comparing 2 tables

[SirChick]

I am wondering if something like this can be done:

 

 

SELECT userid from userstable

 

Then check a table named Hospital... for userid

 

Then what i need to do is a while loop which will "only" while loop through users that are not in the hospital table but are in the user table..... meaning only people out of hospital will be queried but "all" out of hospital users must be ran through the while loop..

 

is this possible in one query?

[Psycho]

You only need 1 query. Using a loop for queries is extremely inefficient and can create performance issues.

 

The following query should return all users that exist in the user table which do not have any associated records in the hospital table.

 

SELECT u.*

FROM userstable u

  LEFT JOIN hospitals h
    ON u.userid = h.userid

WHERE h.AnyColumInHospitalTable = NULL

[SirChick]

I tried this :

 

$Get = mysql_query("SELECT u.* FROM userregistration u LEFT JOIN hospital h ON u.UserID = h.UserID WHERE h.IllnessType = NULL")
or die (mysql_error());

while(mysql_fetch_assoc($Get)){
echo 'test';
}

 

 

but it didn't work =/ It should have done 4 echo's for 'test'

[Branden Wagner]

What did it do?

did you get an error?

 

[SirChick]

no it didnt echo basically.... just a blank screen

[Psycho]

OK, try changing the "= NULL" to "IS NULL".

 

I have tested it and it works for me.