在MYSQL中联接多个表

Question

Here's an example of the kind of join I'm trying to accomplish. http://sqlfiddle.com/#!2/aab00/3 .

You can see that there are records (Name2, Name3) that satisfy the WHERE clause but do not show up. Also the record for Name1 that shows up isn't accurate.

Answer 1

You should do the aggregation before the join s:

SELECT t3.PersonName AS Name,
       COALESCE(t1.NumberOfCars, 0) AS NumberOfCars,
       COALESCE(TMobile, 0) AS TMobileConnections,
       COALESCE(ATT, 0) AS ATTConnections
FROM Table3 t3 LEFT OUTER JOIN
     Table1 t1
     ON t3.PersonSSN = t1.PersonSSN and t1.State = 'FL' LEFT OUTER JOIN
     (select t2.PersonSSN, t2.state,
             max(t2.Carrier = 'T Mobile') as TMobile,
             max(t2.Carrier = 'AT&T') as ATT
      from Table2 t2
      group by t2.PersonSSN, t2.state
     ) t2
     ON t3.PersonSSN = t2.PersonSSN and t2.State = 'FL';

You could also do the filtering by state in the subqueries, but this works for this query.

Note that I changed all the double quotes to single quotes. Always use single quotes for string and date constants -- that is standard SQL. Use double quotes or backticks for quoting identifiers, when needed.