+----+----------+-----+
| 2 | even | 10 |
| 4 | even | 10 |
| 6 | even | 10 |
| 8 | even | 10 |
| 10 | even | 10 |
| 1 | odd | 9 |
| 3 | odd | 9 |
| 5 | odd | 9 |
| 7 | odd | 9 |
| 9 | odd | 9 |
+----+----------+-----+
```

Adding an `ORDER BY`

clause lets you experiment with results that are cumulative or apply to a moving
set of rows (the window). The following examples use

`MAX()`

in an analytic context
(that is, with an `OVER()`

clause) to display the smallest value of `X`

encountered up to each row in the result set. The examples use two columns in the `ORDER BY`

clause to produce a sequence of values that rises and falls, to illustrate how the `MAX()`

result only increases or stays the same throughout each partition within the result set.
The basic `ORDER BY x`

clause implicitly
activates a window clause of `RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW`

,
which is effectively the same as `ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW`

,
therefore all of these examples produce the same results:
```
select x, property,
max(x)
```**over (order by property, x desc)** as 'maximum to this point'
from int_t where property in ('prime','square');
+---+----------+-----------------------+
| x | property | maximum to this point |
+---+----------+-----------------------+
| 7 | prime | 7 |
| 5 | prime | 7 |
| 3 | prime | 7 |
| 2 | prime | 7 |
| 9 | square | 9 |
| 4 | square | 9 |
| 1 | square | 9 |
+---+----------+-----------------------+
select x, property,
max(x) over
(
**order by property, x desc**
**rows between unbounded preceding and current row**
) as 'maximum to this point'
from int_t where property in ('prime','square');
+---+----------+-----------------------+
| x | property | maximum to this point |
+---+----------+-----------------------+
| 7 | prime | 7 |
| 5 | prime | 7 |
| 3 | prime | 7 |
| 2 | prime | 7 |
| 9 | square | 9 |
| 4 | square | 9 |
| 1 | square | 9 |
+---+----------+-----------------------+
select x, property,
max(x) over
(
**order by property, x desc**
**range between unbounded preceding and current row**
) as 'maximum to this point'
from int_t where property in ('prime','square');
+---+----------+-----------------------+
| x | property | maximum to this point |
+---+----------+-----------------------+
| 7 | prime | 7 |
| 5 | prime | 7 |
| 3 | prime | 7 |
| 2 | prime | 7 |
| 9 | square | 9 |
| 4 | square | 9 |
| 1 | square | 9 |
+---+----------+-----------------------+

The following examples show how to construct a moving window, with a running maximum taking into account all rows before
and 1 row after the current row.
Because of a restriction in the Impala `RANGE`

syntax, this type of
moving window is possible with the `ROWS BETWEEN`

clause but not the `RANGE BETWEEN`

clause.
Because of an extra Impala restriction on the `MAX()`

and `MIN()`

functions in an
analytic context, the lower bound must be `UNBOUNDED PRECEDING`

.
```
select x, property,
max(x) over
(
```**order by property, x**
**rows between unbounded preceding and 1 following**
) as 'local maximum'
from int_t where property in ('prime','square');
+---+----------+---------------+
| x | property | local maximum |
+---+----------+---------------+
| 2 | prime | 3 |
| 3 | prime | 5 |
| 5 | prime | 7 |
| 7 | prime | 7 |
| 1 | square | 7 |
| 4 | square | 9 |
| 9 | square | 9 |
+---+----------+---------------+
-- Doesn't work because of syntax restriction on RANGE clause.
select x, property,
max(x) over
(
**order by property, x**
**range between unbounded preceding and 1 following**
) as 'local maximum'
from int_t where property in ('prime','square');
ERROR: AnalysisException: RANGE is only supported with both the lower and upper bounds UNBOUNDED or one UNBOUNDED and the other CURRENT ROW.