官网：https://www.cloudera.com/documentation/enterprise/latest/topics/impala_math_functions.html
转载链接1：https://blog.csdn.net/qq_24699959/article/details/79863664
转载链接2：https://blog.csdn.net/qq_24699959/article/details/80090050
https://my.oschina.net/weiqingbin/blog/189413#OSC_h2_2
Impala数据类型：https://blog.csdn.net/yidu_fanchen/article/details/78295499
1、字符串截取substr，从1开始，而不是0；注意一个汉字长度是3
select brand,substr(brand,1,6) from dw_bill_his limit 10;
2、cast函数
cast(expr AS type), 类型转换函数, 比如将number转成string, 或相反.
select cast(length as int) len from dw_bill_his where length != '无' and startdate='2018-09-01' order by cast(length as int);
3、max,min,avg函数：length字段是字符串类型
select max(cast(length as int)) len from dw_bill_his where length!='无' and startdate='2018-09-01';
select min(cast(length as int)) len from dw_bill_his where length!='无' and startdate='2018-09-01';
select avg(cast(length as int)) len from dw_bill_his where length!='无' and startdate='2018-09-01';
4、截取数值，四舍五入
select dround(2.14123,3) result;
select dround(2.14123,2) result;
取整
select dround(2.14123) result;
5、删除所有小数点以后的数或删除N位小数
select truncate(3.45);
select truncate(3.456,1)
6、返回表达式列表中的最大值：greatest
select greatest(5,16,2) as greatest;
7、返回表达式列表中的最小值： least
select least(5,16,2) as least;
8、like 模糊查询
select count(*) from dw_bill_his where city='北京' and broadcastdate like '2018/03%';
9、字符串截取，substr(str,startindex,length) startindex从1开始
select substr('2018-08-20',1,4) year1;
10、字符串连接 concat(string a,string b…)
拼接多个字符串
--连接hello和world两个字符串
select concat('hello','world') as concat
concat_ws(string sep,string a,string b…)
拼接多个字符串，由指定分隔符分割
--通过'-'连接两个字符串
select concat_ws('-','hello','world') as concat_ws;
11、字符串长度 length(string a)
select length('world') as len;
12、给表增加一列：
ALTER TABLE name ADD COLUMNS (col_spec[, col_spec ...])
比如给表dw_bill增加一个float类型的week列
ALTER TABLE dw_bill ADD COLUMNS(week FLOAT);
13、删除一列
ALTER TABLE name DROP [COLUMN] column_name
比如删除dw_bill的week列表
ALTER TABLE dw_bill DROP week;
14、字符串去空格
去左空格: select ltrim(' hello ');
去右空格: select rtrim(' hello ');
去左右空格： select trim(' hello ');
15、查询某个字段为null的记录条数
select count(1) from dw_bill where brand is null;
不为null的记录条数
select count(1) from dw_bill where brand is not null;