Hive（五）—

Hive（五）——查询

SELECT… FROM语句，一般情况不再赘述，以下展示复合数据结构如何取值

# 先提供几条数据与建表语句,方便随手练习
John Doe!100000.0!Mary Smith$Todd Jones!Federal Taxes,0.2$State Taxes,0.05$Insurance,0.1!1 Michigan Ave.$Chicago$IL$60600
Mary Smith!80000.0!Bill King!Federal Taxes,0.2$State Taxes,0.05$Insurance,0.1!100 Ontario St.$Chicago$IL$60601
Todd Jones!70000.0!lili!Federal Taxes,0.15$State Taxes,0.03$Insurance,0.1!200 Chicago Ave.$Oak Park$IL$60700
Bill King!60000.0!Huahua$Xixi!Federal Taxes,0.15$State Taxes,0.03$Insurance,0.1!300 Obscure Dr.$Obscuria$IL$60100CREATE TABLE employees (name STRING,salary FLOAT,subordinates ARRAY<STRING>,deductions MAP<STRING, FLOAT>,address STRUCT<street:STRING, city:STRING, state:STRING, zip:INT>) ROW FORMAT DELIMITED FIELDS TERMINATED BY '!' COLLECTION ITEMS TERMINATED BY '$' MAP KEYS TERMINATED BY ',' LINES TERMINATED BY '\n' STORED AS TEXTFILE;

接下来就是查询复合数据类型的值

# 先看下描述信息
hive (default)> desc employees;
name                    string
salary                  float
subordinates            array<string>
deductions              map<string,float>
address                 struct<street:string,city:string,state:string,zip:int># 查询array类型数据
hive (default)> select subordinates from employees;
["Mary Smith","Todd Jones"]
["Bill King"]
["lili"]
["Huahua","Xixi"]
hive (default)> select subordinates[0] from employees;
Mary Smith
Bill King
lili
Huahua# 查询map类型数据
hive (default)> select deductions from employees;
{"Federal Taxes":0.2,"StateTaxes":0.05,"Insurance":0.1}
{"Federal Taxes":0.2,"State Taxes":0.05,"Insurance":0.1}
{"Federal Taxes":0.15,"State Taxes":0.03,"Insurance":0.1}
{"Federal Taxes":0.15,"State Taxes":0.03,"Insurance":0.1}
hive (default)> select deductions['State Taxes'] from employees;
0.05
0.05
0.03
0.03# 查询struct类型数据
hive (default)> select address from employees;
{"street":"1 Michigan Ave.","city":"Chicago","state":"IL","zip":60600}
{"street":"100 Ontario St.","city":"Chicago","state":"IL","zip":60601}
{"street":"200 Chicago Ave.","city":"Oak Park","state":"IL","zip":60700}
{"street":"300 Obscure Dr.","city":"Obscuria","state":"IL","zip":60100}
hive (default)> select address.city from employees;
Chicago
Chicago
Oak Park
Obscuria

使用正则表达式来指定列

# 需要先设置属性才能使用正则
set hive.support.quoted.identifiers=none;# 查询所有`s`开头的列的数据
hive (default)> select name,`s.*` from employees;
John Doe        100000.0        ["Mary Smith","Todd Jones"]
Mary Smith      80000.0 ["Bill King"]
Todd Jones      70000.0 ["lili"]
Bill King       60000.0 ["Huahua","Xixi"]

使用列值进行计算
```
hive (default)> select upper(name),salary,deductions['Federal Taxes'],round(salary*(1-deductions['Federal Taxes'])) from employees;
JOHN DOE        100000.0        0.2     80000.0
MARY SMITH      80000.0 0.2     64000.0
TODD JONES      70000.0 0.15    59500.0
BILL KING       60000.0 0.15    51000.0
```
当进行算术运算时，需要注意数据溢出或数据下溢问题，如果用户比较担心溢出和下溢，那么可以考虑在表模式中定义使用范围更广的数据类型。不过这样做的缺点是每个数据值会占用更多额外的内存。也可以使用特定的表达式将值转换为范围更广的数据类型。
使用函数
- 查看month 相关的函数
  
  show functions like ‘month’
- 查看 add_months 函数的用法
desc function add_months;
- 查看 add_months 函数的详细说明并举例
desc function extended add_months;

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