返回以2为底，number的对数。此函数为MaxCompute 2.0扩展函数。
命令格式
double log2(<number>)参数说明
number：必填。DOUBLE、BIGINT、INT、SMALLINT、TINYINT、FLOAT、DECIMAL、STRING类型。
返回值说明
返回DOUBLE类型。number值为0、负数或NULL时，返回NULL。
示例数据
为便于理解各函数的使用方法，本文为您提供源数据，基于源数据提供函数相关示例。创建表mf_math_fun_t，并添加数据，命令示例如下。
create table if not exists mf_math_fun_t(
int_data int,
bigint_data bigint,
double_data double,
decimal_data decimal,
float_data float,
string_data string
);
insert into mf_math_fun_t values
(null, -10, 0.525, 0.525BD, cast(0.525 as float), '10'),
(-20, null, -0.1, -0.1BD, cast(-0.1 as float), '-10'),
(0, -1, null, 20.45BD, cast(-1 as float), '30'),
(-40, 4, 0.89, null, cast(0.89 as float), '-30'),
(5, -50, -1, -1BD, null, '50'),
(-60, 6, 1.5, 1.5BD, cast(1.5 as float), '-50'),
(-1, -70, -7.5, -7.5BD, cast(-7.5 as float),null ),
(-80, 1, -10.2, -10.2BD, cast(-10.2 as float), '-1' ),
(9, -90, 2.58, 2.58BD, cast(2.58 as float), '0'),
(-100, 10, -5.8, -5.8BD, cast(-5.8 as float), '-90');查询表mf_math_fun_t中的数据，命令示例如下：
select * from mf_math_fun_t;
--返回结果。
+------------+-------------+-------------+--------------+------------+-------------+
| int_data | bigint_data | double_data | decimal_data | float_data | string_data |
+------------+-------------+-------------+--------------+------------+-------------+
| NULL | -10 | 0.525 | 0.525 | 0.525 | 10 |
| -20 | NULL | -0.1 | -0.1 | -0.1 | -10 |
| 0 | -1 | NULL | 20.45 | -1.0 | 30 |
| -40 | 4 | 0.89 | NULL | 0.89 | -30 |
| 5 | -50 | -1.0 | -1 | NULL | 50 |
| -60 | 6 | 1.5 | 1.5 | 1.5 | -50 |
| -1 | -70 | -7.5 | -7.5 | -7.5 | NULL |
| -80 | 1 | -10.2 | -10.2 | -10.2 | -1 |
| 9 | -90 | 2.58 | 2.58 | 2.58 | 0 |
| -100 | 10 | -5.8 | -5.8 | -5.8 | -90 |
| NULL | -10 | 0.525 | 0.525 | 0.525 | 10 |
| -20 | NULL | -0.1 | -0.1 | -0.1 | -10 |
| 0 | -1 | NULL | 20.45 | -1.0 | 30 |
| -40 | 4 | 0.89 | NULL | 0.89 | -30 |
| 5 | -50 | -1.0 | -1 | NULL | 50 |
| -60 | 6 | 1.5 | 1.5 | 1.5 | -50 |
| -1 | -70 | -7.5 | -7.5 | -7.5 | NULL |
| -80 | 1 | -10.2 | -10.2 | -10.2 | -1 |
| 9 | -90 | 2.58 | 2.58 | 2.58 | 0 |
| -100 | 10 | -5.8 | -5.8 | -5.8 | -90 |
+------------+-------------+-------------+--------------+------------+-------------+使用示例：静态数据示例
--返回NULL。
select log2(null);
--返回NULL。
select log2(0);
--返回3.0。
select log2(8);使用示例：表数据示例
基于示例数据，计算所有列以2为底的对数，命令示例如下。
--开启2.0新类型。此命令需要与SQL语句一起提交。
set odps.sql.type.system.odps2=true;
select log2(int_data) as int_new, log2(bigint_data) as bigint_new, log2(double_data) as double_new, log2(decimal_data) as decimal_new, log2(float_data) as float_new, log2(string_data) as string_new from mf_math_fun_t;返回结果如下。
+--------------------+--------------------+----------------------+--------------------+----------------------+--------------------+
| int_new | bigint_new | double_new | decimal_new | float_new | string_new |
+--------------------+--------------------+----------------------+--------------------+----------------------+--------------------+
| NULL | NULL | -0.929610672108602 | -0.929610672108602 | -0.9296107376258038 | 3.3219280948873626 |
| NULL | NULL | NULL | NULL | NULL | NULL |
| NULL | NULL | NULL | 4.354028938054387 | NULL | 4.906890595608519 |
| NULL | 2.0 | -0.16812275880832692 | NULL | -0.16812278199699915 | NULL |
| 2.321928094887362 | NULL | NULL | NULL | NULL | 5.643856189774724 |
| NULL | 2.584962500721156 | 0.5849625007211562 | 0.5849625007211562 | 0.5849625007211562 | NULL |
| NULL | NULL | NULL | NULL | NULL | NULL |
| NULL | 0.0 | NULL | NULL | NULL | NULL |
| 3.1699250014423126 | NULL | 1.3673710656485296 | 1.3673710656485296 | 1.367371022986166 | NULL |
| NULL | 3.3219280948873626 | NULL | NULL | NULL | NULL |
+--------------------+--------------------+----------------------+--------------------+----------------------+--------------------+相关函数
LOG2函数属于数学函数，更多数据计算、数据转换的相关函数请参见数学函数。