sql server字符串相加为NULL

declare @errorMsg nvarchar(500);
declare @tmpErrorMsg nvarchar(500);
declare @pkCount int;
declare @PKTable table
(TABLE_QUALIFIER varchar(100),
TABLE_OWNER varchar(100),
TABLE_NAME varchar(100),
COLUMN_NAME varchar(100),
KEY_SEQ int,
PK_NAME varchar(100));
--在有字符串拼接的情況下，必須要初始化字符串
set @errorMsg = N‘‘;
insert into @PKTable EXEC sp_pkeys @table_name=‘role‘;
set @pkCount=(select COUNT(*) from @PKTable);
if(@pkCount <= 0)
begin
    set @errorMsg += N‘表 role 沒有主鍵‘+‘.‘; 
end
delete from @PKTable;