SQL练习

SQL练习：

1、count(*)、count(1) 、count('字段名') 区别

2、HQL 执行优先级：

from、where、 group by 、having、order by、join、select 、limit

3、where 条件里不支持子查询，实际上是支持 in、not in、exists、not exists

-- 列出与“SCOTT”从事相同工作的所有员工。
select  t1.EMPNO
        ,t1.ENAME
        ,t1.JOB
from emp t1
where t1.ENAME != "SCOTT" and t1.job in(
    select  job
    from emp
    where ENAME = "SCOTT");

select  t1.EMPNO
        ,t1.ENAME
        ,t1.JOB
from emp t1
where t1.ENAME != "SCOTT" and exists(
    select  job
    from emp t2
    where ENAME = "SCOTT"
    and t1.job = t2.job
);

4、hive中大小写不敏感

5、在hive中，数据中如果有null字符串，加载到表中的时候会变成 null （不是字符串）

如果需要判断 null，使用 某个字段名 is null 这样的方式来判断

或者使用 nvl() 函数，不能 直接 某个字段名 == null

6、使用explain查看SQL执行计划

explain select  t1.EMPNO
        ,t1.ENAME
        ,t1.JOB
from emp t1
where t1.ENAME != "SCOTT" and t1.job in(
    select  job
    from emp
    where ENAME = "SCOTT");
    
# 查看更加详细的执行计划，加上extended
explain extended select  t1.EMPNO
        ,t1.ENAME
        ,t1.JOB
from emp t1
where t1.ENAME != "SCOTT" and t1.job in(
    select  job
    from emp
    where ENAME = "SCOTT");

Hive JDBC

创建一个Maven项目，并添加依赖

<dependency>
    <groupId>org.apache.hadoop</groupId>
    <artifactId>hadoop-common</artifactId>
    <version>2.7.6</version>
</dependency>
<dependency>
    <groupId>org.apache.hive</groupId>
    <artifactId>hive-jdbc</artifactId>
    <version>1.2.1</version>
</dependency>

启动hiveserver2

hiveserver2

hive --service hiveserver2

编写并运行代码

import java.sql.*;

public class HiveJDBCDemo {
    public static void main(String[] args) throws ClassNotFoundException, SQLException {
        // 加载驱动
        Class.forName("org.apache.hive.jdbc.HiveDriver");
        // 创建连接(需要先启动 hiveserver2)
        // hive --service hiveserver2
        Connection conn = DriverManager.getConnection("jdbc:hive2://master:10000/test2");
        // 不需要用户名密码，直接创建statement
        Statement stat = conn.createStatement();
        ResultSet rs = stat.executeQuery("select id,name,age,gender,clazz from students limit 10");
        while (rs.next()) {
            int id = rs.getInt(1);
            String name = rs.getString(2);
            int age = rs.getInt(3);
            String gender = rs.getString(4);
            String clazz = rs.getString(5);
            System.out.println(id + "," + name + "," + age + "," + gender + "," + clazz);
        }

        rs.close();
        stat.close();
        conn.close();
    }
}

Hive数据类型

整型：TINYINT、SMALLINT、INT、BIGINT

浮点：FLOAT、DOUBLE

布尔类型：BOOL （False/True）

字符串：STRING

时间类型：

• 时间戳 timestamp
• 日期 date

create table testDate(
    ts timestamp
    ,dt date
) row format delimited fields terminated by ',';

// 2021-01-14 14:24:57.200,2021-01-11

复杂数据类型：

• array

create table testArray(
    name string,
    weight array<string>
)row format delimited 
fields terminated by '\t'
COLLECTION ITEMS terminated by ',';

select name,weight[0] from testArray;

杨老板	140,160,180
张志凯	160,200,180

• map

  key:value,key2:v2,k3:v3

create table scoreMap(
    name string,
    score map<string,int> 
)ROW FORMAT DELIMITED
FIELDS TERMINATED BY '\t'
COLLECTION ITEMS TERMINATED BY ','
MAP KEYS TERMINATED BY ':';

select name,score['语文'] from scoreMap;

小明    语文:91,数学:110,英语:40
小红    语文:100,数学:130,英语:140

• struct

create table scoreStruct(    name string,    score struct<course:string,score:int,course_id:int,tearcher:String> )ROW FORMAT DELIMITEDFIELDS TERMINATED BY '\t'COLLECTION ITEMS TERMINATED BY ',';select name,score.course,score.score from scoreStruct;小明    语文,91,000001,余老师小红    数学,100,000002,体育老师

https://blog.csdn.net/woshixuye/article/details/53317009

Hive HQL

DDL

DML

select id,name from tb t where ... and .... group by xxx having xxxx order by xxx asc/desc limit n;

• where ：过滤数据、！！！分区裁剪！！！

• join：left join、right join、join 注意MapJoin

  

• group by : 通常结合聚合函数一起使用

• order by：全局排序

  

• sort by：局部排序

  

• distribute by：分区

  

• cluster by

https://zhuanlan.zhihu.com/p/93747613 order by、distribute by、sort by、cluster by详解