Migrating OUTER APPLY from SQL Server to Snowflake: A Deep Dive

As a technical blogger, it’s essential to address the challenges of migrating queries between different databases. In this article, we’ll explore the complexities of converting an OUTER APPLY query from SQL Server to Snowflake. We’ll delve into the details of Snowflake’s syntax and provide actionable advice for overcoming common hurdles.

Understanding OUTER APPLY in SQL Server

Before diving into the Snowflake equivalent, let’s recap how OUTER APPLY works in SQL Server:

SELECT ...
FROM table1 t1
OUTER APPLY (
    SELECT top 1 column1
    FROM table2 t2
    WHERE condition
) t2;

In this example, table1 is joined to table2 using an OUTER APPLY. The outer query selects columns from t1, and the inner query returns a single row from t2. This allows you to perform complex calculations or aggregations on data that doesn’t exist in one table.

Snowflake’s Equivalent: LEFT JOIN

When migrating to Snowflake, you may be tempted to replace OUTER APPLY with a LEFT JOIN. However, this approach is not entirely accurate. A LEFT JOIN can return more rows than the number of non-matching rows in the outer table, whereas OUTER APPLY ensures that only one row from the inner table is returned.

To mimic the behavior of OUTER APPLY in Snowflake, you’ll need to create a Common Table Expression (CTE) with multiple subqueries. This approach allows you to perform calculations on data that doesn’t exist in one table while ensuring consistent results.

The CTE-Based Approach

Let’s examine the provided Snowflake code and identify areas for improvement:

WITH T1 AS (
    -- ...
),
T2 AS (
    -- calculate IdealRunRate here
)
SELECT 
    T1.CalculationID,
    T1.SourceEquipmentID,
    T1.Shift,
    T1.BeginTime,
    T1.EndTime,
    T1.UpTime,
    T1.DownTime,
    T1.IdealInput,
    T1.ActualInput,
    T1.ActualOutput,
    T1.BaseUoM,
    T1.ActualOutputOperationsUoM,
    T1.OperationsUoM,
    T1.RejectedOutput,
    T1.ScheduledOutput,
    T1.Failures,
    T1.POnumber,
    T1.Production_Date,
    T2.IdealRunRate
FROM T1
LEFT OUTER JOIN T2 ON T1.CalculationID = T2.CalculationID;

To fix the invalid identifier 'PS.IDEALRUNRATE' error, you need to ensure that the column name is correctly referenced. In this case, we’ll use the table alias (T2) to qualify the column name.

Additional Tips for Migrating OUTER APPLY

When migrating from SQL Server to Snowflake:

1. Be prepared for performance differences: Snowflake’s CTEs can be slower than SQL Server’s OUTER APPLY due to the way data is processed.
2. Understand data types and precision: Ensure that column names and data types match between SQL Server and Snowflake.
3. Use Snowflake-specific functions and operators: Take advantage of Snowflake’s optimized functions, such as DATE_TRUNC and EXTRACT.
4. Test thoroughly: Verify the accuracy of your migrated queries to avoid unexpected results.

By following these tips and understanding the intricacies of Snowflake’s syntax, you’ll be well-equipped to migrate complex OUTER APPLY queries with confidence.

Last modified on 2023-09-06