Why Choosing the Right Data Type Matters in Database Design

 

Why Choosing the Right Data Type Matters in SQL Server

Storage is a crucial component in any RDBMS. To address this, Microsoft provides a variety of data types in SQL Server, each designed to optimize storage and performance. Whether you're working with OLTP or OLAP databases, how queries are processed and data is stored directly impacts performance and infrastructure costs.

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VARCHAR vs NVARCHAR

Choosing between VARCHAR and NVARCHAR is a common decision developers face. Here are key considerations:

• Use NVARCHAR for Unicode or multilingual data.

• If your data contains only English characters and numbers, VARCHAR is generally more storage-efficient.

Important considerations before choosing:

• NVARCHAR Consumes 2 bytes per character, increasing storage requirements.

• Indexed columns use additional bytes depending on the data type.

• When using stored procedures, always match data types between parameters and variables. Mixing VARCHAR and NVARCHAR can lead to performance degradation due to implicit conversions.

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DATE vs DATETIME (and other date/time types)

Each SQL Server date/time data type serves a different purpose and uses different storage sizes. Choosing the correct one improves both performance and clarity.

Data Type	Format	Range	Accuracy	Storage (bytes)	Fractional Seconds	Time Zone Offset
TIME	hh:mm:ss	00:00:00.0000000 – 23:59:59.9999999	100 nanoseconds	3 to 5	Yes	No
DATE	YYYY-MM-DD	0001-01-01 – 9999-12-31	1 day	3	No	No
SMALLDATETIME	YYYY-MM-DD hh:mm:ss	1900-01-01 – 2079-06-06	1 minute	4	No	No
DATETIME	YYYY-MM-DD hh:mm:ss	1753-01-01 – 9999-12-31	~3 milliseconds	8	No	No
DATETIME2	YYYY-MM-DD hh:mm:ss	0001-01-01 – 9999-12-31 23:59:59.9999999	100 nanoseconds	6 to 8	Yes	No
DATETIMEOFFSET	YYYY-MM-DD hh:mm:ss [+	-]hh:mm	0001-01-01 – 9999-12-31 (with time zone awareness)	100 nanoseconds	8 to 10	Yes

Tip: Be cautious when using date/time types in WHERE clauses. Using the wrong data type or format may yield unexpected or inefficient results.

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Data Type Conversion in SQL Server

SQL Server allows both explicit and implicit conversions between system-defined data types such as xml, bigint, and sql_variant.

• Note: While implicit conversion to sql_variant is supported, implicit conversion from sql_variant is not.

SQL Server data type conversion chart

Reference:
Data Type Conversion - SQL Server Docs