SQL Server string addition is NULL

SQL Server TSQL in the case of string concatenation, it is necessary to initialize the string,

Set @errorMsg + = N ' table role has no key ' + '. ' If the @errormsg is not assigned an initial value, the Select @errorMgs will always get null;

Declare @errorMsg nvarchar( -);Declare @tmpErrorMsg nvarchar( -);Declare @pkCount int;Declare @PKTable Table(Table_qualifiervarchar( -), Table_ownervarchar( -), table_namevarchar( -), column_namevarchar( -), Key_seqint, Pk_namevarchar( -));--in the case of string concatenation, it is necessary to initialize the stringSet @errorMsg =N"';Insert  into @PKTable EXECSp_pkeys@table_name='role';Set @pkCount=(Select COUNT(*) from @PKTable);if(@pkCount <= 0)begin    Set @errorMsg +=N'table role does not have a key'+'.'; EndDelete  from @PKTable;

SQL Server string addition is NULL