Varchar variable is not working in WHERE clause

Question

Why does this work...

DECLARE @MyInt int = 12345;
SELECT * FROM MyTable WHERE MyId = @MyInt; --Returns 1 row
SELECT * FROM MyTable WHERE MyId = 12345;  --Returns 1 row

but this doesn't?

DECLARE @MyVarchar varchar = 'ABCDEF';
SELECT * FROM MyTable WHERE MyId = @MyVarchar; --Returns 0 rows
SELECT * FROM MyTable WHERE MyId = 'ABCDEF';   --Returns 1 row

SQL Server version is 10.50.1746

Answer 1

Because when you declare, default varchar length is 1. So @MyVarchar ends up being 'A'.

This is different to cast(something as varchar), where default length is 30.

The right thing is

DECLARE @MyVarchar varchar(10) = 'ABCDEF';

where 10 is the length of the column in the table.