declare @role_type int
select @role_type=role_type from accounts_userroles where user_id=1
if @role_type=1
begin
 select * from project
end
else
begin
 select * from project2 where userid=1
end

------解决方案--------------------

SQL code
DECLARE @i INT
SET @i = 1  --可以修改为2

SELECT  *
FROM    ( SELECT    * ,
                    1 AS role_type
          FROM      project
          UNION ALL
          SELECT    project2.* ,
                    2 AS role_type
          FROM      project2
          INNER JOIN accounts_userroles C ON C.USER_ID = project2.USER_id
        ) B
        INNER JOIN accounts_userroles A ON A.role_type = B.role_type
WHERE   A.user_id = @i