Question

How do you write a query that executes for each different result in a query? To explain, I run one query to find all the “punch_periods” within a date range:

$search_date = mysql_query("SELECT punch_period 
                            FROM timecards 
                            WHERE emp_id ='1' && 
                            punch_time BETWEEN '$start' AND '$end' 
                            ORDER BY punch_time"); 

while($periods = mysql_fetch_array($search_date)) {

    // run another query for each different punch_period

    }

The result might be 1,1,1,1,2,2. Instead of running a second query 6 times (for each result), I need to run a second query 2 times (for each DIFFERENT punch_period). It will be something like:

$punches = mysql_query("SELECT * FROM timecards 
                        WHERE emp_id ='1' && 
                        punch_period = [RESULT FROM 1ST QUERY]"); 

The reason I’m doing it this way is because I can’t search by date alone- someone may clock in at 11:30pm one day and clock out at 1:00am the following “day”. By searching by unique punch periods, I can get total hours worked in a “workday” even if it overlaps a true calendar “day”.

Answer 1

I fail to see the purpose of the while loop.
You are just pinging the database server.
It be much faster to get all rows in one query using a subselect

SELECT DISTINCT tc.* FROM timecards tc
WHERE tc.emp_id = '1' 
  AND tc.punch_period IN (SELECT DISTINCT tc2.punch_period 
                       FROM timecards tc2
                       WHERE tc2.emp_id ='1' 
                         AND tc2.punch_time BETWEEN '$start' AND '$end')
ORDER BY punch_time

Obviously you can simplify this query to:

SELECT DISTINCT tc.* FROM timecards tc
WHERE tc.emp_id = '1'
  AND tc.punch_time BETWEEN '$start' AND '$end'

SELECT * is not recommended
Note that SELECT * is considered an anti-pattern and it is better to explicitly name the fields that you want to select.

SELECT tc.id, tc.punch_time, tc.punch_period ....

Mixing && and AND
Don’t mix && and AND, just use AND the mix is confusing and && is not standard SQL IIRC.