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

Merge date and time fields in MS SQL Server

When working with data that contains separate date and time columns, you need to merge them into a single datetime field. This article will explore how to achieve this in MS SQL Server.

Use zero padding to merge fields

If the time component of the "Date" column is all zero and the date component of the "Time" column is all zero (i.e. base date: January 1, 1900), we can simply add them to get the correct result :

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

This works because the date and time are stored as two 4-byte integers, with the left 4 bytes representing the date and the right 4 bytes representing the time. By adding them together, we effectively combine the date and time components.

Merge fields using SQL Server 2008 types

In SQL Server 2008 and later, the Date and Time data types were introduced. If you use these types, you can add them by casting them to datetime:

<code class="language-sql">SELECT Combined = CAST(MyDate AS DATETIME) + CAST(MyTime AS DATETIME) FROM MyTable</code>

Prevent accuracy loss

In SQL Server 2008 and later, adding date and time values ​​may result in loss of precision. To prevent this, one of the values ​​should be promoted to datetime2 type using the CONVERT function:

<code class="language-sql">SELECT Combined = CONVERT(datetime2, MyDate) + MyTime FROM MyTable</code>

By following these methods, you can effectively combine date and time columns into a single datetime field in MS SQL Server, regardless of the data type used.

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