具有内部联接条件的SQL查询

Question

The context is this: a machine is in a location or in a position inside a location. We have here 2 cases, the machine can have a value in the positionId or if the location hold only one machine, it has 0 as a value in positionsId. I wrote a query to return values for serialNo, name, address, dscription and identifier taking in consideration the cases of existiong or not a position:

SELECT machine.`serialNumber`, location.`name`, location.`address`, product.`identifier`,
(CASE WHEN EXISTS(SELECT `description`
                 FROM `positions` AS pos
                 WHERE pos.`locationId` = `machine`.`id`) 
    THEN `description`
    ELSE 0
END) AS existPos    
FROM machine 
INNER JOIN location 
ON machine.`locationId` = location.`id` 
INNER JOIN positions
INNER JOIN product
ON machine.`id` = product.`machineId`  
WHERE machine.`id` = 5

The result is multiple lines of the same data. Where am i doing it wrong?

Answer 1

Can you try the following:

SELECT 
  machine.`serialNumber`, 
  location.`name`, 
  location.`address`, 
  product.`identifier`,
  (CASE WHEN positions.Id is not null 
    THEN `description`
    ELSE 0
  END) AS existPos    
FROM 
  machine 
INNER JOIN 
  location 
  ON machine.`locationId` = location.`id` 
LEFT JOIN 
  positions
  on positions.`Id` = machine.`positionId`
INNER JOIN 
  product
  ON machine.`id` = product.`machineId`  
WHERE machine.`id` = 5

Answer 2

I guess you're using MySQL. As its documentation says,

In MySQL, JOIN, CROSS JOIN, and INNER JOIN are syntactic equivalents (they can replace each other). In standard SQL, they are not equivalent. INNER JOIN is used with an ON clause, CROSS JOIN is used otherwise.

It means your INNER JOIN positions is actually CROSS JOIN because there isn't ON clause. This is why you get duplicated lines. If you also select some column from positions table, you will see difference in it.

In addition to it, because a machine is in a location or in a position inside a location , you should not use INNER JOIN for location and positions . In your code, once location is inner joined, machines without locationId - FK are all lost. If this is not what you expect, then both table should be joined by LEFT JOIN.

LEFT JOIN location 
ON machine.`locationId` = location.`id` 
LEFT JOIN positions
ON machine.`positionId` = positions.`id`

One more thing that looks weird for me is WHERE pos.locationId = machine.id . This relationship is not described in the diagram you provided above. I guessed you might want to change it to WHERE pos.id = machine.positionId .