How could I simplify this mysql statement? - mysql

Question

I use follow mysql statement to get some info from mysql via php.
( SELECT *
FROM mytable
WHERE qid NOT IN ({$used['used']})
AND level = 1
ORDER BY RAND()
LIMIT 5)
UNION
( SELECT *
FROM app_mytable _qt
WHERE qid NOT IN ({$used['used']})
AND level = 2
ORDER BY RAND()
LIMIT 5)
UNION
( SELECT *
FROM app_mytable _qt
WHERE qid NOT IN ({$used['used']})
AND level = 3
ORDER BY RAND()
LIMIT 5)
UNION
( SELECT *
FROM app_mytable _qt
WHERE qid NOT IN ({$used['used']})
AND level = 4
ORDER BY RAND()
LIMIT 5)
$used['used'] is a set of qid that likes 23,31,653,147,146,134,6.....
How could I simplify this mysql statement?

Answer 1

You can use user variables to keep running totals by groups. This is untested but something like the following should work:
select *,
#running:=#previous:=NULL
from (
select *,
#running:=if(#previous=inside.level,#running,0)+1 as TOTAL,
#previous:=inside.level
from (
select *
from mytable
where qid NOT IN ({$used['used']})
order by level, rand()
) as inside
)as outside
where TOTAL < 5;

Answer 2

Some dynamic sql can eliminate the copy pasting for ya... that way you can easily extend this out for as many levels as you like, just mod the max_level.
DECLARE current_level INT; SET current_level = 1;
DECLARE max_level INT; SET max_level = 4;
DECLARE full_sql VARCHAR(1000); SET full_sql = '';
DECLARE base_sql VARCHAR(1000); SET base_sql = '(SELECT * FROM mytable WHERE qid NOT IN ({$used['used']}) AND level = ? ORDER BY RAND() limit 5) ';
WHILE current_level <= max_level
SET full_sql = CONCAT(full_sql, REPLACE(base_sql, '?', current_level));
IF current_level < max_level THEN
SET full_sql = CONCAT(full_sql, ' UNION ');
END IF
SET current_level = current_level + 1;
END WHILE
PREPARE s1 FROM full_sql;
EXECUTE s1;
DEALLOCATE PREPARE s1;