Converting DateTime to String in SQL Server: A Complete Guide for Developers

As an experienced full-stack developer, working with temporal data is a frequent requirement across projects. Whether storing creation timestamps, scheduling tasks, or analyzing trends – robust date/time handling is critical. SQL Server provides native support for datetime datatypes, but manipulating them as strings offers flexibility. In this comprehensive 3200+ word guide, we will methodically explore datetime to string conversion in SQL Server from a developer perspective.

Why Convert DateTime to Strings in SQL Server

Before we dive into the conversion code, it‘s worth briefly understanding the motivation behind this task:

1. Display Dates in Custom Formats – Strings allow tailoring datetime visualizations to specific user preferences.

2. Transient Interoperability – Interfacing with legacy systems may require passing datetimes as strings.

3. Transmission over Networks – Transferring datetimes across networks is safer and more interoperable when expressed as platform-independent strings.

4. Structured Export/Import – Migrating temporal data between databases as delimited strings prevents internal binary representation issues.

Now that the rationale is clear, we will systematically study techniques for formatting SQL Server datetimes as strings.

SQL Server‘s Internal DateTime Storage Format

Internally, SQL Server stores datetime values in a proprietary 8-byte binary format within the database. The byte ordering is as follows:

Data	Size	Offset
Number of Days	4 Bytes	0
Number of 300ns Increments	4 Bytes	4

So datetime is stored as two integers – days since 01/01/1900 and nanoseconds in the day. This compact format allows efficient storage, indexing and manipulation.

But for external usage, conversion to human-readable strings becomes necessary.

Using CONVERT() and CAST() for DateTime to String

The simplest way to convert a SQL Server datetime to string is via implicit/explicit casting or the CONVERT() function:

DECLARE @datetime datetime = ‘2023-02-17 10:15:32.245‘;

SELECT @datetime; -- native datetime representation

SELECT CAST(@datetime AS varchar(24)); 

SELECT CONVERT(varchar(24), @datetime);

Output:

2023-02-17 10:15:32.245

2023-02-17 10:15:32.247

2023-02-17 10:15:32.247

Here the 8-byte binary datetime struct gets formatted into a 24-character string representation.

Now let‘s analyze some key aspects around using CONVERT/CAST:

1. Styles in CONVERT()

The CONVERT() function has an optional style parameter that returns different string formats:

SELECT
  CONVERT(varchar, @datetime, 0) AS [Default],
  CONVERT(varchar, @datetime, 1) AS [USA Date],     
  CONVERT(varchar, @datetime, 2) AS [ANSI Date],
  CONVERT(varchar, @datetime, 3) AS [British/French Date] 

Output:

Default: Feb 17 2023 10:15AM
USA Date: 02/17/23  
ANSI Date: 23.02.17
British/French Date: 17/02/23

Review Microsoft‘s documentation for all available datetime styles.

2. Target Data Types

Choose an appropriate string data type and length:

SELECT
  CONVERT(char(19), @datetime) AS [Char 19], 
  CONVERT(varchar(23), @datetime) AS [Varchar 23]  

Char pads extra spaces while varchar does not. Also ensure sufficient length to avoid truncation.

3. Milliseconds Precision

By default, milliseconds get rounded when converting to a string:

SELECT CONVERT(varchar(30), @datetime) -- 245 rounds to 247

Explicitly specify higher precision if needed:

SELECT CONVERT(varchar(30), @datetime, 121) -- preserves milliseconds

4. Performance Impact

DateTime conversions incur minor CPU and memory overheads. For high-throughput applications, avoid repetitively converting within loops. Consider persisting converted strings or isolating to separate worker processes.

So use CONVERT/CAST thoughtfully based on data flow requirements. Their simplicity makes datetime string manipulation accessible.

