Why do I get "Procedure expects parameter '@statement' of type 'ntext/nchar/nvarchar'." when I try to use sp_executesql?

Inner workings of sp_executesql

sp_executesql is Microsoft's generous offering to SQL Server enthusiasts that let's one execute a T-SQL statement multiple times. Think of it as getting multiple rides with a single ticket. However, this ticket to ride accepts only Unicode strings. Here's where we wave goodbye to VARCHAR and roll out red carpet for NVARCHAR.

The NVARCHAR Trump Card

sp_executesql demands NVARCHAR because of its stellar ability to handle multilingual data. The Unicode format of NVARCHAR supports every character in every language. Unsurprisingly, VARCHAR can't brag about this!

The VARCHAR Trap

While VARCHAR may seem to be an innocent choice, it can invite not just data type mismatch errors, but also subtle, sneaky bugs. Imagining filtering data with non-English parameters in dynamic SQL. If you used VARCHAR, your non-English characters might go awry within the query.

VARCHAR to NVARCHAR Conversion 101

If you're tangled up in VARCHAR, converting to NVARCHAR needs no magic potion. Simply, flip the data type and add the N prefix to your strings like below:

-- The sneaky VARCHAR
DECLARE @SQL VARCHAR(100);
-- The segment begins here
DECLARE @SQL NVARCHAR(100);
SET @SQL = N'SELECT TOP 1 * FROM sys.tables';
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So, here's the golden rule: Prefix dynamic SQL strings with N when bouncing them to NVARCHAR variables.

Expanding SQL vocabulary with sp_executesql

Different chores require different tools. In the sp_executesql world, whether it's passing parameters, fencing within procedural logic, or sailing through nested queries, NVARCHAR is the non-negotiable currency.

• Addings parameters to dynamic SQL aren't just bells and whistles, they need to be declared as NVARCHAR and prefixed with N:

  DECLARE @SQL NVARCHAR(MAX), @ParamDef NVARCHAR(MAX), @TableName NVARCHAR(128);
  SET @TableName = N'YourTable';
  SET @SQL = N'SELECT * FROM ' + @TableName + N' WHERE YourCondition = @Value'; // Seems like three's not a crowd here!
  SET @ParamDef= N'@Value INT';
  EXEC sp_executesql @SQL, @ParamDef, @Value = 10;
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• Even if you're spicing up your dynamic SQL with control-of-flow language like IF...ELSE or doing loops, NVARCHAR remains indispensable.
• When sp_executesql calls are nested like an onion, every layer still cries out for NVARCHAR.