Converting Date Stored as VARCHAR to datetime in SQL

As a technical blogger, it’s not uncommon to encounter databases that store date and time data as strings rather than as actual datetime values. This can make filtering and querying the data more challenging. In this article, we’ll explore how to convert date stored as VARCHAR to datetime in SQL, focusing on a specific example using the Stack Overflow post provided.

Introduction

In many real-world applications, dates and times are not always stored in the most convenient format. For instance, some databases or programming languages might store dates and times as strings rather than as actual datetime values. This can make filtering and querying the data more difficult. In this article, we’ll discuss how to convert date stored as VARCHAR to datetime in SQL.

Understanding the Problem

Let’s take a closer look at the example provided by the Stack Overflow user. They are trying to filter records from a table named timetrack where the date and time column (startDateAndTime) is stored as a VARCHAR string in the format “dd-mm-yyyy, hh:mm”. The user wants to apply a filter on this column to retrieve only records that fall within a certain date range.

The query they provided looks like this:

SELECT *
FROM timetrack
WHERE startDateAndTime >= '30-11-2017,7:30'

However, the problem with this query is that SQL Server (or any other RDBMS) does not have an automatic way to convert a VARCHAR string to a datetime value without explicit conversion.

Converting Date Stored as VARCHAR to datetime

To fix this issue, we can use the PARSE function in SQL Server (available starting from Management Studio 2012). This function allows us to specify a format for converting a VARCHAR string to a datetime value.

Here’s how it works:

• We use the PARSE function with three arguments: startDateAndTime, datetimeFormat, and culture.
• The startDateAndTime parameter is the string we want to convert, which in our case is “30-11-2017,7:30”.
• The datetimeFormat parameter specifies the format of the datetime value. In this example, we use 'it-IT', which corresponds to the Italian date and time format.
• The culture parameter allows us to specify a culture that determines how the conversion is performed.

Here’s an example query:

SELECT *
FROM timetrack
WHERE parse(startDateAndTime as datetime using 'it-IT') >= '2017-11-30 07:30:00.000'

In this query, we’re applying a filter on the startDateAndTime column to retrieve only records where the parsed datetime value is greater than or equal to the specified date and time.

Understanding Date Formats

Before we dive into the example query, let’s quickly review how different cultures format dates. In our example, we used 'it-IT', which corresponds to the Italian date and time format. Here are a few examples of other common formats:

• en-US (English-speaking United States)
• fr-FR (French-speaking France)
• de-DE (German-speaking Germany)

Working with Different Date Formats

If you need to work with dates in different formats, you’ll want to consider the following:

1. Specify your own format: Always specify a format for the conversion, even if it’s just ‘default’. This ensures that the conversion is accurate.
2. Use culture-specific values: Choose a culture-specific value when using the parse function to ensure the correct formatting for your data.
3. Test with sample data: Verify that the conversion works correctly with sample data and different formats.

Best Practices

Here are some best practices to keep in mind when converting date stored as VARCHAR to datetime:

1. Use meaningful column names: Use descriptive column names, like startDateAndTime, instead of abbreviations or shortened versions.
2. Validate user input: Always validate user input for dates and times to ensure accuracy.
3. Document your schema: Document the structure and formats of your database schema to avoid confusion.

Common Issues

Here are some common issues you might encounter when converting date stored as VARCHAR to datetime:

1. Incorrect formatting: Make sure to specify a format that matches the actual data in your table.
2. Missing or null values: Handle missing or null values correctly, especially if they’re not immediately apparent.
3. Invalid dates: Be prepared for invalid dates and handle them accordingly.

Alternative Solutions

If you can’t use the parse function (for example, if you’re using an older version of SQL Server), here are some alternative solutions:

1. Use a custom date conversion function: Create a user-defined function that takes a VARCHAR string as input and returns the corresponding datetime value.
2. Cast to a compatible type: If your database supports it, cast the VARCHAR column to a datetime-compatible data type, like datetime2.
3. Split the string into separate columns: Split the VARCHAR string into two separate columns for date and time, then apply filtering or conversion as needed.

By understanding how to convert date stored as VARCHAR to datetime in SQL, you can improve your database queries and make data analysis more efficient. Remember to always specify a format and use meaningful column names to avoid confusion.

Last modified on 2024-04-04