T-SQL: Safe and Accurate Date String Comparison with datetime

Dates can be tricky to compare because:

• Dates in strings might have different formats (e.g., "YYYY-MM-DD" vs "MM/DD/YYYY").
• You might want to compare only the date part (without time) of a datetime value.

Converting the String:

There are two main approaches to compare a date string with a datetime value:

Comparing Dates:

Once you have a datetime value from the string (implicit or explicit conversion), you can use standard comparison operators like =, <, >, etc., to compare it with the datetime column in your query.

Key Points:

• Be mindful of date format consistency between your string and the stored datetime data.
• Explicit conversion with CONVERT or CAST is generally recommended for clarity and avoiding conversion errors.
• You can use functions like DATE or YEAR on the datetime column to extract only the date part for comparison if needed.

-- Assuming your date string is in 'YYYY-MM-DD' format and matches database format
SELECT *
FROM MyTable
WHERE MyDateColumn = '2024-07-09';

Explicit Conversion (Recommended):

-- Converting the string to datetime with specified format
SELECT *
FROM MyTable
WHERE MyDateColumn = CONVERT(datetime, '2024-07-09', 120);

-- Another option using CAST with format specifier
SELECT *
FROM MyTable
WHERE MyDateColumn = CAST('2024-07-09' AS datetime USING 'YYYY-MM-DD');

Comparing Only Date Part:

-- Assuming your datetime column is 'MyDateTimeColumn'
SELECT *
FROM MyTable
WHERE DATEADD(day, 0, DATEDIFF(day, 0, MyDateTimeColumn)) = '2024-07-09';

-- This extracts the date part from MyDateTimeColumn and compares it to '2024-07-09'

While not generally recommended for date comparisons due to potential formatting issues, you can use the LIKE operator with wildcard characters (%). This is only suitable if your string format is very specific and unlikely to change.

SELECT *
FROM MyTable
WHERE MyDateColumn LIKE '2024-07-%';  -- Matches dates starting with '2024-07-'

TRY_CONVERT Function (Error Handling):

The TRY_CONVERT function allows you to attempt conversion and handle potential errors gracefully. It returns NULL if the conversion fails.

SELECT *
FROM MyTable
WHERE TRY_CONVERT(datetime, MyDateColumn) >= '2024-07-09';

CASE Expression (Conditional Logic):

You can use a CASE expression to define different logic based on whether the conversion is successful.

SELECT *,
  CASE WHEN TRY_CONVERT(datetime, MyDateColumn) IS NOT NULL
       THEN TRY_CONVERT(datetime, MyDateColumn) >= '2024-07-09'
       ELSE 0  -- Handle invalid conversion (optional)
  END AS IsValidDate
FROM MyTable;

CAST with TRY...CATCH Block (Advanced Error Handling):

For more robust error handling, you can combine CAST with a TRY...CATCH block to capture conversion exceptions.

SELECT *
FROM MyTable
TRY
  WHERE CAST(MyDateColumn AS datetime) >= '2024-07-09'
CATCH
  -- Handle conversion error (optional)