Show Results not in joining table

Question

I have three tables:

1. children
2. activities
3. signupActivity

Table 'children' contains details of children.

Table 'activities' contains details of an activity.

When a child signs up for an activity an entry is added to the table with the activityID and childID. I am trying to get a list of children that haven't signed up to an activity. I have tried the following query but I am getting an error:

"Not unique table/alias: 'children'"

SELECT children.childrenEmailAddress
FROM children 
    INNER JOIN activities ON signupActivity.SignupActivityID = activities.activityID
    INNER JOIN children ON signupActivity.signupActivitychildID = children.childrenID
LEFT JOIN signupActivity ON children.childrenID = signupActivity.signupActivitychildID
WHERE activities.activityID = 8

The recordset would also need to include a result based on joining a 'section' in the tables Children and Activity. children.childrenSection = activities.activitySection as well as filtering the recordset by activities.activityID

This was the final that I adapted from Arth:

 SELECT c.childrenEmailAddress
     FROM children c
     JOIN activities a
       ON a.activitySection = c.childrenSection
        AND a.activityID = 8
LEFT JOIN signupActivity sa 
       ON sa.signupActivitychildID = c.childrenID
    WHERE sa.signupActivitychildID IS NULL

Answer 1

I'd use a LEFT JOIN and a NULL check

   SELECT c.childrenEmailAddress
     FROM children c
LEFT JOIN signupActivity sa 
       ON sa.signupActivitychildID = c.childrenID
      AND sa.SignupActivityID = 8
    WHERE sa.signupActivitychildID IS NULL

Personally I'm not a fan of repeating the table name in each of the column names.. I find it unnecessary and frustrating to work with. Your query could be as simple as:

   SELECT c.emailAddress
     FROM child c
LEFT JOIN signupActivity sa 
       ON sa.childId = c.id 
      AND sa.activityID = 8
    WHERE sa.childId IS NULL 

UPDATE

   SELECT c.childrenEmailAddress
     FROM children c
     JOIN activities a
       ON a.activitySection = c.childrenSection
      AND a.activityID = 8
LEFT JOIN signupActivity sa 
       ON sa.signupActivitychildID = c.childrenID
      AND sa.signupActivityactivityID = a.activityID
    WHERE sa.signupActivitychildID IS NULL