HiveQL: Joining on a Column Created in Your Select Statement

Introduction

Hive is an open-source data warehousing and SQL-like query language for Hadoop. When working with Hive, it’s common to create temporary columns or expressions during your queries. In this article, we’ll explore how to join tables based on a column created in your SELECT statement.

Understanding the Problem

The provided Stack Overflow question illustrates a scenario where a user wants to join two tables based on a calculated column created in their SELECT statement. The challenge arises when trying to join on this new column, as it doesn’t match any existing column name in either table.

SELECT 
    prvdr_num, 
    CONCAT(SUBSTR(CLM_ADMSN_DT, 5, 2),"/",SUBSTR(CLM_ADMSN_DT, 7, 
    2),"/",SUBSTR(CLM_ADMSN_DT, 1, 4)) as Calendar_Admit_date,
    AdmitDate,
    CMSId
FROM table1
INNER JOIN  table2
ON table1.CMSId = table2_lds.prvdr_num
AND table1.AdmitDate = 
table2.CONCAT(SUBSTR(CLM_ADMSN_DT, 5, 2),"/",SUBSTR(CLM_ADMSN_DT, 7, 
2),"/",SUBSTR(CLM_ADMSN_DT, 1, 4))

Solution: Casting as Date

The answer provided in the Stack Overflow question suggests casting the calculated column as a date type on the join. This approach allows Hive to recognize the new column as a date field and establish a valid join condition.

SELECT 
    prvdr_num, 
    CONCAT(SUBSTR(CLM_ADMSN_DT, 5, 2),"/",SUBSTR(CLM_ADMSN_DT, 7, 
    2),"/",SUBSTR(CLM_ADMSN_DT, 1, 4)) as Calendar_Admit_date,
    AdmitDate,
    CMSId
FROM table1
INNER JOIN  table2
ON table1.CMSId = table2_lds.prvdr_num
AND table1.AdmitDate = 
cast(CONCAT(SUBSTR(table2.CLM_ADMSN_DT, 5, 2),"/",SUBSTR(table2.CLM_ADMSN_DT, 7, 
    2),"/",SUBSTR(table2.CLM_ADMSN_DT, 1, 4)) as date)

How It Works

When casting the calculated column as a date, Hive converts it into a format that can be compared with other dates in the query. This is possible because dates are stored internally by Hive in a fixed format.

The key insight here is that Hive doesn’t care about the actual values inside the Calendar_Admit_date column; it only cares about its type and structure. By casting this column as a date, we’re telling Hive to treat it as a date value, which allows us to join on it effectively.

Additional Considerations

It’s essential to keep in mind that when creating calculated columns or expressions in your queries, you should consider the following:

Data Type: Ensure that the data type of the new column matches the expected format. In this case, we cast Calendar_Admit_date as a date.
Format: Be mindful of the date format used in your calculated column and ensure it aligns with the expected format in the other table.
Query Optimization: Keep in mind that using casts or calculations in your join conditions may impact query performance. It’s essential to balance data consistency with optimal query execution.

Conclusion

Joining tables based on a column created in your SELECT statement can be challenging, but understanding how to cast calculated columns as dates and applying the correct data types and formats can help you achieve successful joins. By following these guidelines and considering additional factors, you’ll be better equipped to tackle complex queries and extract meaningful insights from your data.

Common Pitfalls and Alternatives

Pitfall 1: Failing to cast calculated columns as dates when joining on them.
- Alternative: Always include casts or ensure that the new column matches an existing column in both tables.
Pitfall 2: Ignoring data type mismatches between calculated columns and existing table columns.
- Alternative: Verify that the data types align, and consider modifying column formats if necessary.

Best Practices

Use casts or modifications to ensure data consistency across your queries.
Consider query optimization techniques when joining tables with complex conditions.
Always verify that the data types of new columns match expected formats in both tables.

Last modified on 2023-09-14