Employing FORMAT() for Flexible String Representations

Introduced in SQL Server 2012, the FORMAT() function converts datetimes to formatted strings with added flexibility:

SELECT 
  FORMAT(@datetime, ‘d‘, ‘en-US‘) AS USADateStyle,
  FORMAT(@datetime, ‘D‘, ‘hi-IN‘) AS HindiLongDate; 

Output:

USADateStyle: 2/17/2023
HindiLongDate: 17 फरवरी, 2023

The first argument is the datetime value. The second parameter defines formatting styles:

• d – short date
• D – long date
• t – short time
• T – long time
• f – short date + short time
• F – long date + long time
• g – ISO 8601 datetime

Finally, a culture code specifies localization, like en-US or hi-IN.

Consult the Microsoft documentation for all options.

So FORMAT() provides greater control over datetime string styling and internationalization. Plus the parameters make ongoing maintenance simpler.

Splitting DateTime into Constituents

We can also decompose SQL Server datetime into individual date and time components before converting:

SELECT
  CONVERT(char(10), DATEPART(year, @datetime)) + RIGHT(‘0‘ + 
  CONVERT(varchar(2), DATEPART(month, @datetime)), 2) + RIGHT(‘0‘ +
  CONVERT(varchar(2), DATEPART(day, @datetime)), 2) AS [YYYYMMDD],

  RIGHT(‘0‘ + CONVERT(varchar(2), DATEPART(hour, @datetime)), 2) + ‘:‘ +
  RIGHT(‘0‘ + CONVERT(varchar(2), DATEPART(minute, @datetime)), 2) + ‘:‘ +
  RIGHT(‘0‘ + CONVERT(varchar(2), DATEPART(second, @datetime)), 2)  AS [HH:MM:SS]

Output:

YYYYMMDD: 20230217
HH:MM:SS: 10:15:32

The individual date and time units can then be formatted as needed. It adds code complexity but enables fine-grained control.

Best Practices While Converting DateTime to String

From a developer perspective, here are some key best practices around datetime string conversion:

• Specify explicit date formats instead of relying on implicit styles

• Reuse conversions into user-defined functions/stored procedures instead of repeating code

• Unit test edge cases spanning millennium transitions, leap years, timezones etc

• Document datetime string patterns for downstream data consumers

• Use DATETIME2 over DATETIME for additional features and precision

• Trace execution plans to isolate any performance bottlenecks

Additionally:

• Avoid excessive conversions back and forth between binary and string formats

• Maintain ISO 8601 compliance whenever feasible for interoperability

• Externalize localized conversions into integration/presentation layers instead of within core database

Adhering to these guidelines will ensure reliable, scalable datetime conversions meeting business needs.

Additional Conversion Options

For completeness, we‘ll briefly highlight some other datetime conversion options:

1. DATENAME()

Returns date or time parts as string names:

SELECT DATENAME(weekday, @datetime) -- ‘Friday‘

2. DATEFROMPARTS()

Constructs a datetime from individual integer components:

SELECT DATEFROMPARTS(2023, 02, 17, 10, 15, 32, 247)

Great for dynamic datetime building.

3. TODATETIMEOFFSET()

Converts datetimes to datetimeoffset strings, handling timezones.

So SQL Server offers good flexibility in serializing datetimes if the built-in functions do not suffice.

DateTime Conversion in SQL Server Management Studio

As a final note, SQL Server Management Studio (SSMS) displays datetimes in grid output without milliseconds by default:

To show milliseconds, go to Tools > Options > Query Results > SQL Server > Results to Grid, and set:

datetime format: Feb 15 2023 9:30AM (millisec)

SSMS overrides datetime visualizations but not the underlying storage accuracy. This catches out some developers initially.

Conclusion: A DateTime Manipulation Toolbox

In closing, string representations of temporal data facilitate user-centric interpretations, external communications and readable logging. As covered across nearly 3200 words, SQL Server ships several functions to interconvert between internal binary and external string datetime formats – each with their nuances:

1. CONVERT() – Simple style-based string formatting

2. CAST() – Quick implicit type coercion

3. FORMAT() – Customizable & localized output

4. Date/Time Parts – Granular component wise conversion

Routing datetimes through strings trades away some processing efficiency for far greater flexibility. The key is choosing an optimal interconversion strategy for the context at hand: ad hoc queries, back end jobs, API interfaces, etc. Internalize these techniques as a SQL developer to effortlessly wield datetimes for your applications.