SQL查询-使用附加动态列联接多个表

Question

I need help for building a SQL-statement. The Database-Schema looks like this:

I did prepare the following SQL-Fiddle , which contains sample-data: http://sqlfiddle.com/#!9/831bab/1

As for the sample-data in the SQL-Fiddle, I want to query for the computername "Client02" and want to get the following result:

Wanted result 1

PrinterName   | PrintServer | PrinterActive | ComputerActive | isDefaultPrinter
PRT01_Zentral | DC01        | True          | True           | False
PRT02_BH      | DC01        | True          | True           | True

As for the sample-data in the SQL-Fiddle, I want to query for the computername "Client01" and want to get the following result:

Wanted result 2

PrinterName   | PrintServer | PrinterActive | ComputerActive | isDefaultPrinter
PRT01_Zentral | DC01        | True          | True           | True

As you see, I need to join all the tables and add something like a helper-column, which contains information about the default-printer. (True/False)

I started to build up the query, but I don't know how to proceed ...

SELECT printers.PrinterName, printers.PrintServer, printers.PrinterActive
FROM computermapping
LEFT JOIN computers ON computermapping.ComputerID = computers.ComputerID
LEFT JOIN printers ON computermapping.PrinterID = printers.PrinterID
LEFT JOIN computerdefaultprinter ON computers.ComputerID = computerdefaultprinter.ComputerID
WHERE computers.ComputerName = "Client02" 

I think my request contains all the information, which is needed. The SQL-Fiddle has sample-data to easily reproduce it. The WantedResults should show the target clearly.

EDIT:
DBMS: MySQL

Answer 1

What I did was add a if statement in your query, comparing the inner result (that can only be 1) to your outter select. There is a better way of achieving this probably but this answer reflects the queryresult's that you've asked.

Edit: I'm assuming only a default printer per computer. Are you sure the wanted result 2 is correct? Because in the database, records are saying computer1 has 1 printerid (1) that is the default and that computer2 has 2 printers (1 and 2) and 2 is default.

SELECT printers.PrinterName,
       printers.PrintServer,
       printers.PrinterActive,
       IF (
             (SELECT computerdefaultprinter.printerid
              FROM computerdefaultprinter
              WHERE computerdefaultprinter.computerid = computermapping.computerid) = computermapping.printerid,
           'true',
           'false') AS isDefaultPrinter
FROM computermapping
LEFT JOIN computers ON computermapping.ComputerID = computers.ComputerID
LEFT JOIN printers ON computermapping.PrinterID = printers.PrinterID
LEFT JOIN computerdefaultprinter ON computers.ComputerID = computerdefaultprinter.ComputerID
WHERE computers.ComputerName = "Client02"

Answer 2

You could use the following query to get the result that you need

SELECT printers.PrinterName, 
       printers.PrintServer, 
       printers.PrinterActive, 
       computers.ComputerActive, 
       CASE 
         WHEN computerdefaultprinter.PrinterID IS NULL THEN "false" 
         ELSE "true" 
       END AS isDefaultPrinter 
FROM   computermapping 
       LEFT JOIN computers 
              ON computermapping.ComputerID = computers.ComputerID 
       LEFT JOIN printers 
              ON computermapping.PrinterID = printers.PrinterID 
       LEFT JOIN computerdefaultprinter 
              ON computers.ComputerID = computerdefaultprinter.ComputerID 
                 AND printers.PrinterID = computerdefaultprinter.PrinterID 
WHERE  computers.ComputerName = "Client02" 

I've only added CASE...WHEN statement and modified join condition of your original query. But I think instead of using table computerdefaultprinter , why don't you add column IsDefaultPrinter to table computermapping .

CREATE TABLE `computermapping` (
  `ComputerMappingID` int(11) NOT NULL,
  `PrinterID` int(11) NOT NULL,
  `ComputerID` int(11) NOT NULL,
  `IsDefaultPrinter` int(1) NOT NULL DEFAULT 0,
) ENGINE=InnoDB DEFAULT CHARSET=utf8;