Merging Results from Multiple Columns into One: A SQL Server 2012 Solution

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As a developer, working with complex databases and queries can be daunting. In this article, we will delve into the world of SQL Server 2012 and explore how to merge results from three columns into one. We’ll examine the code snippets provided in the original Stack Overflow post, understand the challenges faced by the user, and discuss potential solutions using UNION, UNION ALL, and other techniques.

Understanding the Challenge

The problem statement is quite straightforward: combine the values from three columns (Column 1, Column 2, and Column 3) into a single column. The catch lies in understanding how to handle NULL values and ensure that the resulting column has consistent data types.

Let’s break down the original query:

SELECT
    CASE
        WHEN r.scode IS NULL THEN ''
        ELSE r.scode
    END 'Column 1',
    CASE 
        WHEN ( u.scode IS NULL AND c.itype = 4)  THEN ''
        ELSE u.scode
    END 'Column 2',
    CASE 
        WHEN ( f.scode IS NULL AND c.itype = 10)  THEN ''
        ELSE f.scode
    END 'Column 3'
FROM CapsConfig c
INNER JOIN CapsXref x
    ON c.hmy = x.hcapsconfig
INNER JOIN Prop p
    ON c.hprop = p.hmy
LEFT OUTER JOIN ChgTyp t
    ON x.hchgcode = t.hmy
LEFT OUTER JOIN Tenant r
    ON c.hpointer = r.hmyperson
LEFT OUTER JOIN Unit u
    ON c.hpointer = u.hmy
LEFT OUTER JOIN UnitType f
    ON c.hpointer = f.hmy;

The query joins multiple tables and uses LEFT OUTER JOINs to include NULL values from the joined tables. However, the user wants to combine these columns into a single column without NULL values.

Examining the Proposed Solution

The answer provided suggests using a slightly modified version of the original query:

SELECT
    CASE
        WHEN r.scode IS NULL THEN ''
        ELSE r.scode
    END 'Column 1',
    CASE 
        WHEN ( u.scode IS NULL AND c.itype = 4)  THEN ''
        ELSE u.scode
    END 'Column 2',
    CASE 
        WHEN ( f.scode IS NULL AND c.itype = 10)  THEN ''
        ELSE f.scode
    END 'Column 3'
FROM CapsConfig c
INNER JOIN CapsXref x
    ON c.hmy = x.hcapsconfig
INNER JOIN Prop p
    ON c.hprop = p.hmy
LEFT OUTER JOIN ChgTyp t
    ON x.hchgcode = t.hmy
LEFT OUTER JOIN Tenant r
    ON c.hpointer = r.hmyperson
LEFT OUTER JOIN Unit u
    ON c.hpointer = u.hmy
LEFT OUTER JOIN UnitType f
    ON c.hpointer = f.hmy;

At first glance, this seems like a direct copy-paste solution. However, there’s an important distinction between the original query and this proposed solution.

UNION vs. UNION ALL

The key difference lies in how NULL values are handled during the merging process.

• UNION eliminates duplicate rows and returns only distinct results.
• UNION ALL also returns duplicate rows and keeps all results, including NULL values.

In this case, we want to combine the values from Column 1, Column 2, and Column 3 into a single column without NULL values. This implies that we need to use UNION ALL instead of UNION.

However, using UNION ALL alone might not be enough. We also need to handle the case where one or more columns have NULL values.

Handling NULL Values

To ensure that our final column has consistent data types and no NULL values, we can use a combination of COALESCE and ISNULL functions.

SELECT
    COALESCE(r.scode, '') AS Combined,
    COALESCE(u.scode, '') AS Column 2,
    COALESCE(f.scode, '') AS Column 3
FROM CapsConfig c
INNER JOIN CapsXref x
    ON c.hmy = x.hcapsconfig
INNER JOIN Prop p
    ON c.hprop = p.hmy
LEFT OUTER JOIN ChgTyp t
    ON x.hchgcode = t.hmy
LEFT OUTER JOIN Tenant r
    ON c.hpointer = r.hmyperson
LEFT OUTER JOIN Unit u
    ON c.hpointer = u.hmy
LEFT OUTER JOIN UnitType f
    ON c.hpointer = f.hmy;

In this revised query, COALESCE returns the first non-NULL value from the arguments. If all values are NULL, it returns the second argument (in this case, an empty string). This ensures that our final column has consistent data types and no NULL values.

Conclusion

Merging results from multiple columns into one can be a challenging task, especially when dealing with NULL values. By understanding how to handle NULL values using UNION ALL and combining functions like COALESCE, we can create a single column with consistent data types and no NULL values.

In this article, we examined the original query provided in the Stack Overflow post, discussed potential solutions using UNION and UNION ALL, and explored alternative approaches for handling NULL values. With these techniques, you should be able to tackle similar challenges when working with complex databases and queries.

Last modified on 2023-05-13