Dropping rows based on the previous selection statmnet

Question

I have a database with following tables: temp_1, temp_2.... and cached_tbl. Inside cached_tbl I have column table_name and in corresponding rows I store all the temp table names I mentioned above. SO what I wanted to do is to create event scheduler that would drop temp tables that were created more than a day ago...For that purpose I am writing something like

SELECT (DROP) 
    *
FROM
    information_schema.TABLES
WHERE 
    table_schema = 'db' and CREATE_TIME < (NOW()-INTERVAL 24 HOUR)

But I also wanted to delete the rows containing just removed table names inside cached_tbl table... Please advise how I can do it... In the regular programming I would just through all the selected table_names from previous script into the array and go through the loop comparing this table names to the table names in cached_tbls... But I have no idea how implement it in mysql

Answer 1

To get a dynamically formed SQL statement you need to get it into a user variable, @drop, and then execute this:

SELECT CONCAT('DROP TABLE ',
              GROUP_CONCAT(CONCAT(TABLE_SCHEMA,'.',TABLE_NAME))) INTO @drop
FROM information_schema.TABLES
WHERE  table_schema = 'db' AND
       table_name like 'temp%' AND
       CREATE_TIME < (NOW()-INTERVAL 24 HOUR);
PREPARE dropstmt FROM @drop;
EXECUTE dropstmt;
DEALLOCATE PREPARE dropstmt;

To clean up cached_tbl, delete where there isn't match on the join:

DELETE c FROM cached_tbl c 
LEFT JOIN information_schema.TABLES t ON t.TABLE_NAME = c.table_name
WHERE t.TABLE_NAME IS NULL