Convert SQL Server varchar column to date based on a format

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29-09-2024

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Converting VARCHAR to DATE in SQL Server: A Practical Guide

Often, you encounter scenarios where data in a SQL Server database is stored as a VARCHAR column but represents a date. This can occur when you import data from external sources, receive data in a specific format, or simply have legacy databases with inconsistent data types. This article guides you on converting these VARCHAR columns to DATE format using SQL Server's built-in functions and offers best practices to ensure data integrity.

Let's assume we have a table named "Customer" with a column "BirthDate" storing dates as VARCHAR in the format 'yyyy-mm-dd'. Here's an example:

CREATE TABLE Customer (
    CustomerID INT PRIMARY KEY,
    CustomerName VARCHAR(50),
    BirthDate VARCHAR(10)
);

INSERT INTO Customer (CustomerID, CustomerName, BirthDate) VALUES
    (1, 'Alice', '1990-01-15'),
    (2, 'Bob', '1985-06-20'),
    (3, 'Charlie', '1978-12-05');

To convert this VARCHAR column to DATE format, we can use the CONVERT function in conjunction with a specific style code.

ALTER TABLE Customer
ADD BirthDate_Date DATE;

UPDATE Customer
SET BirthDate_Date = CONVERT(DATE, BirthDate, 120);

SELECT * FROM Customer;

Explanation:

1. ALTER TABLE Customer ADD BirthDate_Date DATE; - We create a new column named BirthDate_Date of type DATE to store the converted values.
2. CONVERT(DATE, BirthDate, 120); - We use the CONVERT function to perform the conversion.
   • DATE: Specifies the target data type (DATE in this case).
   • BirthDate: The column containing the date values in VARCHAR format.
   • 120: This style code represents the format 'yyyy-mm-dd'. Refer to SQL Server documentation for a complete list of style codes.

Key Points to Consider:

• Validating the Data: Before converting, it's essential to verify that all values in the VARCHAR column are valid dates in the expected format. You can use the ISDATE() function for this purpose.
• Handling Invalid Dates: If the VARCHAR column contains invalid dates, you need to decide how to handle them. You can either:
  • Reject the invalid rows: Prevent the conversion from happening for invalid dates.
  • Replace invalid dates: Set invalid dates to a default value, like '1900-01-01', or use a custom logic based on your requirements.
• Data Type Precision: If you need to store time information along with the date, use the DATETIME or DATETIME2 data type instead of DATE.

Example with Handling Invalid Dates:

UPDATE Customer
SET BirthDate_Date = 
  CASE 
    WHEN ISDATE(BirthDate) = 1 THEN CONVERT(DATE, BirthDate, 120)
    ELSE '1900-01-01'
  END;

In this example, if the BirthDate is a valid date, it's converted to a DATE value. Otherwise, it's replaced with '1900-01-01'.

Conclusion:

Converting VARCHAR columns to DATE in SQL Server is a common task. By understanding the CONVERT function, style codes, and data validation techniques, you can ensure a smooth and accurate conversion process. Remember to choose the appropriate data type for your needs and handle invalid dates effectively.