Difference between different string types in SQL Server?

Question

What is the difference between char, nchar, ntext, nvarchar, text and varchar in SQL?

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Answer 1

text and ntext are deprecated, so lets omit them for a moment. For what is left, there are 3 dimensions:

• Unicode (UCS-2) vs. non-unicode: N in front of the name denotes Unicode
• Fixed length vs. variable length: var denotes variable, otherwise fixed
• In-row vs. BLOB: (max) as length denotes a BLOB, otherwise is an in-row value

So with this, you can read any type's meaning:

• CHAR(10): is an in-row fixed length non-Unicode of size 10
• NVARCHAR(256): is an in-row variable length Unicode of size up-to 256
• VARCHAR(MAX): is a BLOB variable length non-Unicode

The deprecated types text and ntext correspond to the new types varchar(max) and nvarchar(max) respectively.

When you go to details, the meaning of in-row vs. BLOB blurs for small lengths as the engine may optimize the storage and pull a BLOB in-row or push an in-row value into the 'small BLOB' allocation unit, but this is just an implementation detail. See Table and Index Organization.

From a programming point of view, all types: CHAR, VARCHAR, NCHAR, NVARCHAR, VARCHAR(MAX) and NVARCHAR(MAX), support an uniform string API: String Functions. The old, deprecated, types TEXT and NTEXT do not support this API, they have a separate, deperated, TEXT API to manipulate. You should not use the deprecated types.

BLOB types support efficient in-place updates by using the UPDATE table SET column.WRITE(@value, @offset) syntax.

The difference between fixed-length and variable length types vanishes when row-compression on a table. With row-compression enabled, fixed lenght types and variable length are stored in the same format and trailing spaces are not stored on disk, see Row Compression Implementation. Note that page-compression implies row-compression.