Complex select statement - php

Question

I have 4 tables
Tbl_Items:
Asset Id,
Asset_name,
Workshop,
PAT_PASS
(workshop, and PAT_PASS are either YES or NO. Workshop = YES and PAT_PASS = NO means this item is not available for hire..)
Tbl_Project_details:
Project_id,
Project_start_date,
project_end_date
TBL_Project_items:
Project_id,
Asset_id,
Start_date,
End_date
I need to do a select statement that allows me to show all the items in tbl_Items EXCEPT:
1) Those that are already in use on another project on the dates that I need them.
2) Are in the workshop. (Workshop = YES)
3) Are not PAT Tested (PAT_PASS = NO)
So far I have started with a simple statement like this:
$post_project_id = "13-1309.01"; //this is just an example number..
$sql =
"SELECT * FROM Project_details
Where Project_id = '$post_project_id' ";
$result = mysql_query($sql);
while ($row = mysql_fetch_array($result))
{
$hire_start_date = $row['Start_date']; // we find out our projects start date
$hire_end_date = $row['End_date']; // we find out our projects end date
}
$result=mysql_query("SELECT Items.Asset_id, Items.Name
FROM
Items,Project_items
WHERE
Project_details.Start_date < '$hire_start_date'
AND
Project_details.End_date > '$hire_end_date'")
or die(mysql_error());
while($row = mysql_fetch_array($result))
{
// fill up an option box with the results
}
Even this basic script doesn't seem to work! This is the first time I have tried selecting from multiple tables, so this is a steep learning curve!
Thanks!

Answer 1

I would go with a subquery:
SELECT * FROM Tbl_Items AS i WHERE
this should matches your (1.) condition: "take only those records, which with that select dont have any occurences":
0 = (
SELECT COUNT(1) FROM TBL_Project_items AS pi
WHERE i.Asset_Id = pi.Asset_Id
AND Start_date < $hire_start_date
AND End_date > $hire_end_date
)
workshop condition (2.):
AND WORKSHOP != "YES"
pat tested condition (3):
AND PAT_PASS != "NO"

Answer 2

$StartDate and $EndDate are your date constraints:
SELECT ITEM.asset_id,
ITEM.asset_name,
ITEM.workshop,
ITEM.pat_pass
FROM Tbl_Items ITEM
LEFT JOIN Tbl_Project_Items PJITEM
ON PJITEM.asset_id = ITEM.asset_id
AND PJITEM.start_date BETWEEN $StartDate AND $EndDate
OR PJITEM.end_date BETWEEN $StartDate AND $EndDate
WHERE ITEM.workshop = 'no'
AND ITEM.pat_pass = 'yes'
AND PJITEM.asset_id IS NULL
Who's Pat?

Answer 3

One problem:
WHERE
Project_details.Start_date < '$hire_start_date'
AND
Project_details.End_date > '$hire_end_date'")
The columns "Start_date" and "End_date" aren't in the "Project_details" table. In that table, those columns seem to be named "Project_start_date" and "Project_end_date".