Question

The following code is supposed to find electors that do not have a postal vote, do have a telephone number are not in possession of certain codes are listed in another table called voting intentions with certain pledges or are not listed in that table at all.

FROM electors,voting_intention 
WHERE
  electors.telephone > 0 
  AND electors.postal_vote != 1 
  AND (
    electors.mosaic NOT IN ('E1','E2','E3') 
    OR (
      electors.ID = voting_intention.elector 
      AND voting_intention.pledge IN ('U','W','X')
    ) 
    OR electors.ID != voting_intention.elector
  )

At the moment it is producing a count of over 2 million records many times the number in the database. Obviously something is wrong but I can’t see my error.

Answer 1

FROM electors e
        LEFT JOIN voting_intention v1 ON e.ID = v1.elector AND v1.pledge IN ('U','W','X') 
        LEFT JOIN voting_intention v2 ON e.ID = v2.elector
WHERE 
    e.telephone > 0 
AND e.postal_vote != 1 /* do not have a postal vote?? */
AND (e.mosaic NOT IN ('E1','E2','E3') 
     OR v1.elector IS NOT NULL
     OR v2.elector IS NULL)