--  方式 1
    DECLARE cnt INT DEFAULT 0;
    select count(*) into cnt from test_tbl;
    select cnt;
    
--  方式 2
    set @cnt = (select count(*) from test_tbl);
    select @cnt;
    
--  方式 3
    select count(*) into @cnt1 from test_tbl;
    select @cnt1;
    
--  多个列的情况下似乎只能用 into 方式
    select max(status), avg(status) into @max, @avg from test_tbl;
    select @max, @avg;