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base64-decode-transact-sql.md

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title BASE64_DECODE (Transact-SQL)
description BASE64_DECODE converts a Base64 encoded varchar into the corresponding varbinary.
author abledenthusiast
ms.author aaronpitman
ms.reviewer wiassaf, randolphwest
ms.date 01/13/2026
ms.service sql
ms.subservice t-sql
ms.topic reference
ms.custom
ignite-2025
f1_keywords
BASE64_DECODE
BASE64_DECODE_TSQL
helpviewer_keywords
Base64 decode [SQL Server], Base64 decode
BASE64_DECODE function
Base64 decoding [SQL Server]
dev_langs
TSQL
monikerRange =azuresqldb-current || =fabric || =fabric-sqldb

BASE64_DECODE (Transact-SQL)

[!INCLUDE sqlserver2025-asdb-asmi-fabricse-fabricdw-fabricsqldb]

BASE64_DECODE converts a Base64-encoded varchar expression into the corresponding varbinary expression.

:::image type="icon" source="../../includes/media/topic-link-icon.svg" border="false"::: Transact-SQL syntax conventions

Syntax

BASE64_DECODE ( expression )

Arguments

expression

An expression of type varchar(n) or varchar(max).

Return types

  • varbinary(8000) if the input is varchar(n).
  • varbinary(max) if the input is varchar(max).
  • If the input expression is NULL, the output is NULL.

Remarks

The encoded string's alphabet must be that of RFC 4648 Table 1 and might include padding, though padding isn't required. The URL-safe alphabet specified within RFC 4648 Table 2 is also accepted. This function ignores whitespace characters: \n, \r, \t, and .

  • When the input contains characters not contained within the standard or URL-safe alphabets specified by RFC 4648, the function returns the following error:

    Msg 9803, Level 16, State 20, Line 15, Invalid data for type "Base64Decode"

  • If the data has valid characters but is incorrectly formatted, the function returns error Msg 9803, State 21.

  • If the input contains more than two padding characters or padding characters followed by extra valid input, the function returns error Msg 9803, State 23.

Examples

A. Standard BASE64_DECODE

In the following example, the Base64 encoded string is decoded back into varbinary.

SELECT BASE64_DECODE('qQ==');

[!INCLUDE ssResult_md]

0xA9

B. BASE64_DECODE a standard Base64 string

In the following example, the string is Base64 decoded. Note the string contains URL-unsafe characters = and /.

SELECT BASE64_DECODE('yv7K/g==');

[!INCLUDE ssResult_md]

0xCAFECAFE

C. BASE64_DECODE varchar url_safe Base64 string

Unlike example B, this example uses RFC 4648 Table 2 (url_safe) to encode the Base64 string. However, you can decode it the same way as example B.

SELECT BASE64_DECODE('yv7K_g');

[!INCLUDE ssResult_md]

0xCAFECAFE

D. BASE64_DECODE varchar contains characters not in the Base64 alphabet

This example contains characters that aren't valid Base64 characters.

SELECT BASE64_DECODE('qQ!!');

[!INCLUDE ssResult_md]

Msg 9803, Level 16, State 20, Line 223
Invalid data for type "Base64Decode".

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