Converting String Dates to Standard Format with Standard SQL's PARSE

Standard SQL String to Date Conversion

Standard SQL provides various functions and techniques to convert string representations of dates into a standard date format. In this article, we will explore the PARSE_DATE() function, its usage, and best practices for converting string dates in different SQL dialects.

Understanding the Problem

The problem at hand is to convert a string date formatted as “YYYYMMDD” (20190101) to the ISO 8601 format (“YYYY-MM-DD”). The goal is to achieve this conversion using standard SQL.

Legacy SQL Dialects

In legacy SQL dialects, such as MySQL and Oracle, it was possible to use the DATE_FORMAT() function or simple concatenation with date functions (e.g., date('20190101')) to achieve the desired result. However, these approaches are not portable across different SQL vendors.

Standard SQL Solutions

In standard SQL, we can leverage the PARSE_DATE() function to convert string dates to a standard format. This function is part of the SQL Standard and is supported by most modern SQL dialects, including PostgreSQL, Microsoft SQL Server, and others.

The PARSE_DATE() Function

The PARSE_DATE() function takes two arguments:

A format mask: This specifies the format of the input string date.
The input string date: This is the string date that needs to be converted.

The format mask is used to define the expected structure and formatting of the input string date. In this case, we use the %Y%m%d format mask, which represents:

YYYY: Four-digit year
m: Two-digit month (01-12)
d: Two-digit day (01-31)

Here is an example of how to use PARSE_DATE() in SQL:

SELECT PARSE_DATE('%Y%m%d', '20190101');

This will return the date “2019-01-01” in ISO 8601 format.

Format Masks

The format mask used with PARSE_DATE() can vary depending on the SQL dialect. Here are some common formats and their equivalents:

%Y: Four-digit year
%y: Two-digit year
%m: Two-digit month (01-12)
%d: Two-digit day (01-31)
%H: Two-digit hour (00-23)
%I: Two-digit hour (01-12)
%M: Two-digit minute (00-59)
%S: Two-digit second (00-59)

For example, to use the PARSE_DATE() function with a different format mask, such as “%Y-%m-%d”, you would modify the format string accordingly:

SELECT PARSE_DATE('%Y-%m-%d', '20190101');

This will also return the date “2019-01-01” in ISO 8601 format.

Handling Errors and Invalid Dates

When using PARSE_DATE(), it’s essential to consider potential errors and invalid dates. If the input string does not match the specified format mask, or if the date is outside the valid range (e.g., February 30th), PARSE_DATE() will return NULL or an incorrect value.

To handle such situations, you can use conditional statements or error-handling mechanisms to identify and handle errors:

SELECT 
    PARSE_DATE('%Y%m%d', '20190101') AS date_value,
    CASE WHEN PARSE_DATE('%Y%m%d', '20190101') IS NULL THEN 'Invalid Date' ELSE 'Valid Date' END AS result;

This will return the converted date value and an additional column indicating whether the input string is valid or not.

Best Practices

Here are some best practices to keep in mind when using PARSE_DATE():

Use the correct format mask: Make sure to use the correct format mask for your specific SQL dialect. Consult your database documentation for more information.
Handle errors and invalid dates: Implement proper error handling and validation mechanisms to identify and handle incorrect or invalid date values.
Test thoroughly: Test your queries with different input strings, formats, and date ranges to ensure accurate results.

Conclusion

In this article, we explored the standard SQL PARSE_DATE() function for converting string dates in different formats. We covered format masks, best practices, and error handling techniques to help you effectively convert string dates using standard SQL.

Last modified on 2023-12-17