Understanding SQL Insert Queries with Case Statements: A Comprehensive Guide

Understanding SQL Insert Queries with Case Statements

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When it comes to inserting data from one table into another, using a case statement can be an effective way to map values from the original table to specific columns in the target table. In this article, we’ll explore how to use case statements in SQL insert queries and provide a detailed example of how to achieve this.

Background on Case Statements

A case statement is a control structure used in SQL that allows you to execute different blocks of code based on conditions. The general syntax for a case statement is as follows:

CASE condition THEN value WHEN condition THEN value ELSE value END;

In the context of an insert query, a case statement can be used to map values from one column to multiple columns in another table. This allows you to transform the data in real-time without having to explicitly define every possible mapping.

The Problem with Case Statements

The problem arises when we try to use a single case statement to map all possible combinations of values to specific columns. In this case, SQL does not support this directly, as it requires converting the columns into rows and back into columns.

Converting Columns to Rows

To solve this issue, we can convert the columns into rows using an outer apply clause. This allows us to pivot the data, applying different values from each column to a specific column based on conditions.

PIVOT Clause

The PIVOT clause is used to rotate the data so that the columns become the new row headers. The general syntax for a pivot clause is as follows:

SELECT [column1], [column2], ... 
FROM table_name
PIVOT (
    MAX(value) FOR position IN ([column1], [column2], ...)
) AS pivotTable;

In this example, we’re selecting multiple columns and pivoting the data so that each column becomes a new row. The MAX function is used to aggregate the values.

Converting Rows Back into Columns

After pivoting the data, we need to convert it back into columns using another outer apply clause.

ROW_NUMBER() Function

The ROW_NUMBER() function assigns a unique number to each row within a result set.

SELECT ROW_NUMBER() OVER (ORDER BY pos) AS position, val FROM (
    SELECT 1 pos, CASE WHEN slp_c = 'Y' THEN 'SLP' ELSE NULL END val
    UNION ALL SELECT 2 pos, CASE WHEN ot_c = 'Y' THEN 'OT' ELSE NULL END val 
    UNION ALL SELECT 3 pos, CASE WHEN SpecEd_c = 'Y' THEN 'SE' ELSE NULL END val
    UNION ALL SELECT 4 pos, CASE WHEN medical_c = 'Y' THEN 'MED' ELSE NULL END val
    UNION ALL SELECT 5 pos, CASE WHEN pt_c = 'Y' THEN 'PT' ELSE NULL END val
    UNION ALL SELECT 6 pos, CASE WHEN sw_c = 'Y' THEN 'SW' ELSE NULL END val
    UNION ALL SELECT 7 pos, CASE WHEN psych_c = 'Y' THEN 'PSY' ELSE NULL END val
    UNION ALL SELECT 8 pos, CASE WHEN other_c = 'Y' THEN 'OTH' ELSE NULL END val
) AS allDisciplines 
WHERE val IS NOT NULL;

This allows us to select the position and value of each row in a specific column.

Putting It All Together

Now that we’ve covered how to convert columns into rows using PIVOT and how to convert rows back into columns using ROW_NUMBER(), we can put it all together to create an insert query with a case statement.

Here’s the complete example:

SELECT uniqueid_c,
    [1] AS Discipline1_c, 
    [2] AS Discipline2_c,
    [3] AS Discipline3_c            
FROM @tb_cdsa_eligibility 
OUTER APPLY (
    SELECT [1], [2], [3] FROM (
        SELECT ROW_NUMBER() OVER (ORDER BY pos) AS position, val FROM (
            SELECT 1 pos, CASE WHEN slp_c = 'Y' THEN 'SLP' ELSE NULL END val
            UNION ALL SELECT 2 pos, CASE WHEN ot_c = 'Y' THEN 'OT' ELSE NULL END val 
            UNION ALL SELECT 3 pos, CASE WHEN SpecEd_c = 'Y' THEN 'SE' ELSE NULL END val
            UNION ALL SELECT 4 pos, CASE WHEN medical_c = 'Y' THEN 'MED' ELSE NULL END val
            UNION ALL SELECT 5 pos, CASE WHEN pt_c = 'Y' THEN 'PT' ELSE NULL END val
            UNION ALL SELECT 6 pos, CASE WHEN sw_c = 'Y' THEN 'SW' ELSE NULL END val
            UNION ALL SELECT 7 pos, CASE WHEN psych_c = 'Y' THEN 'PSY' ELSE NULL END val
            UNION ALL SELECT 8 pos, CASE WHEN other_c = 'Y' THEN 'OTH' ELSE NULL END val
        ) AS allDisciplines 
        WHERE val IS NOT NULL
    ) AS notNullDisciplines
    PIVOT (
        MAX(val) FOR position IN ([1], [2], [3])
    ) AS pivotTable
) AS pivotedDisciplines
ORDER BY uniqueid_c;

This query will transform the data in @tb_cdsa_eligibility and insert it into a new table, applying different values from each column to a specific column based on conditions.

Conclusion

In conclusion, using case statements in SQL insert queries can be an effective way to map values from one column to multiple columns. However, when dealing with complex data transformations, converting the columns into rows and back into columns using PIVOT and ROW_NUMBER() functions is often a more suitable approach.

By following this article, you should now have a better understanding of how to use case statements in SQL insert queries and how to transform data using pivot tables.

Last modified on 2024-05-19