How to pivot two rows into two columns - sql

Question

I have the following SQL Query:
select
distinct
Equipment_Reserved.Equipment_Attached_To,
Equipment.Name
from
Equipment,
Studies,
Equipment_Reserved
where
Studies.Study = 'MAINT19-01'
and
Equipment.idEquipment = Equipment_Reserved.Equipment_idEquipment
and
Studies.idStudies = Equipment_Reserved.Studies_idStudies
and
Equipment.Type = 'Probe'
This query produces the following results:
Equipment_Attached_To Name
2297 R1-P1
2297 R1-P2
2299 R1-P3
I would like to change it to the following:
Equipment_Attached_To Name1 Name2
2297 R1-P1 R1-P2
2299 R1-P3 NULL
Thanks for your help!

Answer 1

I'd first change your query from the old, legacy JOIN syntax to an explicit join as it makes the query easier to understand:
SELECT
DISTINCT
Equipment_Reserved.Equipment_Attached_To,
Equipment.Name
FROM
Equipment
INNER JOIN Equipment_Reserved ON Equipment_Reserved.Equipment_idEquipment = Equipment.idEquipment
INNER JOIN Studies ON Studies.idStudies = Equipment_Reserved.Studies_idStudies
WHERE
Studies.Study = 'MAINT19-01'
AND
Equipment.Type = 'Probe'
I don't think you actually need a PIVOT - I think you can do this with a nested query with the ROW_NUMBER function. I've seen that PIVOT queries often have worse query execution plans than nested-queries.
Let's add ROW_NUMBER (which require an ORDER BY as it's a windowing-function) and a matching ORDER BY in the whole query to make it consistent). Let's also use PARTITION BY so it resets the row-number for each Equipment_Attached_To value:
SELECT
DISTINCT
Equipment_Reserved.Equipment_Attached_To,
Equipment.Name,
ROW_NUMBER() OVER (PARTITION BY Equipment_Attached_To ORDER BY [Name]) AS RowNumber
FROM
Equipment
INNER JOIN Equipment_Reserved ON Equipment_Reserved.Equipment_idEquipment = Equipment.idEquipment
INNER JOIN Studies ON Studies.idStudies = Equipment_Reserved.Studies_idStudies
WHERE
Studies.Study = 'MAINT19-01'
AND
Equipment.Type = 'Probe'
ORDER BY
Equipment_Attached_To,
[Name]
This will give output like this:
Equipment_Attached_To Name RowNumber
2297 R1-P1 1
2297 R1-P2 2
2299 R1-P3 1
This can then be split out into explicit columns like so below. The use of MAX() is arbitrary (we could use MIN() instead) and only because we're dealing with a GROUP BY and because the CASE WHEN... restricts the input set to just 1 row anyway.
SELECT
Equipment_Attached_To,
MAX( CASE WHEN RowNumber = 1 THEN [Name] END ) AS Name1,
MAX( CASE WHEN RowNumber = 2 THEN [Name] END ) AS Name2
FROM
(
-- the query from above
)
GROUP BY
Equipment_Attached_To
ORDER BY
Equipment_Attached_To,
Name1,
Name2
So the final query is:
SELECT
Equipment_Attached_To,
MAX( CASE WHEN RowNumber = 1 THEN [Name] END ) AS Name1,
MAX( CASE WHEN RowNumber = 2 THEN [Name] END ) AS Name2
FROM
(
SELECT
DISTINCT
Equipment_Reserved.Equipment_Attached_To,
Equipment.Name,
ROW_NUMBER() OVER (PARTITION BY Equipment_Attached_To ORDER BY [Name]) AS RowNumber
FROM
Equipment
INNER JOIN Equipment_Reserved ON Equipment_Reserved.Equipment_idEquipment = Equipment.idEquipment
INNER JOIN Studies ON Studies.idStudies = Equipment_Reserved.Studies_idStudies
WHERE
Studies.Study = 'MAINT19-01'
AND
Equipment.Type = 'Probe'
)
GROUP BY
Equipment_Attached_To
ORDER BY
Equipment_Attached_To,
Name1,
Name2

Answer 2

Let's start with some basics.
To facilitate reading the code, I added alias to the tables using their initials.
Then, I converted the old join syntax which is partly deprecated to use the standard syntax since 1992 (27 years and people still use the old syntax).
Finally, since there are only 2 possible values, we can use MIN and MAX to separate them in 2 columns.
And because we're using aggregate functions, we remove the DISTINCT and use GROUP BY
The code now looks like this:
SELECT er.Equipment_Attached_To,
--Gets the first row for the id
MIN( e.Name) AS Name1,
--If the MAX is equal to the MIN, returns a NULL. If not, it returns the second value.
NULLIF( MAX(e.Name), MIN( e.Name)) AS Name2
FROM Equipment e
JOIN Studies s ON s.idStudies = er.Studies_idStudies
JOIN Equipment_Reserved er ON e.idEquipment = er.Equipment_idEquipment
WHERE s.Study = 'MAINT19-01'
AND e.Type = 'Probe'
GROUP BY er.Equipment_Attached_To;