How Can I Combine Separate Date and Time Fields into a Single DateTime Column in MS SQL Server?

Merging Date and Time Columns in MS SQL Server

Data extracts sometimes present dates and times in separate columns. This article demonstrates how to efficiently combine these into a single datetime column within MS SQL Server.

The Solution:

A straightforward method involves directly adding the date and time fields. However, this requires two key assumptions:

1. The date column's time component must always be midnight (00:00:00).
2. The time column's date component must be the base date (January 1, 1900).

If these conditions are met, the following SQL query will successfully merge the data:

<code class="language-sql">SELECT CombinedDateTime = MyDate + MyTime FROM MyTable;</code>

Explanation:

This works because of the internal representation of the datetime data type. It uses two 4-byte integers; one for the date and one for the time. Adding them directly produces the correct combined datetime value.

Important Notes:

• SQL Server 2008 and later: If using separate date and time data types (available from SQL Server 2008 onwards), explicitly cast both fields to datetime before addition: CAST(MyDate AS DATETIME) CAST(MyTime AS DATETIME).
• Precision Loss: In SQL Server 2008 and later, adding datetime and time might lead to precision loss. For a solution that preserves precision, refer to [relevant documentation/article](link_to_relevant_article_or_documentation_here - replace with actual link if available). This would involve using functions like DATETIMEFROMPARTS for more granular control.

This approach provides a simple and effective solution for combining date and time fields under the specified conditions. For scenarios with more complex data, alternative methods may be necessary.

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