set @user = 123456;

set @group = (select GROUP from USER where User = @user);

select * from USER where GROUP = @group;

 

SET @user := 123456;

SELECT @group := `group` FROM user WHERE user = @user;

SELECT * FROM user WHERE `group` = @group;

SET @user := 123456;

SELECT `group` FROM user LIMIT 1 INTO @group;

SELECT * FROM user WHERE `group` = @group;

refer:https://stackoverflow.com/questions/3888735/mysql-set-user-variable-from-result-of-query