Currently MySQL does not support ROW_NUMBER[] function that can assign a sequence number within a group, but as a workaround we can use MySQL session variables.
These variables do not require declaration, and can be used in a query to do calculations and to store intermediate results.
@current_country := country
This code is executed for each row and stores the value of country column to @current_country variable.
@country_rank := IF[@current_country = country, @country_rank + 1, 1]
In this code, if @current_country is the same we increment rank, otherwise set it to 1. For the first row @current_country is NULL, so rank is also set to 1.
For correct ranking, we need to have ORDER BY country, population DESC
So if we just execute the subquery:
SELECT city, country, population,
@country_rank := IF[@current_country = country, @country_rank + 1, 1] AS country_rank,
@current_country := country
We get the list of cities ranked by population within the country:
city country population country_rank current_country Paris France 2181000 1 France Marseille France 808000 2 France Lyon France 422000 3 France London United Kingdom 7825300 1 United Kingdom Birmingham United Kingdom 1016800 2 United Kingdom Leeds United Kingdom 770800 3 United Kingdom New York United States 8175133 1 United States Los Angeles United States 3792621 2 United States Chicago United States 2695598 3 United States
- Selecting Range
When we have a rank assigned to each city within its country, we can retrieve the required range:
Get top 2 for each country SELECT city, country, population FROM [/subquery above/] ranked WHERE country_rank set hive.support.quoted.identifiers=NONE;
And also enable property to print the columns as follows
hive> set hive.cli.print.header=true;
Here we are going to use below query to excluding the quit date column as follows and also we use the regulate expression to exclude columns
How do I get the first row of every group in SQL?
To select the first row of each group in SQL, you can use the ' GROUP BY ' clause with the ' MIN ' or ' MAX ' aggregate function.
What is the difference between SQL and Hive?
Hive and SQL DifferencesHive is better for analyzing complex data sets. SQL is better for analyzing less complicated data sets very quickly. SQL supports Online Transactional Processing [OLTP]. Hive doesn't support OLTP.
What is Hive in big data?
Hive is a data warehouse system that is used to query and analyze large datasets stored in the HDFS. Hive uses a query language called HiveQL, which is similar to SQL. As seen from the image below, the user first sends out the Hive queries.
What is querying data in Hive?
Hive enables data summarization, querying, and analysis of data. Hive queries are written in HiveQL, which is a query language similar to SQL. Hive allows you to project structure on largely unstructured data. After you define the structure, you can use HiveQL to query the data without knowledge of Java or MapReduce.