Introduction to Difference Tables

In this blog post, we’ll explore the concept of difference tables and how they can be used to optimize database queries. A difference table is a type of database schema that stores the changes made to data over time, allowing for efficient retrieval of historical data.

Problem Statement

Suppose we have two tables: T_Name and T_nameAttributes. The first table contains information about individuals (Name, NameID), while the second table contains attributes associated with each individual (AttributeID, Value). Initially, we retrieve data from these tables using a join operation.

SELECT *
FROM T_Name
LEFT JOIN T_nameAttributes
    ON T_name.NameID = T_nameAttributes.NameID
    AND AttributeID = 3 AND NameID = 2;

However, as the data changes over time, we need to update the queries to reflect these changes. This is where difference tables come into play.

Difference Tables

A difference table is a table that stores the changes made to data over time. It contains two columns: Date and either NameID or AttributeID, depending on which column is being tracked.

For our example, we’ll create three difference tables:

1. T_NameLog
2. T_nameAttributesLog

These tables will store the changes made to the T_Name and T_nameAttributes tables over time.

CREATE TABLE T_NameLog (
    Date DATE,
    Name VARCHAR(255),
    NameID INT
);

CREATE TABLE T_nameAttributesLog (
    Date DATE,
    AttributeID INT,
    NameID INT,
    Value VARCHAR(255)
);

Optimizing Queries

To optimize our queries and retrieve the most up-to-date rows from these tables, we’ll use a combination of techniques:

1. Generate a row per date: We’ll create a new table that contains one row for each date, with columns for Date, Name, and Attribute.
2. Cross Apply: We’ll use the CROSS APPLY operator to join this new table with the difference tables.
3. TOP (1) WITH TIES: We’ll use the TOP (1) WITH TIES clause to retrieve the most up-to-date rows from each table.

Query

Here’s the optimized query:

WITH days AS (
    SELECT cast('2023-10-02' as date) as dt,
           cast('2023-10-05' as date) as last_dt
    UNION ALL
    SELECT dateadd(day, 1, dt),
           last_dt
    FROM days
    WHERE dt < last_dt
)
SELECT days.dt, n.name, na.value
FROM days
CROSS APPLY (
    SELECT TOP (1) WITH TIES
        nl.name,
        nl.nameid
    FROM T_NameLog nl
    WHERE nl.[Date] <= days.dt
    -- and nl.nameid = 2
    ORDER BY RANK() OVER (PARTITION BY nl.nameid ORDER BY nl.[Date] DESC)
) n
CROSS APPLY (
    SELECT TOP (1) WITH TIES
        nal.value,
        nal.attributeid
    FROM T_nameAttributesLog nal
    WHERE nal.nameid = n.nameid
    AND nal.[Date] <= days.dt
    -- and nal.attributeid = 3
    ORDER BY RANK() OVER (PARTITION BY nal.attributeid ORDER BY nal.[Date] DESC)
) na
ORDER BY days.dt, n.nameid, na.attributeid;

Explanation

This query works as follows:

1. Generate a row per date: The days CTE generates a row for each date from ‘2023-10-02’ to ‘2023-10-05’.
2. Cross Apply: We use the CROSS APPLY operator to join this new table with the difference tables (T_NameLog and T_nameAttributesLog).
3. TOP (1) WITH TIES: The TOP (1) WITH TIES clause retrieves the most up-to-date rows from each table, handling ties in case of identical dates.

Example

Here’s an example of how this query works:

The query will return the most up-to-date rows for each date:

Conclusion

In this blog post, we explored the concept of difference tables and how they can be used to optimize database queries. We optimized a query to retrieve historical data from two tables with minimal changes over time. By using a combination of techniques such as generating rows per date, cross applying, and TOP (1) WITH TIES, we were able to improve the performance and efficiency of our query.

Resources

For more information on difference tables and optimized queries, you can refer to the following resources:

• Difference Tables
• Optimizing Queries with Difference Tables
• [SQL Server Query Optimizer](https://docs.microsoft.com/en-us/sql/relational-databases/performance/tuning-the-query optimizer?view=sql-server-ver15)

Last modified on 2025-02-07