sql server convert varchar to datetime

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## SQL Server: Converting Varchar to Datetime - A Comprehensive Guide

This tutorial provides a detailed guide on how to convert `varchar` data to `datetime` (or `date`, `smalldatetime`, etc.) datatypes in SQL Server. It covers various aspects, including:

* Why conversion is necessary
* Using `CONVERT` and `CAST` functions
* Understanding datetime styles
* Handling potential errors
* Choosing the right target datatype
* Performance considerations
* Best practices

**1. Why Convert Varchar to Datetime?**

SQL Server stores data in various datatypes. While `varchar` is suitable for storing strings, it's inefficient and problematic for storing date and time information. Dates and times stored as `varchar`:

* **Cannot be used in datetime-specific calculations:** You can't easily calculate the difference between two dates, add or subtract days/months, or extract specific date parts (year, month, day).
* **Are prone to data entry errors:** Users can enter dates in inconsistent or invalid formats, leading to data quality issues.
* **Hinder sorting and filtering:** Sorting `varchar` dates lexicographically (alphabetically) will not produce the correct chronological order (e.g., '2023-01-10' will come before '2023-02-01').
* **Waste storage space:** Datetime datatypes are typically more storage-efficient than storing dates as strings.

Converting `varchar` data to a proper datetime datatype solves these problems, allowing you to leverage SQL Server's built-in datetime functions and ensure data integrity.

**2. Functions for Conversion: CONVERT and CAST**

SQL Server provides two primary functions for datatype conversion:

* **`CONVERT()`:** A versatile function with more options, especially when dealing with datetime conversions. It allows you to specify a *style* that indicates the format of the input string.
* **`CAST()`:** A more ANSI-standardized function with simpler syntax. It's generally preferred for simple conversions when you don't ...

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